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<body>
<h1>Graphical Definitions in R Graphics</h1>
<p>
<span style="font-style: italic">by Paul Murrell</span>
<a href="http://orcid.org">
<img alt="" src="https://www.stat.auckland.ac.nz/~paul/ORCID/ORCiD.png" style="width: 16px; height: 16px; vertical-align: middle">
</a>
<span style="font-family: mono; font-size: small">
<a href="http://orcid.org/0000-0002-3224-8858">http://orcid.org/0000-0002-3224-8858</a>
</span>
</p>
<p class="date">
Version 1: original publication
</p>
<hr>
<p><a rel="license" href="http://creativecommons.org/licenses/by/4.0/"><img class="CC" alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by/4.0/88x31.png"></a><br><span xmlns:dct="http://purl.org/dc/terms/" property="dct:title">This document</span>
by <span xmlns:cc="http://creativecommons.org/ns#" property="cc:attributionName">Paul
Murrell</span> is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/4.0/">Creative
Commons Attribution 4.0 International License</a>.
</p>
<hr>
<p>
This document describes a plan to expand the R graphics engine
to support graphical definitions: graphical features like gradients,
patterns, masks, and clipping paths.
</p>
<p>
The code changes are being
tracked in a
subversion branch called <code>R-defs</code>.
</p>
<p>
This is a living document that will evolve as the code is developed.
</p>
<p>
The <a href="#features">Features</a> Section outlines the features that I
would like to target and the
<a href="#demos">Demonstrations</a> Section provides a glimpse of how some
new features are currently implemented at the user
level (in the <code>R-defs</code> branch).
</p>
<p>
The <a href="#internals">Internal details</a> Section has more on the internal design and
internal code changes
that have been made.
</p>
<p>
The <a href="#issues">Issues</a> Section contains some of the major design
issues that I have identified so far and
the <a href="#discussion">Discussion</a> Section contains some questions and
(some) answers that I have already thought a bit about,
including the changes that would be imposed
on graphics device maintainers.
</p>
<div>
<h2>Table of Contents:</h2>
<ul style="list-style: none">
<li>
<a href="#intro">1. Introduction</a>
</li>
<li>
<a href="#issues">2. Issues</a>
</li>
<li>
<a href="#features">3. Features</a>
</li>
<li>
<a href="#demos">4. Demonstrations</a>
</li>
<li>
<a href="#internals">5. Internal details</a>
</li>
<li>
<a href="#discussion">6. Discussion</a>
</li>
</ul>
</div>
<h2>
<a name="intro">1. Introduction</a>
</h2>
<p>
The R graphics engine only supports a limited set of graphical
features: basic shapes like lines, rectangles, and circles;
paths (including shapes with holes);
text; raster images; and rectangular clipping regions.
These features are sufficient to produce a wide range of statistical plots,
but there are many things that R graphics cannot do.
</p>
<p>
For example, the image below contains a set of filled contours within
a map of New Zealand. The natural way to achieve this result would
be to draw the filled contours and apply a clipping path to the
filled contours based on
the boundary of New Zealand. That is
not something that can
be achieved in R graphics (because R graphics only supports
rectangular clipping regions).<a href="#clippingpathworkaround"><sup>1</sup></a>
</p>
<p>
<img src="gore-nzoverlaypath.png" alt="contour lines clipped to a map">
</p>
<p>
The plan is to add new graphical
features to the R graphics engine, such as clipping paths,
in order to expand the range of graphical images that can be produced
with R.
</p>
<h2>
<a name="issues">2. Issues</a>
</h2>
<p>
There are a number of issues that need to be resolved
when adding a new graphics feature to the R graphics engine.
</p>
<dl>
<dt>
Device-independence:
</dt>
<dd>
<p>
Can we generate the same output on all R graphics devices?
</p>
<p>
This
horse has already bolted.
The output on different R graphics devices has never been identical
and there are already graphics features
that are not supported on all R graphics devices (e.g.,
translucent colours are not supported on the PostScript device).
The plan is to double-down on this approach and just allow
graphics devices to opt out of any or all of the new features.
</p>
<p>
This is partly because not all devices will be capable
of supporting the new features and partly because it makes it
easy to roll out the new features; graphics devices can
initially opt out of all new features and add support as
and when they are able.
</p>
</dd>
<dt>
<code>SEXP</code>s in graphics devices:
</dt>
<dd>
<p>
Can we avoid sending R's <code>SEXP</code> structures to
graphics devices ?
</p>
<p>
In the early days, there was an effort to avoid using R's internal
C structures within graphics devices (so that they only had to
include R's graphics-related header files and they did not have to
worry about complications like R's memory management).
</p>
<p>
This horse has also bolted. From R 2.13.2 (at least), when the
<code>cap</code> graphics device procedure was added (to allow
raster "screen shots" of current device state), devices have
been able to at least return R structures.
</p>
<p>
The plan is again to double-down. Some of the new features
will require sending relatively complex information to the graphics
device (e.g., gradient information and
pattern information), so rather than
define complex new C structures, it makes sense to make use
of R's existing flexible structures.
</p>
<p>
One problem is that an SEXP in C is basically an opaque pointer.
How does the device know about changes to the SEXP structure?
The proposal is to
write a C API for interacting with the SEXP structures
and encourage
devices to use that. An example has been developed for
gradient fills (see the
<a href="#demos">Demonstrations</a> Section).
</p>
</dd>
<dt>
The feature set:
</dt>
<dd>
<p>
How to decide which graphics features to support?
</p>
<p>
This is almost the inverse of the device-independence problem.
Do we shoot for the union of all features supported by all
possible graphics devices ? Or do we target the feature set
of one particular device or format (e.g., SVG) ?
