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<h6 id="header">RMagick User's Guide and Reference</h6>
<div class="nav">
&laquo;&nbsp;<a href="constants.html">Prev</a> | <a href=
"index.html">Contents</a> | <a href=
"rvg.html">Next</a>&nbsp;&raquo;
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<h1>Drawing with RVG</h1>
<div id="toc">
<h2>A tutorial</h2>
</div>
<div style="position:relative;">
<p><img src="ex/images/duck.gif" alt="duck|type" width="180"
height="180" /></p>
<div style="position:absolute; left: 200px;top:1em">
<h3>Introduction</h3>
<p>RVG is the newest addition to RMagick. RVG (Ruby Vector
Graphics) is a facade for RMagick's <a href=
"draw.html">Draw</a> class that supplies a drawing API based
on the <a href="http://www.w3.org/Graphics/SVG/">Scalable
Vector Graphics</a> W3C recommendation.</p>
<p>RVG is a <em>scalable</em> <em>vector</em> drawing
library. <em>Scalable</em> means that drawings are not fixed
to a single size in pixels. The same drawing can be rendered
for a screen display or for printing. <em>Vector</em> images
are drawn using geometric objects like lines and circles.
Unlike raster images, vector images don't get "pixelated"
when you make them bigger.</p>As an introduction to the RVG
library, let's see how to draw this little duck on the left.
Here is the complete program.
</div>
</div>
<pre class="example">
1 require 'rvg/rvg'
2 include Magick
3
4 RVG::dpi = 72
5
6 rvg = RVG.new(2.5.in, 2.5.in).viewbox(0,0,250,250) do |canvas|
7 canvas.background_fill = 'white'
8
9 canvas.g.translate(100, 150).rotate(-30).styles(:fill=&gt;'yellow',
10 :stroke=&gt;'black', :stroke_width=&gt;2) do |body|
11 body.ellipse(50, 30)
12 body.rect(45, 20, -20, -10).skewX(-35)
13 end
14
15 canvas.g.translate(130, 83).styles(:stroke=&gt;'black', :stroke_width=&gt;2) do |head|
16 head.circle(30).styles(:fill=&gt;'yellow')
17 head.circle(5, 10, -5).styles(:fill=&gt;'black')
18 head.polygon(30,0, 70,5, 30,10, 62,25, 23,20).styles(:fill=&gt;'orange')
19 end
20
21 foot = RVG::Group.new do |_foot|
22 _foot.path('M0,0 v30 l30,10 l5,-10, l-5,-10 l-30,10z').
23 styles(:stroke_width=&gt;2, :fill=&gt;'orange', :stroke=&gt;'black')
24 end
25 canvas.use(foot).translate(75, 188).rotate(15)
26 canvas.use(foot).translate(100, 185).rotate(-15)
27
28 canvas.text(125, 30) do |title|
29 title.tspan("duck|").styles(:text_anchor=&gt;'end', :font_size=&gt;20,
30 :font_family=&gt;'Helvetica', :fill=&gt;'black')
31 title.tspan("type").styles(:font_size=&gt;22,
32 :font_family=&gt;'Times', :font_style=&gt;'italic', :fill=&gt;'red')
33 end
34 canvas.rect(249,249).styles(:stroke=&gt;'blue', :fill=&gt;'none')
35 end
36
37 rvg.draw.write('duck.gif')
</pre>
<h2>Summary</h2>
<p>All drawings follow the same 3 steps:</p>
<ol>
<li>Create an RVG object. Specify the width and height of the
final image. The <code>RVG.new</code> method yields to a
block.</li>
<li>Within the block, call methods on the RVG object to specify
a background, add shapes, text, or raster images, or add groups
of shapes, text, or raster images.</li>
<li>Call the <code>draw</code> method to draw the shapes, text,
or raster images onto the background.</li>
</ol>
<p>I'll step through the example line-by-line.</p>
<h2>Lines 1-3</h2>
<pre class="example">
1 require 'rvg/rvg'
2 include Magick
</pre>
<p>These are just the usual Ruby code to load the RVG extension.
