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<span id="Rendering-Engine"></span><div class="header">
<p>
Previous: <a href="Render-Settings.html" accesskey="p" rel="prev">Render Settings</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="Rendering-Engine-1"></span><h4 class="subsection">5.3.17 Rendering Engine</h4>
<p>The <code>(chickadee graphics engine)</code> module provides a Scheme
abstraction to the state of the GPU driver. When the Chickadee game
loop launches, it takes care to initialize the engine. All draw calls
and state changes happen within the context of this engine.
</p>
<p>Performing a custom draw call could look something like this:
</p>
<div class="example">
<pre class="example">(with-graphics-state ((g:blend-mode blend:alpha)
(g:texture-0 my-texture))
(shader-apply my-shader #:foo 1))
</pre></div>
<span id="Render-States"></span><h4 class="subsubsection">5.3.17.1 Render States</h4>
<p>Render states represent individual state values on the GPU. For
example, the current shader. As a naming convention, Chickadee
prefixes variables containing render states with <code>g:</code>. Render
states can be manipulated using the <code>with-graphics-state</code> macro.
</p>
<dl>
<dt id="index-with_002dgraphics_002dstate">Syntax: <strong>with-graphics-state</strong> <em>((name value) …) body …</em></dt>
<dd><p>Evaluate <var>body</var> with render states defined by <var>name</var> changed
to <var>value</var>. The render states are restored to their previous
values afterwards.
</p></dd></dl>
<p>One additional piece of state that the rendering engine has, that is
not part of the GPU state, is the current projection matrix:
</p>
<dl>
<dt id="index-current_002dprojection">Procedure: <strong>current-projection</strong></dt>
<dd><p>Return the currently bound projection matrix (see <a href="Matrices.html">Matrices</a>).
</p></dd></dl>
<dl>
<dt id="index-with_002dprojection">Syntax: <strong>with-projection</strong> <em>projection body …</em></dt>
<dd><p>Evaluate <var>body</var> with the current projection matrix bound to
<var>projection</var> (see <a href="Matrices.html">Matrices</a>).
</p></dd></dl>
<span id="Rendering"></span><h4 class="subsubsection">5.3.17.2 Rendering</h4>
<p>Chickadee likens a GPU draw call to a Scheme procedure call. A shader
(see <a href="Shaders.html">Shaders</a>) is like a procedure for the GPU to apply. Shaders
are passed arguments: The positional arguments are vertex array
attributes (see <a href="Buffers.html">Buffers</a>) and the keyword arguments correspond to
the shader’s uniform variables. Scheme uses <code>apply</code> to call a
procedure, so Chickadee uses <code>shader-apply</code> to call a shader.
</p>
<dl>
<dt id="index-shader_002dapply">Syntax: <strong>shader-apply</strong> <em>shader vertex-array [#:uniform-key uniform-value …]</em></dt>
<dt id="index-shader_002dapply_002a">Syntax: <strong>shader-apply*</strong> <em>shader vertex-array count [#:uniform-key uniform-value …]</em></dt>
<dd>
<p>Render <var>vertex-array</var> using <var>shader</var> with the uniform values
specified in the following keyword arguments.
</p>
<p>While <code>shader-apply</code> will draw every vertex in <var>vertex-array</var>,
<code>shader-apply*</code> will only draw <var>count</var> vertices.
</p></dd></dl>
<dl>
<dt id="index-shader_002dapply_002finstanced">Syntax: <strong>shader-apply/instanced</strong> <em>shader vertex-array n [#:uniform-key uniform-value …]</em></dt>
<dt id="index-shader_002dapply_002finstanced-1">Syntax: <strong>shader-apply/instanced</strong> <em>shader vertex-array count n [#:uniform-key uniform-value …]</em></dt>
<dd>
<p>Render <var>vertex-array</var> <var>n</var> times using <var>shader</var> with the
uniform values specified in the following keyword arguments.
</p>
<p>Instanced rendering is very beneficial for rendering the same object
many times with only small differences for each one. For example, the
particle effects described in <a href="Particles.html">Particles</a> use instanced rendering.
</p>
<p>While <code>shader-apply/instanced</code> will draw every vertex in
<var>vertex-array</var>, <code>shader-apply*</code> will only draw <var>count</var>
vertices.
</p></dd></dl>
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