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<title>Blending (The Chickadee Game Toolkit)</title>
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<span id="Blending"></span><div class="header">
<p>
Next: <a href="Framebuffers.html" accesskey="n" rel="next">Framebuffers</a>, Previous: <a href="3D-Models.html" accesskey="p" rel="prev">3D Models</a>, Up: <a href="Graphics.html" accesskey="u" rel="up">Graphics</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html" title="Index" rel="index">Index</a>]</p>
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<hr>
<span id="Blending-1"></span><h4 class="subsection">2.3.10 Blending</h4>
<p>Rendering a scene often involves drawing layers of objects that
overlap each other. Blending determines how two overlapping pixels
are combined in the final image that is rendered to the screen.
</p>
<p>Chickadee provides the following blend modes:
</p>
<dl>
<dt id="index-blend_003aalpha">Variable: <strong>blend:alpha</strong></dt>
<dd><p>Blend pixels according to the values of their alpha channels. This is
the most commonly used blend mode.
</p></dd></dl>
<dl>
<dt id="index-blend_003areplace">Variable: <strong>blend:replace</strong></dt>
<dd><p>Overwrite the output pixel color with the color being drawn.
</p></dd></dl>
<dl>
<dt id="index-blend_003aadd">Variable: <strong>blend:add</strong></dt>
<dd><p>Add all pixel color values together. The more colors blended
together, the more white the final color becomes.
</p></dd></dl>
<dl>
<dt id="index-blend_003asubtract">Variable: <strong>blend:subtract</strong></dt>
<dd><p>Subtract all pixel color values. The more colors blended together,
the more black the final color becomes.
</p></dd></dl>
<dl>
<dt id="index-blend_003amultiply">Variable: <strong>blend:multiply</strong></dt>
</dl>
<dl>
<dt id="index-blend_003adarken">Variable: <strong>blend:darken</strong></dt>
</dl>
<dl>
<dt id="index-blend_003alighten">Variable: <strong>blend:lighten</strong></dt>
</dl>
<dl>
<dt id="index-blend_003ascreen">Variable: <strong>blend:screen</strong></dt>
</dl>
<p>Custom blend modes can be created using the <code>make-blend-mode</code> procedure:
</p>
<dl>
<dt id="index-make_002dblend_002dmode">Procedure: <strong>make-blend-mode</strong> <em>equation source-function destination-function</em></dt>
<dd><p>Return a new custom blend mode that applies <var>source-function</var> to
the source color, <var>destination-function</var> to the destination color,
and finally applies <var>equation</var> to the transformed
source/destination color values. These arguments are <em>not</em>
procedures, but symbolic representations of the functions that OpenGL
supports.
</p>
<p>Valid values for <var>equation</var> are:
</p>
<ul>
<li> <code>add</code>
</li><li> <code>subtract</code>
</li><li> <code>reverse-subtract</code>
</li><li> <code>min</code>
</li><li> <code>max</code>
</li><li> <code>alpha-min</code>
</li><li> <code>alpha-max</code>
</li></ul>
<p>Valid values for <var>source-function</var> are:
</p>
<ul>
<li> <code>zero</code>
</li><li> <code>one</code>
</li><li> <code>destination-color</code>
</li><li> <code>one-minus-destination-color</code>
</li><li> <code>source-alpha-saturate</code>
</li><li> <code>source-alpha</code>
</li><li> <code>one-minus-source-alpha</code>
</li><li> <code>destination-alpha</code>
</li><li> <code>one-minus-destination-alpha</code>
</li><li> <code>constant-color</code>
</li><li> <code>one-minus-constant-color</code>
</li><li> <code>constant-alpha</code>
</li><li> <code>one-minus-constant-alpha</code>
</li></ul>
<p>Valid values for <var>destination-function</var> are:
</p>
<ul>
<li> <code>zero</code>
</li><li> <code>one</code>
</li><li> <code>source-color</code>
</li><li> <code>one-minus-source-color</code>
</li><li> <code>source-alpha</code>
</li><li> <code>one-minus-source-alpha</code>
</li><li> <code>destination-alpha</code>
</li><li> <code>one-minus-destination-alpha</code>
</li><li> <code>constant-color</code>
</li><li> <code>one-minus-constant-color</code>
</li><li> <code>constant-alpha</code>
</li><li> <code>one-minus-constant-alpha</code>
</li></ul>
</dd></dl>
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