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  <h6 id="header">RMagick 0.0.0 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=
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  </div>

  <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) do |body|
10          body.styles(:fill=&gt;'yellow', :stroke=&gt;'black', :stroke_width=&gt;2)
11          body.ellipse(50, 30)
12          body.rect(45, 20, -20, -10).skewX(-35)
13      end
14
15      canvas.g.translate(130, 83) do |head|
16          head.styles(:stroke=&gt;'black', :stroke_width=&gt;2)
17          head.circle(30).styles(:fill=&gt;'yellow')
18          head.circle(5, 10, -5).styles(:fill=&gt;'black')
19          head.polygon(30,0, 70,5, 30,10, 62,25, 23,20).styles(:fill=&gt;'orange')
20      end
21
22      foot = RVG::Group.new do |_foot|
23          _foot.path('m0,0 v30 l30,10 l5,-10, l-5,-10 l-30,10z').
24                styles(:stroke_width=&gt;2, :fill=&gt;'orange', :stroke=&gt;'black')
25      end
26      canvas.use(foot).translate(75, 188).rotate(15)
27      canvas.use(foot).translate(100, 185).rotate(-15)
28
29      canvas.text(125, 30) do |title|
30          title.tspan("duck|").styles(:text_anchor=&gt;'end', :font_size=&gt;20,
31                         :font_family=&gt;'helvetica', :fill=&gt;'black')
32          title.tspan("type").styles(:font_size=&gt;22,
33                 :font_family=&gt;'times', :font_style=&gt;'italic', :fill=&gt;'red')
34      end
35      canvas.rect(249,249).styles(:stroke=&gt;'blue', :fill=&gt;'none')
36  end
37
38  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) do |body|
10          body.styles(:fill=&gt;'yellow', :stroke=&gt;'black', :stroke_width=&gt;2)
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 its container. In this example,
  the group's container is the canvas object created in line 6. The
  graphic objects in the group are drawn as part of drawing the
  container. The <code>translate</code> and <code>rotate</code>
  chained to <code>g</code> modify the group by adding
  <em>coordinate system transforms</em>.</p>

  <p>(Okay, there <em>is</em> a block, but there are 2 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 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. I want
  the styles to apply to all objects within the group so in line 10
  I call <code>styles</code> on the new group object.</p>

  <p>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>

  <div style="position:relative">
    <p><img src="ex/images/duck3.gif" alt=
    "default coordinate system" width="180" height="180" /></p>

    <div style="position:absolute; left:200px;top:0">
      <p><strong>The shape methods</strong></p>

      <p>There are 7 shape methods. In addition to
      <code>ellipse</code> and <code>rect</code>, there's
      <code>circle</code>, <code>line</code>, <code>path</code>,
      <code>polygon</code>, and <code>polyline</code>. You can also
      think of text as a shape. Shapes are stroked and filled, and
      can be modified by the transform methods and the
      <code>styles</code> method.</p>
    </div>
  </div>

  <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
  moving on. I said earlier that container constructors (such as
  <code>g</code>) yield to a block if present. In this case,
  though, the <code>translate</code> and <code>rotate</code>
  methods intervene between <code>g</code> and the block. All the
  transform methods 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 15-20</h2>
  <pre class="example">
15      canvas.g.translate(130, 83) do |head|
16          head.styles(:stroke=&gt;'black', :stroke_width=&gt;2)
17          head.circle(30).styles(:fill=&gt;'yellow')
18          head.circle(5, 10, -5).styles(:fill=&gt;'black')
19          head.polygon(30,0, 70,5, 30,10, 62,25, 23,20).styles(:fill=&gt;'orange')
20      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>In line 16 I define the <code>stroke</code> and
  <code>stroke_width</code> styles on the group. Styles defined on
  the group propogate to shapes within the group unless you
  override them. To do that, call <code>styles</code> on the
  shapes. In this group each shape has its own fill color. 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 22-25</h2>
  <pre class="example">
22      foot = RVG::Group.new do |_foot|
23          _foot.path('m0,0 v30 l30,10 l5,-10, l-5,-10 l-30,10z').
24                styles(:stroke_width=&gt;2, :fill=&gt;'orange', :stroke=&gt;'black')
25      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. This
  creates a group object that is not contained within the canvas.
  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 26-27</h2>
  <pre class="example">
26       canvas.use(foot).translate(75, 188).rotate(15)
27       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 29-34</h2>
  <pre class="example">
29      canvas.text(125, 30) do |title|
30          title.tspan("duck|").styles(:text_anchor=&gt;'end', :font_size=&gt;20,
31                         :font_family=&gt;'helvetica', :fill=&gt;'black')
32          title.tspan("type").styles(:font_size=&gt;22,
33                 :font_family=&gt;'times', :font_style=&gt;'italic', :fill=&gt;'red')
34      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 35</h2>
  <pre class="example">
35       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 38</h2>
  <pre class="example">
38   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>

  <div class="nav">
    &laquo; <a href="constants.html">Prev</a> | <a href=
    "index.html">Contents</a> | <a href="rvg.html">Next</a> &raquo;
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