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<span id="Shaders"></span><div class="header">
<p>
Next: <a href="Framebuffers.html" accesskey="n" rel="next">Framebuffers</a>, Previous: <a href="Buffers.html" accesskey="p" rel="prev">Buffers</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="Shaders-1"></span><h4 class="subsection">5.3.13 Shaders</h4>
<p>Shaders are programs that the GPU can evaluate that allow the
programmer to completely customized the final output of a GPU draw
call. The <code>(chickadee graphics shader)</code> module provides an API for
building custom shaders.
</p>
<p>Shaders are written in the OpenGL Shading Language, or GLSL for short.
Chickadee aspires to provide a domain specific language for writing
shaders in Scheme, but we are not there yet.
</p>
<p>Shader programs consist of two components: A vertex shader and a
fragment shader. A vertex shader receives vertex data (position
coordinates, texture coordinates, normals, etc.) and transforms them
as desired, whereas a fragment shader controls the color of each
pixel.
</p>
<p>Sample vertex shader:
</p>
<div class="example">
<pre class="verbatim">#version 130
in vec2 position;
in vec2 tex;
out vec2 fragTex;
uniform mat4 mvp;
void main(void) {
fragTex = tex;
gl_Position = mvp * vec4(position.xy, 0.0, 1.0);
}
</pre></div>
<p>Sample fragment shader:
</p>
<div class="example">
<pre class="verbatim">#version 130
in vec2 fragTex;
uniform sampler2D colorTexture;
void main (void) {
gl_FragColor = texture2D(colorTexture, fragTex);
}
</pre></div>
<p>This manual will not cover GLSL features and syntax as there is lots
of information already available about this topic.
</p>
<p>One way to think about rendering with shaders, and the metaphor
Chickadee uses, is to think about it as a function call: The shader is
a function, and it is applied to some “attributes” (positional
arguments), and some “uniforms” (keyword arguments).
</p>
<div class="lisp">
<pre class="lisp"><span class="syntax-open">(</span><span class="syntax-special">define</span> <span class="syntax-symbol">my-shader</span> <span class="syntax-open">(</span><span class="syntax-symbol">load-shader</span> <span class="syntax-string">"vert.glsl"</span> <span class="syntax-string">"frag.glsl"</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-special">define</span> <span class="syntax-symbol">vertices</span> <span class="syntax-open">(</span><span class="syntax-symbol">make-vertex-array</span> <span class="syntax-symbol">...</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">shader-apply</span> <span class="syntax-symbol">my-shader</span> <span class="syntax-symbol">vertices</span> <span class="syntax-keyword">#:color</span> <span class="syntax-symbol">red</span><span class="syntax-close">)</span>
</pre></div>
<p>See <a href="Rendering-Engine.html">Rendering Engine</a> for more details about the <code>shader-apply</code>
procedure.
</p>
<p>Shaders are incredibly powerful tools, and there’s more information
about them than we could ever fit into this manual, so we highly
recommend searching the web for more information and examples. What
we can say, though, is how to use our API:
</p>
<dl>
<dt id="index-strings_002d_003eshader">Procedure: <strong>strings->shader</strong> <em>vertex-source fragment-source</em></dt>
<dd><p>Compile <var>vertex-source</var>, the GLSL code for the vertex shader, and
<var>fragment-source</var>, the GLSL code for the fragment shader, into a
GPU shader program.
</p></dd></dl>
<dl>
<dt id="index-load_002dshader">Procedure: <strong>load-shader</strong> <em>vertex-source-file fragment-source-file</em></dt>
<dd><p>Compile the GLSL source code within <var>vertex-source-file</var> and
<var>fragment-source-file</var> into a GPU shader program.
</p></dd></dl>
<dl>
<dt id="index-make_002dshader">Procedure: <strong>make-shader</strong> <em>vertex-port fragment-port</em></dt>
<dd><p>Read GLSL source from <var>vertex-port</var> and <var>fragment-port</var> and
compile them into a GPU shader program.
</p></dd></dl>
<dl>
<dt id="index-shader_003f">Procedure: <strong>shader?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> is a shader.
</p></dd></dl>
<dl>
<dt id="index-null_002dshader">Variable: <strong>null-shader</strong></dt>
<dd><p>Represents the absence shader program.
</p></dd></dl>
<dl>
<dt id="index-shader_002duniform">Procedure: <strong>shader-uniform</strong> <em>shader name</em></dt>
<dd><p>Return the metadata for the uniform <var>name</var> in <var>shader</var>.