To save some typing, I've included the Magick module into
Object's namespace.</p>
<h2>Lines 4-6</h2>
<pre class="example">
4 RVG::dpi = 72
5
6 rvg = RVG.new(2.5.in, 2.5.in).viewbox(0,0,250,250) do |canvas|
</pre>
<p><code>RVG::dpi</code> enables the use of <em>unit methods</em>
in RVG. When you set <code>RVG::dpi</code> to a non-nil value,
RVG adds a number of conversion methods to the Fixnum and Float
classes . These methods allow you to specify measurements in
units such as inches, millimeters, and centimeters. <em>DPI</em>
stands for "dots per inch," the image resolution. Here I set
<code>RVG::dpi</code> to 72, a common value for displays.</p>
<p>The <code>RVG.new</code> method accepts 2 parameters. These
parameters specify the width and height of the final image in
pixels. Since I've defined <code>RVG::dpi</code>, I can specify
these values in inches using the <code>in</code> conversion
method. At 72dpi, the final image will be 2.5*72=180 pixels on a
side.</p>
<p>By default, RVG uses pixels as its unit of measurement, but
since I'm drawing a scalable picture I don't want to confine
myself to pixels. The <code>viewbox</code> method defines a
coordinate system with a logical unit. <code>Viewbox</code> takes
4 parameters, <code>min_x</code>, <code>min_y</code>,
<code>width</code>, and <code>height</code>. On line 6 I define
my coordinate system to have its origin at (0,0) and a width and
height of 250 units. By using my own coordinate system, I can
later change the size of the image to 5 inches square or 1 inch
square just by changing the arguments to <code>new</code>.</p>
<div style="position:relative">
<p><img src="ex/images/duck0.gif" alt=
"default coordinate system" width="180" height="180" /></p>
<div style="position:absolute; left:200px;top:0;">
<p><strong>The default coordinate system</strong></p>
<p>By default, the RVG coordinate system has its origin in
the upper-left corner. The x-axis proceeds to the right. The
y-axis proceeds downwards. The image on the left shows the
axes of this coordinate system. I've added a light-blue
"graph-paper" background to the example images to help
associate the coordinate arguments to the actual locations in
the image. Just remember that the axes and graph-paper
background are not actually part of the image I'm
producing.</p>
</div>
</div>
<p>The RVG class is one of the <em>container</em> classes defined
by RVG. Container objects can contain graphic objects such as
circles and lines, text, raster images, and other container
objects. The outermost container is always an RVG object. I will
add all the graphic objects that form the duck to this
container.</p>
<p>Container constructors normally yield to a block. However,
here I've chained <code>viewbox</code> to <code>new</code>, so
<code>viewbox</code> takes responsibility for yielding and passes
the new instance of RVG to the <code>canvas</code> argument.</p>
<h2>Line 7</h2>
<pre class="example">
7 canvas.background_fill = 'white'
</pre>
<p>By default, RVG graphics are drawn on a transparent
background. This is convenient when you want to display your
image over another image. You can override the default background
color by assigning a color to the <code>background_fill=</code>
attribute. Here I set the background color to "white."</p>
<h2>Lines 9-13</h2>
<pre class="example">
9 canvas.g.translate(100, 150).rotate(-30).styles(:fill=&gt;'yellow',
10 :stroke=&gt;'black', :stroke_width=&gt;2) do |body|
11 body.ellipse(50, 30)
12 body.rect(45, 20, -20, -10).skewX(-35)
13 end
</pre>
<p>There's a lot going on in these few lines - seven method calls
- so let's take it one method at a time.</p>
<h3>Groups</h3>
<p><code>Group</code> is the second container class in RVG. The
purpose of a group is to associate a set of coordinate system
transformations and a set of styles with the graphic objects
within the group. To create a Group object within another
container, call the <code>g</code> method on the container. The
<code>g</code> method yields if a block is present. In this
example, there is no block associated with <code>g</code>, so
<code>g</code> returns the new group. The <code>g</code> method
adds the group to the content of the container, so when the
container is drawn the graphic objects in the group will be drawn
as well. The three methods chained to <code>g</code> -
<code>translate</code>, <code>rotate</code>, and
<code>styles</code> - modify the group by adding <em>coordinate
system transforms</em> and by modifying the default
<em>styles</em>.</p>
<p>(Okay, there <em>is</em> a block, but there are 3 method calls
between <code>g</code> and the block. I'll explain more
later.)</p>
<h3>Transforms</h3>
<p>I'm going to use this group to contain the ellipse that forms
the duck's body and the rectangle that forms the wing. I could
just specify x- and y-coordinates to position these shapes
relative to the origin, but it's easier to move the origin to
where I want to draw the shapes. This is the purpose of the
<code>translate</code> method. This method moves the origin to
the (x,y) position specified by its arguments. I call
<code>translate</code> on the group object, and since the content
of the group gets the coordinate system transformations specified
for the group, the ellipse and the rectangle will be drawn on a
coordinate system with the origin at (100, 150) relative to the