</p>
<p>
The plan is to take ideas from everywhere.
There are features that will only be supported by some
formats (e.g., SVG has filters, but Cairo has only very
limited filter support, whereas Cairo has mesh patterns,
but SVG does not).
There are also subtleties between formats, like the
'gradientUnits' in SVG that are not
supported in Cairo, but we can hopefully make an interface
for these (at least in 'grid') even if the graphics engine
only goes as far as Cairo support.
</p>
<p>
The PDF feature set is probably the most sophisticated (?).
For example, most graphics formats have linear and radial gradients,
Cairo graphics has "mesh patterns" that SVG does not,
but PDF has seven types of "shaders"
(type 2=Linear gradients, 3=Radial gradients,
4&5=Gouraud-shaded triangle meshes,
6=Coons patch meshes, and 7=Tensor-product patch meshes)
including Function-based shaders (type 1) that define the
colour of every point in the image using a mathematical function.
Gradient meshes have been proposed for SVG, but it
<a href="http://libregraphicsworld.org/blog/entry/gradient-meshes-and-hatching-to-be-removed-from-svg-2-0">does
not look like they will make it into SVG 2.0</a>.
</p>
</dd>
<dt>
'grid' versus 'graphics'
</dt>
<dd>
<p>
Will the new features be implemented for 'graphics' or 'grid'
or both ?
</p>
<p>
The new features will be implemented within the graphics engine.
This means that, in theory, interfaces can be made for both
'graphics' and 'grid'.
</p>
<p>
The plan is to focus, initially at least, on 'grid'.
This is because
'grid' allows more flexible and interesting definitions,
for example, the use of units in locating gradient stops.
'grid' also allows for things like adding features
by post-editing because we are able to address graphical
objects by name.
</p>
<p>
It is also possible in the meantime
to apply the new features to 'graphics'
output via <code>gridGraphics::grid.echo</code>
and functions like <code>grid::grid.edit</code>.
</p>
</dd>
<dt>
Graphics devices
</dt>
<dd>
<p>
Which graphics devices should implement which features ?
</p>
<p>
Obviously, only some features can be added to some graphics devices
because of limitations in graphics formats and/or graphics
libraries. This issue relates more to <em>when</em> support
gets added to different graphics devices. The problem is that
implementing all possible features on all possible devices
is a big job (made more difficult by the development environment
on some platforms, like Windows).
</p>
<p>
The proposal is to be feature-greedy and just
focus initially on the Cairo graphics devices.
This gives a reasonable coverage (raster on Linux at least, PDF,
and SVG) from a single set of code changes.
The idea would be to introduce as many features as possible,
on the Cairo devices, for 4.0.0 and then fill in the gaps.
Once the Cairo changes are implemented, they will hopefully
provide a template so that others
could contribute with patches
for other devices.
</p>
<p>
A danger is that only implementing for Cairo may "bias" the
graphics engine implementation towards that device and cause
problems for implementations on other graphics devices.
Perhaps implementing for two devices (e.g., also the PDF device)
rather than one
would be more prudent.
</p>
<p>
Another issue is that the default, "graphapp", Windows
device is NOT going to be able to support these features
very easily (if at all). GraphApp does not support any
of these features (?) (even in the
<a href="http://enchantia.com/graphapp/doc/manual/">latest version</a>
and not even the underlying Windows GDI supports gradient fills
(though it looks like clipping paths are possible).
OTOH, implementing features on the Cairo device makes them
available on all platforms (?).
I am unsure about whether the RStudio plot pane (RStudioGD) will
be able to support these features (across platforms).
</p>
<p>
Third-party graphics devices can of course update as much as they
want when they want, though they will have to immediately
provide some minimal stubs for the changes to the graphics
device API (<code>GraphicsDevice.h</code>).
</p>
</dd>
</dl>
<h2>
<a name="features">3. Features</a>
</h2>
<p>
This section briefly describes
the set of graphical features that are currently
being considered.
</p>
<dl>
<dt>
Gradients
</dt>
<dd>
<p>
This includes linear gradients and radial gradients.
Gradients can be applied to both fills and strokes
(at least on some devices).
</p>
<p>
Gradient meshes are also supported on several devices
(Cairo and PDF).
</p>
</dd>
<dt>
Patterns
</dt>
<dd>
<p>
Pattern fills are similar to gradient fills in that they
define a "paint" for filling a region. The difference is
that a pattern fill is based on "tiling" a smaller image.
</p>
<p>
There are other sorts of pattern fills in some formats
(e.g., function-based shaders in PDF).
</p>
</dd>
<dt>
Clipping paths
</dt>
<dd>
<p>
Allow clipping to an arbitrary path rather than just a
rectangle. This relies on being able to specify a
"path" separate from drawing (stroking or filling) it.
</p>
</dd>
<dt>
Masks
</dt>
<dd>
<p>
Similar to clipping except that affect translucency of
rendered output (rather than just visible or not).
Again, relies on being able to describe paths without
drawing them.
</p>
</dd>
<dt>
Filters
</dt>
<dd>
<p>
These are essentially raster operations, but a format like
SVG allows them to be defined as part of a vector image
(they are applied as part of the rasterization of the
vector image, for example, when drawing to screen).
</p>
<p>
Implementing these in the graphics engine is probably too hard.
I don't think any of the standard graphics devices will
support them (including the Cairo svg device).
</p>
<p>
These may have to remain the domain of packages like
'gridSVG' (which provides full access to SVG features)
or 'magick' (which allows us to apply raster operations
to raster images), possibly in combination with
the 'rasterize' package.
</p>
<p>
It looks like Thomas Lin Pedersen has been thinking along similar lines
(to the latter) with a
<a href="https://github.com/thomasp85/ggfx">'ggfx'</a> package.
</p>
<!-- Only very limited support in Cairo? No support in PDF?
(see ../../Examples/Filters) -->
</dd>
</dl>
<h2>
<a name="demos">4. Demonstrations</a>
</h2>
<p>
A new <a href="https://svn.r-project.org/R/branches/R-defs/"><code>R-defs</code></a>
branch has been created for experimenting.
So far, that includes the addition of support for
linear and radial gradient fills, patterns, clipping paths, and masks
on both Cairo graphics devices and the <code>pdf</code> device
(except for patterns),
with a 'grid' interface. The following set of examples
show how the 'grid' interface has been implemented so far.
</p>
<p>
<strong>NOTE: support for graphics definitions has only been implemented
for Cairo-based graphics devices and the <code>pdf</code> device
so far. Other graphics devices
should run, but will not produce output from graphics definitions.
Third-party graphics devices will hopefully error out (if they make
use of <code>R_GE_checkVersionOrDie</code>), but otherwise they
are likely to segfault until they are updated.</strong>
</p>
<p>
On the positive side, Cairo devices and the <code>pdf</code> device
can be used on all major platforms,
so there is already an option for accessing these new features
on all major platforms.
</p>
<h3>Gradients</h3>
<p>
The <code>fill</code> gpar can now be a linear gradient
(as well as a normal R colour).
</p>
<div class="chunk" id="unnamed-chunk-2"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">library</span><span class="hl std">(grid)</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">()))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-2-1.svg" title="plot of chunk unnamed-chunk-2" alt="plot of chunk unnamed-chunk-2" class="plot" /></div></div>
<p>
A linear gradient is defined by a start point (default: bottom-left)
and an end point (default: top-right) and a series of "stops".
Each stop is defined by a position on the line from start point to
end point, plus a colour. If the first stop is not at the start
point, or the last stop is not at the end point, we can specify
how the gradient "extends" beyond the first and last stops.
</p>
<p>
The following code creates a linear gradient that starts one inch
off the bottom of the image, proceeds vertically to the top of the image,
and transitions from red to yellow then back to red.
</p>
<!-- produce PNG for this one because SVG does NOT have
spreadMethod="none" (so Cairo SVG just uses "pad") -->
<div class="chunk" id="unnamed-chunk-3"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">(</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"red"</span><span class="hl std">,</span> <span class="hl str">"yellow"</span><span class="hl std">,</span> <span class="hl str">"red"</span><span class="hl std">),</span>
<span class="hl kwd">c</span><span class="hl std">(</span><span class="hl num">0</span><span class="hl std">,</span> <span class="hl num">.5</span><span class="hl std">,</span> <span class="hl num">1</span><span class="hl std">),</span>
<span class="hl kwc">x1</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">y1</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">1</span><span class="hl std">,</span> <span class="hl str">"in"</span><span class="hl std">),</span>
<span class="hl kwc">x2</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">y2</span><span class="hl std">=</span><span class="hl num">1</span><span class="hl std">,</span>
<span class="hl kwc">extend</span><span class="hl std">=</span><span class="hl str">"none"</span><span class="hl std">)))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-3-1.png" title="plot of chunk unnamed-chunk-3" alt="plot of chunk unnamed-chunk-3" class="plot" /></div></div>
<p>
When the gradient fill is specified on a grob,
the start and end points are relative to the grob bounding box.
The following code fills a rectangle in the central quarter of the
image with the default linear gradient; the gradient
starts at the bottom-left of the rectangle and ends at the top-right
of the rectangle.
</p>
<div class="chunk" id="unnamed-chunk-4"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">()))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-4-1.svg" title="plot of chunk unnamed-chunk-4" alt="plot of chunk unnamed-chunk-4" class="plot" /></div></div>
<p>
A gradient fill can also be specified on a viewport, in which case
the gradient is relative to the viewport and all subsequent
drawing makes use of that gradient (unless a new explicit fill is
specified).
</p>
<p>
In the code below, we push a viewport that fills the whole image
and specify the default linear gradient fill, so the gradient
is relative to the whole image.
We then draw a rectangle, with no explicit fill, so it
"inherits" the gradient from the viewport.
</p>
<div class="chunk" id="vppattern"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">())))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">()</span>
</pre></div>
</div><div class="rimage default"><img src="figure/vppattern-1.svg" title="plot of chunk vppattern" alt="plot of chunk vppattern" class="plot" /></div></div>
<p>
In the next example, we have the same viewport, but we draw
the rectangle only in the central quarter of the image. This
shows that the gradient that the rectangle inherits is relative
to the viewport rather than being relative to the rectangle.
</p>
<div class="chunk" id="unnamed-chunk-5"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">())))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-5-1.svg" title="plot of chunk unnamed-chunk-5" alt="plot of chunk unnamed-chunk-5" class="plot" /></div></div>
<p>
The next example shows another variation on inheritance of gpar
settings. This time a viewport is pushed with the default
linear gradient and we draw a rectangle in the left third of the
image that inherits that gradient (relative to the viewport).
Then we push another viewport with a green fill and draw a rectangle
within that viewport (in the centre of the image) and that rectangle
inherits the green fill. Finally, we pop the second viewport and
return to the first viewport and restore its linear gradient
so that when we draw
a rectangle in the right third of the image it again inherits
the linear gradient (relative to the viewport).