old coordinate system.</p>
<p>Also, I want the duck's body to slant upward, so I use the
<code>rotate</code> method to rotate the axes. The argument to
<code>rotate</code> is the number of degrees of rotation. A
negative number indicates counter-clockwise rotation.</p>
<p>After translating and rotating the coordinate system, the axes
look like this:</p>
<div style="position:relative">
<p><img src="ex/images/duck1.gif" width="180" height="180" alt=
"duck body" /></p>
<div style="position:absolute; left:200px;top:0">
<p><strong>The other transform methods</strong></p>
<p>There are six transform methods. In addition to
<code>translate</code> and <code>rotate</code>, there's
<code>scale</code>, <code>skewX</code>, <code>skewY</code>,
and <code>matrix</code>. When groups are nested, any
transforms defined on the inner group(s) are added to the
outer transforms.</p>
</div>
</div>
<h3>Styles</h3>
<p>Recall that the <code>styles</code> method modifies the
default group styles. The <code>styles</code> method takes a hash
as an argument. The hash keys are style names, and the hash
values are, well, style values. In this example there are three
style names. The :fill style sets the fill color to 'yellow'. The
:stroke style sets the outline color to 'black'. The
:stroke_width style sets the width of the outline to 2. The
<code>styles</code> method is a real workhorse. It's defined in
almost every class in RVG and there are many other style names in
addition to these three..</p>
<h3>Basic shapes</h3>
<p>The group contains two basic shapes, an ellipse and a
rectangle. I add the ellipse to the group with the
<code>ellipse</code> method. <code>Ellipse</code> has four
parameters. The first two, the radius on the x-axis and the
radius on the y-axis, are required. The last two are the (x,y)
coordinate of the center. When these are omitted, as here, they
default to (0,0). I add the rectangle with the <code>rect</code>
method, which also has four parameters. The first two are the
width and height of the rectangle. The last two are the (x,y)
coordinate of the upper-left corner. Both of these methods return
<code>self</code>, so you can chain other methods to them.</p>
<p>Here's what the group looks like when rendered. The ellipse is
centered on the origin. The upper-left corner of the rectangle is
slightly up and to the left of the origin.</p>
<p><img src="ex/images/duck3.gif" alt="default coordinate system"
width="180" height="180" /></p>
<h3>SkewX</h3>
<p>Everybody knows that a wing doesn't look like a rectangle! A
wing looks like a slanted parallelogram. (Well, it does in this
example!) Fortunately, I can use the transform methods on shapes
as well as containers. The <code>skewX</code> method makes it
easy for us to give the rectangle a slant. The <code>skewX</code>
method is another transform. It takes a single argument, the
number of degrees to skew the x-axis. Since all the shape
constructors, including <code>rect</code>, return
<code>self</code>, I can chain <code>skewX</code> directly to
<code>rect</code> and limit the effect of the transform to just
the rectangle. The result looks like this. (I've drawn in the
axes for the wing coordinate system.)</p>
<p><img src="ex/images/duck4.gif" width="180" height="180" alt=
"duck wing" /></p>
<p>That's it for the body. Let's tie up one loose end before I
move on. I said earlier that container constructors (such as
<code>g</code>) yield to a block if present. In this case,
though, there are three intervening method calls -
<code>translate</code>, <code>rotate</code>, and
<code>styles</code> - between <code>g</code> and the block. The
<code>style</code> method and the transform methods will all
yield when there is an associated block, so I can easily chain
them to a container constructor and still use a block argument to
define the graphic objects in the group. Method chaining is a
common RVG idiom. You'll see it a lot in the examples.</p>
<p>The next group draws the head.</p>
<h2>Lines 15-19</h2>
<pre class="example">
15 canvas.g.translate(130, 83).styles(:stroke=&gt;'black', :stroke_width=&gt;2) do |head|
16 head.circle(30).styles(:fill=&gt;'yellow')
17 head.circle(5, 10, -5).styles(:fill=&gt;'black')
18 head.polygon(30,0, 70,5, 30,10, 62,25, 23,20).styles(:fill=&gt;'orange')
19 end
</pre>
<p>This section is very similar to the previous one. I'm defining
a group to contain the graphic objects that draw the duck's head,
eye, and beak. First I use the translate method to move the
origin to (130,83):</p>
<p><img src="ex/images/duck6.gif" width="180" height="180" alt=
"duck head" /></p>
<p>Notice that I can use <code>styles</code> on shapes as well as
containers. In this group each shape has its own fill color. (The
styles defined on the group propogate to the contained shapes,
unless you override them.) The yellow circle forms the head. The
<code>circle</code> method takes 3 parameters. The first
parameter is the radius of the circle. The other two parameters
are the (x,y) coordinate of the center. If omitted, as here, they
default to (0,0). I use a small black circle for the eye.</p>
<p>Last, I use the <code>polygon</code> method to draw the beak.
This method draws a polygon from a series of (x,y) coordinates.
If the last coordinate is not the same as the first,
<code>polygon</code> implicitly adds it to close the polygon.