</p>
<div class="chunk" id="unnamed-chunk-6"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">())))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">x</span><span class="hl std">=</span><span class="hl num">.2</span><span class="hl std">,</span> <span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.2</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl str">"green"</span><span class="hl std">)))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">x</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.2</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)</span>
<span class="hl kwd">popViewport</span><span class="hl std">()</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">x</span><span class="hl std">=</span><span class="hl num">.8</span><span class="hl std">,</span> <span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.2</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-6-1.svg" title="plot of chunk unnamed-chunk-6" alt="plot of chunk unnamed-chunk-6" class="plot" /></div></div>
<p>
The next example also demonstrates inheritance of gpar settings,
but this time it is inheritance between viewports.
We push a viewport with the default gradient, then push another
viewport in the central quarter of the image, then draw a rectangle.
The second viewport inherits the gradient from the first viewport and
then the rectangle inherits the gradient from the second viewport.
The important point is that the gradient is relative to the first
viewport, which is where it was defined.
</p>
<div class="chunk" id="unnamed-chunk-7"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">())))</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">()</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-7-1.svg" title="plot of chunk unnamed-chunk-7" alt="plot of chunk unnamed-chunk-7" class="plot" /></div></div>
<p>
The next example just shows that gradients work with
translucent colours. We draw the text "Reveal" then over the
top draw a rect with a gradient fill that transitions from
(opaque) white to transparent.
</p>
<div class="chunk" id="unnamed-chunk-8"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"Reveal"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fontface</span><span class="hl std">=</span><span class="hl str">"bold"</span><span class="hl std">,</span> <span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">(</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"white"</span><span class="hl std">,</span> <span class="hl str">"transparent"</span><span class="hl std">),</span>
<span class="hl kwc">x1</span><span class="hl std">=</span><span class="hl num">.2</span><span class="hl std">,</span> <span class="hl kwc">x2</span><span class="hl std">=</span><span class="hl num">.8</span><span class="hl std">,</span> <span class="hl kwc">y1</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">y2</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-8-1.svg" title="plot of chunk unnamed-chunk-8" alt="plot of chunk unnamed-chunk-8" class="plot" /></div></div>
<p>
The next example shows that radial gradients work via
the same interface. It is expected that fill patterns would
work the same way.
</p>
<div class="chunk" id="unnamed-chunk-9"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">radialGradient</span><span class="hl std">()))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-9-1.svg" title="plot of chunk unnamed-chunk-9" alt="plot of chunk unnamed-chunk-9" class="plot" /></div></div>
<p>
As with linear gradients, we have control over the start and end
of the radial gradient, though in the radial case it is a start circle
and an end circle. In the code below, we make a gradient that starts
near the top right of the image (with white) and ends with a circle
that fills the image (black).
</p>
<div class="chunk" id="unnamed-chunk-10"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">radialGradient</span><span class="hl std">(</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"white"</span><span class="hl std">,</span> <span class="hl str">"black"</span><span class="hl std">),</span>
<span class="hl kwc">cx1</span><span class="hl std">=</span><span class="hl num">.8</span><span class="hl std">,</span> <span class="hl kwc">cy1</span><span class="hl std">=</span><span class="hl num">.8</span><span class="hl std">)))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-10-1.svg" title="plot of chunk unnamed-chunk-10" alt="plot of chunk unnamed-chunk-10" class="plot" /></div></div>
<p>
The next example shows that we can add a gradient to an
existing image, in this case a 'ggplot2' plot.
Here we edit the background of the plot panel to add
a grey-to-white gradient.
</p>
<div class="chunk" id="unnamed-chunk-11"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">library</span><span class="hl std">(ggplot2)</span>
<span class="hl kwd">ggplot</span><span class="hl std">(mtcars)</span> <span class="hl opt">+</span>
<span class="hl kwd">geom_point</span><span class="hl std">(</span><span class="hl kwd">aes</span><span class="hl std">(</span><span class="hl kwc">x</span><span class="hl std">=disp,</span> <span class="hl kwc">y</span><span class="hl std">=mpg))</span> <span class="hl opt">+</span>
<span class="hl kwd">theme_bw</span><span class="hl std">()</span>
<span class="hl kwd">grid.force</span><span class="hl std">()</span>
<span class="hl kwd">grid.edit</span><span class="hl std">(</span><span class="hl str">"panel.background"</span><span class="hl std">,</span> <span class="hl kwc">grep</span><span class="hl std">=</span><span class="hl num">TRUE</span><span class="hl std">,</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">(</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"grey"</span><span class="hl std">,</span> <span class="hl str">"white"</span><span class="hl std">),</span> <span class="hl kwc">x1</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">x2</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-11-1.svg" title="plot of chunk unnamed-chunk-11" alt="plot of chunk unnamed-chunk-11" class="plot" /></div></div>
<p>
The next example is similar, but demonstrates that, even with
gradients only implemented in 'grid', we can still add gradients
to 'graphics' plots if we first convert them using the
'gridGraphics' package.
</p>
<div class="chunk" id="unnamed-chunk-12"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">plot</span><span class="hl std">(mpg</span> <span class="hl opt">~</span> <span class="hl std">disp, mtcars,</span> <span class="hl kwc">pch</span><span class="hl std">=</span><span class="hl num">16</span><span class="hl std">)</span>
<span class="hl kwd">library</span><span class="hl std">(gridGraphics)</span>
<span class="hl kwd">grid.echo</span><span class="hl std">()</span>
<span class="hl std">g</span> <span class="hl kwb"><-</span> <span class="hl kwd">grid.grab</span><span class="hl std">()</span>
<span class="hl std">box</span> <span class="hl kwb"><-</span> <span class="hl kwd">grid.grep</span><span class="hl std">(</span><span class="hl str">"box"</span><span class="hl std">, g,</span> <span class="hl kwc">grep</span><span class="hl std">=</span><span class="hl num">TRUE</span><span class="hl std">)</span>
<span class="hl std">g</span> <span class="hl kwb"><-</span> <span class="hl kwd">editGrob</span><span class="hl std">(g, box</span><span class="hl opt">$</span><span class="hl std">name,</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">(</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"grey"</span><span class="hl std">,</span> <span class="hl str">"white"</span><span class="hl std">),</span> <span class="hl kwc">x1</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">x2</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)))</span>
<span class="hl std">g</span> <span class="hl kwb"><-</span> <span class="hl kwd">reorderGrob</span><span class="hl std">(g, box</span><span class="hl opt">$</span><span class="hl std">name)</span>
<span class="hl kwd">grid.newpage</span><span class="hl std">()</span>
<span class="hl kwd">grid.draw</span><span class="hl std">(g)</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-12-1.svg" title="plot of chunk unnamed-chunk-12" alt="plot of chunk unnamed-chunk-12" class="plot" /></div></div>
<h3>Patterns</h3>
<p>
The <code>fill</code> gpar can also be a general pattern that is based
on a 'grid' grob. The <code>pattern</code> function takes a grob
and draws it as the fill for another grob.
</p>
<div class="chunk" id="unnamed-chunk-13"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">pat</span> <span class="hl kwb"><-</span> <span class="hl kwd">pattern</span><span class="hl std">(</span><span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl num">1</span><span class="hl opt">:</span><span class="hl num">3</span><span class="hl opt">/</span><span class="hl num">4</span><span class="hl std">,</span> <span class="hl kwc">r</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">5</span><span class="hl std">,</span> <span class="hl str">"mm"</span><span class="hl std">),</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"red"</span><span class="hl std">,</span> <span class="hl str">"green"</span><span class="hl std">,</span> <span class="hl str">"blue"</span><span class="hl std">))))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=pat))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-13-1.svg" title="plot of chunk unnamed-chunk-13" alt="plot of chunk unnamed-chunk-13" class="plot" /></div></div>
<p>
Patterns on grobs are relative to the grob.
</p>
<div class="chunk" id="unnamed-chunk-14"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">pat</span> <span class="hl kwb"><-</span> <span class="hl kwd">pattern</span><span class="hl std">(</span><span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl num">1</span><span class="hl opt">:</span><span class="hl num">3</span><span class="hl opt">/</span><span class="hl num">4</span><span class="hl std">,</span> <span class="hl kwc">r</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">5</span><span class="hl std">,</span> <span class="hl str">"mm"</span><span class="hl std">),</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"red"</span><span class="hl std">,</span> <span class="hl str">"green"</span><span class="hl std">,</span> <span class="hl str">"blue"</span><span class="hl std">))))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=pat))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-14-1.svg" title="plot of chunk unnamed-chunk-14" alt="plot of chunk unnamed-chunk-14" class="plot" /></div></div>
<p>
Patterns on viewports are relative to the viewport.
</p>
<div class="chunk" id="unnamed-chunk-15"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">pat</span> <span class="hl kwb"><-</span> <span class="hl kwd">pattern</span><span class="hl std">(</span><span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl num">1</span><span class="hl opt">:</span><span class="hl num">3</span><span class="hl opt">/</span><span class="hl num">4</span><span class="hl std">,</span> <span class="hl kwc">r</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">5</span><span class="hl std">,</span> <span class="hl str">"mm"</span><span class="hl std">),</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"red"</span><span class="hl std">,</span> <span class="hl str">"green"</span><span class="hl std">,</span> <span class="hl str">"blue"</span><span class="hl std">))))</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=pat)))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-15-1.svg" title="plot of chunk unnamed-chunk-15" alt="plot of chunk unnamed-chunk-15" class="plot" /></div></div>
<p>
Patterns are full drawings so can have gradients and clipping (see
"Clipping paths" below).
</p>
<div class="chunk" id="unnamed-chunk-16"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">pat</span> <span class="hl kwb"><-</span> <span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl num">1</span><span class="hl opt">:</span><span class="hl num">3</span><span class="hl opt">/</span><span class="hl num">4</span><span class="hl std">,</span> <span class="hl kwc">r</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">5</span><span class="hl std">,</span> <span class="hl str">"mm"</span><span class="hl std">),</span>
<span class="hl kwc">vp</span><span class="hl std">=</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">clip</span><span class="hl std">=</span><span class="hl kwd">rectGrob</span><span class="hl std">(</span><span class="hl kwc">height</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">5</span><span class="hl std">,</span> <span class="hl str">"mm"</span><span class="hl std">)),</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">linearGradient</span><span class="hl std">(</span><span class="hl kwc">y1</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">y2</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">))))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">pattern</span><span class="hl std">(pat)))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-16-1.svg" title="plot of chunk unnamed-chunk-16" alt="plot of chunk unnamed-chunk-16" class="plot" /></div></div>
<p>
Patterns can "extend" like gradient fills; probably the most
typical use of a pattern will draw a simple shape and then
specify <code>extend="repeat"</code>
to fill a region.
To do this, we must specify a size for the pattern (this specifies
a "size" for the "tile" that gets repeated).
For example, the code below fills a rectangle with a polka-dot
pattern by specifying a circle as the pattern and specifying that
the pattern size is just a little bit larger than the circle
and specifying that the pattern repeats.