Again, I use <code>styles</code> to set the fill color to
orange.</p>
<p><img src="ex/images/duck8.gif" width="180" height="180" alt=
"duck head final" /></p>
<h2>Lines 21-24</h2>
<pre class="example">
21 foot = RVG::Group.new do |_foot|
22 _foot.path('M0,0 v30 l30,10 l5,-10, l-5,-10 l-30,10z').
23 styles(:stroke_width=&gt;2, :fill=&gt;'orange', :stroke=&gt;'black')
24 end
</pre>
<p>Here I create a group by directly calling <code>new</code>
instead of calling the <code>g</code> method on a container. The
graphic objects within the group are not (yet) contained within
the canvas and so will not be drawn when the canvas is drawn. You
might think of the foot as not being attached to anything, like
this:</p>
<p><img src="ex/images/duck9.gif" width="180" height="180" alt=
"duck foot" /></p>
<h2>Lines 25-26</h2>
<pre class="example">
25 canvas.use(foot).translate(75, 188).rotate(15)
26 canvas.use(foot).translate(100, 185).rotate(-15)
</pre>
<p>To add the group to the canvas I use the <code>use</code>
method. The use method can accept any container or graphic object
as an argument. Optionally you can specify an (x,y) coordinate
that specifies where to position the objects. In this example,
however, I let those arguments default to (0,0) and use
<code>translate</code> to position the feet. Here's how the left
foot attaches to the duck:</p>
<p><img src="ex/images/duck10.gif" width="180" height="180" alt=
"duck foot 2" /></p>
<p>Of course, the duck is walking, so I have to give the foot a
little slant with <code>rotate</code>:</p>
<p><img src="ex/images/duck11.gif" width="180" height="180" alt=
"duck foot 3" /></p>
<p>Attaching the right foot is easy. Call <code>use</code> again
but give different arguments to <code>translate</code> and
<code>rotate</code>:</p>
<p><img src="ex/images/duck12.gif" width="180" height="180" alt=
"final duck foot" /></p>
<h2>Lines 28-33</h2>
<pre class="example">
28 canvas.text(125, 30) do |title|
29 title.tspan("duck|").styles(:text_anchor=&gt;'end', :font_size=&gt;20,
30 :font_family=&gt;'Helvetica', :fill=&gt;'black')
31 title.tspan("type").styles(:font_size=&gt;22,
32 :font_family=&gt;'Times', :font_style=&gt;'italic', :fill=&gt;'red')
33 end
</pre>
<p>All I need now is a title for the picture. Text in RVG is a
job for the <code>text</code> method. Like the shape methods,
<code>text</code> can be used with any container object.
<code>Text</code> itself is a container, except that it can only
contain text-related objects. The <code>text</code> method takes
2 or 3 arguments, an (x,y) pair and optionally a string. The
(x,y) pair define a <em>current text position</em> at which
rendering starts. If there is a string argument, it will be
rendered starting at the current text position. Rendering text
changes the current text position to the end of the text.</p>
<p>In the example, text is used as a container. Text objects can
contain Tspan objects. Each tspan can specify its own styles. By
default each tspan is rendered starting at the current text
position.</p>
<p>As usual, I can change the appearance of the text with
<code>styles</code>. Here I choose a font, a font style (the
default is "normal"), its size in points, and the color.</p>
<p><img src="ex/images/duck14.gif" width="180" height="180" alt=
"duck title" /></p>
<h2>Line 34</h2>
<pre class="example">
34 canvas.rect(249,249).styles(:stroke=&gt;'blue', :fill=&gt;'none')
</pre>
<p>I'm almost done. All I need to do it add a blue border. (I'm
going to remove the graph paper background because we don't need
it any more.)</p>
<p><img src="ex/images/duck15.gif" width="180" height="180" alt=
"duck with border" /></p>
<h2>Line 37</h2>
<pre class="example">
37 rvg.draw.write('duck.gif')
</pre>
<p>The <code>draw</code> method call doesn't occupy a lot of
space - just 4 letters - but does a lot of work. The
<code>draw</code> method goes through all the graphics objects
that I've added to the outermost RVG container and draws them on
the background. When the drawing is complete, <code>draw</code>
returns the drawing in the form of an RMagick Image object. You
can use any Image class methods on the drawing. Here I simply
write the image to a GIF file.</p>
<h2>Scalable graphics</h2>
<p>Are RVG images really scalable? Let's try. Change the RVG.new
call to make an image that's 4 times as big. That's 5 inches on a
side:</p>
<pre class="example">
6 rvg = RVG.new(5.in, 5.in).viewbox(0,0,250,250) do |canvas|
</pre>
<p>Change nothing else. Run the program again and see what you
get.</p>
<p><img src="ex/images/big-duck.gif" width="360" height="360"
alt="big duck" /></p>
<p class="spacer">&nbsp;</p>
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