</p>
<div class="chunk" id="unnamed-chunk-17"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">pat</span> <span class="hl kwb"><-</span> <span class="hl kwd">pattern</span><span class="hl std">(</span><span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl kwc">r</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">3</span><span class="hl std">,</span> <span class="hl str">"mm"</span><span class="hl std">),</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">col</span><span class="hl std">=</span><span class="hl num">NA</span><span class="hl std">,</span> <span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">)),</span>
<span class="hl kwc">width</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">8</span><span class="hl std">,</span> <span class="hl str">"mm"</span><span class="hl std">),</span>
<span class="hl kwc">height</span><span class="hl std">=</span><span class="hl kwd">unit</span><span class="hl std">(</span><span class="hl num">8</span><span class="hl std">,</span> <span class="hl str">"mm"</span><span class="hl std">),</span>
<span class="hl kwc">extend</span><span class="hl std">=</span><span class="hl str">"repeat"</span><span class="hl std">)</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=pat)))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">)</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-17-1.svg" title="plot of chunk unnamed-chunk-17" alt="plot of chunk unnamed-chunk-17" class="plot" /></div></div>
<h3>Clipping paths</h3>
<p>
The <code>clip</code> argument for the <code>viewport</code> function
can be a 'grid' grob (in addition to the existing
<code>"on"</code>, which only enforces a rectangular clipping region).
The grob defines a path that is used
to set the clipping region.
</p>
<p>
A simple example is clipping output (in this case text) to a circle.
The text and the circle are both drawn in grey and then the text
is drawn black with the circle used as a clipping path.
</p>
<div class="chunk" id="unnamed-chunk-18"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"testing"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">,</span> <span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">))</span>
<span class="hl std">cg</span> <span class="hl kwb"><-</span> <span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl kwc">r</span><span class="hl std">=</span><span class="hl num">.25</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">))</span>
<span class="hl kwd">grid.draw</span><span class="hl std">(cg)</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">clip</span><span class="hl std">=cg))</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"testing"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-18-1.svg" title="plot of chunk unnamed-chunk-18" alt="plot of chunk unnamed-chunk-18" class="plot" /></div></div>
<p>
A slightly more dramatic example involves clipping text to several
circles at once. Note that this still only requires a single
grob to specify the clipping path.
</p>
<div class="chunk" id="unnamed-chunk-19"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"testing"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">,</span> <span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">))</span>
<span class="hl std">cg</span> <span class="hl kwb"><-</span> <span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl kwc">x</span><span class="hl std">=</span><span class="hl num">1</span><span class="hl opt">:</span><span class="hl num">3</span><span class="hl opt">/</span><span class="hl num">4</span><span class="hl std">,</span> <span class="hl kwc">r</span><span class="hl std">=</span><span class="hl num">.1</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">))</span>
<span class="hl kwd">grid.draw</span><span class="hl std">(cg)</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">clip</span><span class="hl std">=cg))</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"testing"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-19-1.svg" title="plot of chunk unnamed-chunk-19" alt="plot of chunk unnamed-chunk-19" class="plot" /></div></div>
<p>
The clipping path is relative to the viewport and
clipping paths are inherited like the existing <code>clip</code>
settings.
</p>
<p>
In the following code (after drawing everything in grey
without any clipping), we push a viewport with three circles as the
clipping path, then we draw the text "test one", which gets clipped
to the circles. Next, we push another viewport in the bottom third
of the image, which inherits the clipping path, and we draw the text
"test two", which also gets clipped to the circles. Finally,
we pop those two viewports and push a third viewport in the top
third of the image; now there is no clipping path in effect (because
we popped the viewport with a clipping path), so when we
draw the text "test three" the text is not clipped at all.
</p>
<div class="chunk" id="unnamed-chunk-20"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">cg</span> <span class="hl kwb"><-</span> <span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl kwc">y</span><span class="hl std">=</span><span class="hl num">1</span><span class="hl opt">:</span><span class="hl num">3</span><span class="hl opt">/</span><span class="hl num">4</span><span class="hl std">,</span> <span class="hl kwc">r</span><span class="hl std">=</span><span class="hl num">.1</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">))</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"test one"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">,</span> <span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">))</span>
<span class="hl kwd">grid.draw</span><span class="hl std">(cg)</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">y</span><span class="hl std">=</span><span class="hl num">0</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">1</span><span class="hl opt">/</span><span class="hl num">3</span><span class="hl std">,</span> <span class="hl kwc">just</span><span class="hl std">=</span><span class="hl str">"bottom"</span><span class="hl std">))</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"test two"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">,</span> <span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">))</span>
<span class="hl kwd">popViewport</span><span class="hl std">()</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">clip</span><span class="hl std">=cg))</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"test one"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">))</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">y</span><span class="hl std">=</span><span class="hl num">0</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">1</span><span class="hl opt">/</span><span class="hl num">3</span><span class="hl std">,</span> <span class="hl kwc">just</span><span class="hl std">=</span><span class="hl str">"bottom"</span><span class="hl std">))</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"test two"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">))</span>
<span class="hl kwd">popViewport</span><span class="hl std">(</span><span class="hl num">2</span><span class="hl std">)</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">y</span><span class="hl std">=</span><span class="hl num">1</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">1</span><span class="hl opt">/</span><span class="hl num">3</span><span class="hl std">,</span> <span class="hl kwc">just</span><span class="hl std">=</span><span class="hl str">"top"</span><span class="hl std">))</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"test three"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-20-1.svg" title="plot of chunk unnamed-chunk-20" alt="plot of chunk unnamed-chunk-20" class="plot" /></div></div>
<p>
Clipping paths are just paths (outlines of shapes), they do not
reflect, for example, stroke width or fill colours.
</p>
<div class="chunk" id="unnamed-chunk-21"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">path</span> <span class="hl kwb"><-</span> <span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl kwc">r</span><span class="hl std">=</span><span class="hl num">.3</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">lwd</span><span class="hl std">=</span><span class="hl num">30</span><span class="hl std">,</span> <span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">,</span> <span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl num">NA</span><span class="hl std">))</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"testing"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">col</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">,</span> <span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">))</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">clip</span><span class="hl std">=path))</span>
<span class="hl kwd">grid.draw</span><span class="hl std">(path)</span>
<span class="hl kwd">grid.circle</span><span class="hl std">(</span><span class="hl kwc">r</span><span class="hl std">=</span><span class="hl num">.3</span><span class="hl std">)</span>
<span class="hl kwd">grid.text</span><span class="hl std">(</span><span class="hl str">"testing"</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">cex</span><span class="hl std">=</span><span class="hl num">3</span><span class="hl std">))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-21-1.svg" title="plot of chunk unnamed-chunk-21" alt="plot of chunk unnamed-chunk-21" class="plot" /></div></div>
<p>
Although a clipping path can be an arbitrary 'grid' grob,
clipping and masks (see below) are disallowed within a clipping path.
Any viewport that sets <code>clip=TRUE</code> or <code>clip=FALSE</code>
or sets <code>clip</code> to be a grob, or that sets <code>mask</code>
to be a grob, will generate a warning and the clipping setting
(or the mask) will be ignored for determining the clipping path.
</p>
<p>
This constraint has been applied for several reasons:
to make life easier for graphics devices;
because it is very difficult or impossible to achieve
some nested clipping effects on some devices;
formats like PDF and SVG enforce limits on what can go
in a clipping path; and fancy clipping shapes can be
achieved by using a mask instead.
</p>
<h3>Masks</h3>
<p>
A 'grid' viewport can now have a <code>mask</code> (in addition to
<code>clip</code>). This value should be either <code>"inherit"</code>
(the default), in which case the viewport inherits the mask from
its parent, <code>"none"</code>, in which case the viewport does not
mask its content, or a 'grid' grob that defines a new mask for the
viewport.
</p>
<p>
The following example defines a circle grob with a
radial gradient fill that
starts at black in the centre and gradually transitions to
transparent at the circumference.
A viewport is pushed with this circle grob as the mask, then
a rectangle is drawn half the width of the viewport and
filled with solid black.
The result is that only the parts of the rectangle where
the mask is non-transparent are drawn; the retangle is filtered
by the mask.
</p>
<div class="chunk" id="unnamed-chunk-22"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">mask</span> <span class="hl kwb"><-</span> <span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">col</span><span class="hl std">=</span><span class="hl num">NA</span><span class="hl std">,</span>
<span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl kwd">radialGradient</span><span class="hl std">(</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"black"</span><span class="hl std">,</span> <span class="hl str">"transparent"</span><span class="hl std">))))</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">mask</span><span class="hl std">=mask))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl str">"black"</span><span class="hl std">))</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-22-1.svg" title="plot of chunk unnamed-chunk-22" alt="plot of chunk unnamed-chunk-22" class="plot" /></div></div>
<p>
The following code blends two 'ggplot2' plots using a mask.
This time the mask is a linear gradient from black 30%
of the way up the page to transparent 70% of the way up the page.
First, we draw a plot with the default ggplot2 them,
then we draw a black-and-white themed version of the same
plot on top, using the mask, so from 30% up the page the
black-and-white plot transitions smoothly into the default themed plot.
</p>
<div class="chunk" id="unnamed-chunk-23"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">gg1</span> <span class="hl kwb"><-</span> <span class="hl kwd">ggplot</span><span class="hl std">(mtcars)</span> <span class="hl opt">+</span>
<span class="hl kwd">geom_point</span><span class="hl std">(</span><span class="hl kwd">aes</span><span class="hl std">(</span><span class="hl kwc">x</span><span class="hl std">=disp,</span> <span class="hl kwc">y</span><span class="hl std">=mpg))</span>
<span class="hl std">gg2</span> <span class="hl kwb"><-</span> <span class="hl std">gg1</span> <span class="hl opt">+</span>
<span class="hl kwd">theme_bw</span><span class="hl std">()</span>
<span class="hl std">grad</span> <span class="hl kwb"><-</span> <span class="hl kwd">linearGradient</span><span class="hl std">(</span><span class="hl kwd">c</span><span class="hl std">(</span><span class="hl str">"black"</span><span class="hl std">,</span> <span class="hl str">"transparent"</span><span class="hl std">),</span> <span class="hl kwc">x1</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">x2</span><span class="hl std">=</span><span class="hl num">.5</span><span class="hl std">,</span> <span class="hl kwc">y1</span><span class="hl std">=</span><span class="hl num">.3</span><span class="hl std">,</span> <span class="hl kwc">y2</span><span class="hl std">=</span><span class="hl num">.7</span><span class="hl std">)</span>
<span class="hl std">mask</span> <span class="hl kwb"><-</span> <span class="hl kwd">rectGrob</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">col</span><span class="hl std">=</span><span class="hl num">NA</span><span class="hl std">,</span> <span class="hl kwc">fill</span><span class="hl std">=grad))</span>
<span class="hl kwd">print</span><span class="hl std">(gg1)</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">mask</span><span class="hl std">=mask))</span>
<span class="hl kwd">print</span><span class="hl std">(gg2,</span> <span class="hl kwc">newpage</span><span class="hl std">=</span><span class="hl num">FALSE</span><span class="hl std">)</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-23-1.svg" title="plot of chunk unnamed-chunk-23" alt="plot of chunk unnamed-chunk-23" class="plot" /></div></div>
<p>
Here is an example of a relatively complex mask that cannot be
achieved with clipping paths (in the current proposal) because
it would require nesting clipping paths.
The mask consists of two narrow rectangles, but the left
rectangle is drawn within a viewport with a clipping path.
The same effect could be achieved by replacing the clipping path
with a mask (nested masks are ok).
</p>
<div class="chunk" id="unnamed-chunk-24"><div class="rcode"><div class="source"><pre class="knitr r"><span class="hl std">mask</span> <span class="hl kwb"><-</span> <span class="hl kwd">gTree</span><span class="hl std">(</span><span class="hl kwc">children</span><span class="hl std">=</span><span class="hl kwd">gList</span><span class="hl std">(</span><span class="hl kwd">rectGrob</span><span class="hl std">(</span><span class="hl kwc">x</span><span class="hl std">=</span><span class="hl num">.25</span><span class="hl std">,</span> <span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.3</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">.8</span><span class="hl std">,</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl str">"black"</span><span class="hl std">),</span>
<span class="hl kwc">vp</span><span class="hl std">=</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">clip</span><span class="hl std">=</span><span class="hl kwd">circleGrob</span><span class="hl std">(</span><span class="hl kwc">r</span><span class="hl std">=</span><span class="hl num">.4</span><span class="hl std">))),</span>
<span class="hl kwd">rectGrob</span><span class="hl std">(</span><span class="hl kwc">x</span><span class="hl std">=</span><span class="hl num">.75</span><span class="hl std">,</span> <span class="hl kwc">width</span><span class="hl std">=</span><span class="hl num">.3</span><span class="hl std">,</span> <span class="hl kwc">height</span><span class="hl std">=</span><span class="hl num">.8</span><span class="hl std">,</span>
<span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl str">"black"</span><span class="hl std">))))</span>
<span class="hl kwd">pushViewport</span><span class="hl std">(</span><span class="hl kwd">viewport</span><span class="hl std">(</span><span class="hl kwc">mask</span><span class="hl std">=mask))</span>
<span class="hl kwd">grid.rect</span><span class="hl std">(</span><span class="hl kwc">gp</span><span class="hl std">=</span><span class="hl kwd">gpar</span><span class="hl std">(</span><span class="hl kwc">fill</span><span class="hl std">=</span><span class="hl str">"grey"</span><span class="hl std">))</span>
<span class="hl kwd">popViewport</span><span class="hl std">()</span>
</pre></div>
</div><div class="rimage default"><img src="figure/unnamed-chunk-24-1.svg" title="plot of chunk unnamed-chunk-24" alt="plot of chunk unnamed-chunk-24" class="plot" /></div></div>
<h2>
<a name="internals">5. Internal details</a>
</h2>
<h3>Gradients</h3>
<p>
A <code>patternFill</code>
property has been added to the <code>R_GE_gcontext</code>
(graphical context)
in the graphics engine.
This property can be either <code>NULL</code>
(indicating no pattern fill) or an R object, which
represents a reference to one of the current pattern fills on
the graphics device.
</p>
<p>
This property is in addition to the existing <code>fill</code>
property. If <code>patternFill</code> is non-<code>NULL</code>
then it overrides <code>fill</code>. This makes it easy
for a graphics device to ignore <code>patternFill</code>
if it wants to.
</p>
<p>
A <code>grDevices::linearGradient</code> function has been added.
This is not meant to be called
directly by users; it is there for 'graphics' and 'grid' to use.
The idea is the 'graphics' and 'grid' have user-level
interfaces to define gradients and those are
converted to graphics engine versions of the gradients
for drawing. There is also a <code>grDevices::radialGradient</code>.
</p>
<p>
A <code>setPattern</code> device function has been added to
allow the graphics engine to ask a graphics device to create
a pattern. The function takes a <code>"Pattern"</code> object
(either a linear or radial gradient so far) and returns an R object
(a reference to be used in a <code>patternFill</code> call).
</p>
<p>
(A graphics device can just always return <code>NULL</code>
if it generates
a new pattern every time <code>setPattern</code> is called ?)
</p>
<p>
There is also a <code>releasePattern</code> device function
so that, e.g., 'grid' can release a pattern that is just set on
a grob (and release all patterns on new page). Specify
<code>NULL</code>
to release all patterns.
</p>
<p>
NOTE that the <code>"Pattern"</code> sent to
<code>setPattern</code> is an SEXP,
which means it is sort of an opaque pointer in C code,
so the graphics engine provides a C API for extracting
information from a <code>"Pattern"</code> object.
For example,
<code>R_GE_isPattern</code> can be called to check for a pattern fill,
<code>R_GE_patternType</code> can be called to get the pattern type, and
<code>R_GE_linearGradientNumStops</code> can be called
to get the number of stops from a <code>LinearGradient</code> SEXP.
</p>
<p>
The Cairo graphics device (<code>x11(type="cairo")</code>,
<code>png(type="cairo")</code>,
<code>cairo_pdf()</code>, and <code>svg()</code>)
has implemented <code>setPattern</code>
and <code>releasePattern</code> and maintains an array
of patterns. It observes the new
<code>R_GE_gcontext.gradientFill</code> setting
and uses the value
as an index into its array of patterns.
</p>
<p>
The 'grid' package provides its own <code>linearGradient</code>
function for defining a linear gradient in 'grid'.
One notable difference is that the start and end points for
the gradient can be defined using 'grid' units.
The <code>gpar</code> function now also allows
a <code>GridPattern</code> as the value for the
<code>fill</code> property.
</p>
<p>
When gpar settings are enforced during drawing a grob, a
pattern fill is resolved to a graphics engine
<code>Pattern</code> (for example, the start and end
points for a linear gradient are converted to locations on the graphics
device).
If the pattern fill is specified on a viewport, the pattern definition
is relative to that viewport; if the pattern fill is specified