</p></dd></dl>
<dl>
<dt id="index-shader_002duniforms">Procedure: <strong>shader-uniforms</strong> <em>shader</em></dt>
<dd><p>Return a hash table of uniforms for <var>shader</var>.
</p></dd></dl>
<dl>
<dt id="index-shader_002dattributes">Procedure: <strong>shader-attributes</strong> <em>shader</em></dt>
<dd><p>Return a hash table of attributes for <var>shader</var>.
</p></dd></dl>
<dl>
<dt id="index-shader_002duniform_002dset_0021">Procedure: <strong>shader-uniform-set!</strong> <em>shader uniform value</em></dt>
</dl>
<span id="Attributes"></span><h4 class="subsubsection">5.3.13.1 Attributes</h4>
<dl>
<dt id="index-attribute_003f">Procedure: <strong>attribute?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> is an attribute.
</p></dd></dl>
<dl>
<dt id="index-attribute_002dname">Procedure: <strong>attribute-name</strong> <em>attribute</em></dt>
<dd><p>Return the variable name of <var>attribute</var>.
</p></dd></dl>
<dl>
<dt id="index-attribute_002dlocation">Procedure: <strong>attribute-location</strong> <em>attribute</em></dt>
<dd><p>Return the binding location of <var>attribute</var>.
</p></dd></dl>
<dl>
<dt id="index-attribute_002dtype">Procedure: <strong>attribute-type</strong> <em>attribute</em></dt>
<dd><p>Return the data type of <var>attribute</var>.
</p></dd></dl>
<span id="Uniforms"></span><h4 class="subsubsection">5.3.13.2 Uniforms</h4>
<dl>
<dt id="index-uniform_003f">Procedure: <strong>uniform?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> is a uniform.
</p></dd></dl>
<dl>
<dt id="index-uniform_002dname">Procedure: <strong>uniform-name</strong> <em>uniform</em></dt>
<dd><p>Return the variable name of <var>uniform</var>.
</p></dd></dl>
<dl>
<dt id="index-uniform_002dtype">Procedure: <strong>uniform-type</strong> <em>uniform</em></dt>
<dd><p>Return the data type of <var>uniform</var>.
</p></dd></dl>
<dl>
<dt id="index-uniform_002dvalue">Procedure: <strong>uniform-value</strong> <em>uniform</em></dt>
<dd><p>Return the current value of <var>uniform</var>.
</p></dd></dl>
<span id="User_002dDefined-Shader-Types"></span><h4 class="subsubsection">5.3.13.3 User-Defined Shader Types</h4>
<p>The shader examples in this manual thus far have only shown uniforms
defined using primitive types. However, GLSL shaders support
user-defined compound structs, such as this one:
</p>
<div class="example">
<pre class="verbatim">struct Light {
bool enabled;
int type;
vec3 position;
vec3 direction;
vec4 color;
float intensity;
float cutOff;
};
uniform Light light;
</pre></div>
<p>While <code>light</code> is declared as a single uniform in the shader code,
OpenGL translates this into <em>seven</em> uniforms in this case: One
uniform each member of the <code>Light</code> struct. This poses a problem
for sending Scheme data to the GPU. How can compound Scheme data
translate into compound uniform data on the GPU? The answer is with
shader types. Shader types are a special kind of Guile struct that
provide a one-to-one mapping between a Scheme data structure and a
shader struct.
</p>
<p>Some example code will explain this concept best. Here is the Scheme
equivalent of the <code>Light</code> struct:
</p>
<div class="lisp">
<pre class="lisp"><span class="syntax-open">(</span><span class="syntax-special">define-shader-type</span> <span class="syntax-symbol"><light></span>
<span class="syntax-symbol">make-light</span>
<span class="syntax-symbol">light?</span>
<span class="syntax-open">(</span><span class="syntax-symbol">bool</span> <span class="syntax-symbol">enabled</span> <span class="syntax-symbol">light-enabled?</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">int</span> <span class="syntax-symbol">type</span> <span class="syntax-symbol">light-type</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">float-vec3</span> <span class="syntax-symbol">position</span> <span class="syntax-symbol">light-position</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">float-vec3</span> <span class="syntax-symbol">direction</span> <span class="syntax-symbol">light-direction</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">float-vec4</span> <span class="syntax-symbol">color</span> <span class="syntax-symbol">light-color</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">float</span> <span class="syntax-symbol">intensity</span> <span class="syntax-symbol">light-intensity</span><span class="syntax-close">)</span>
<span class="syntax-open">(</span><span class="syntax-symbol">float</span> <span class="syntax-symbol">cut-off</span> <span class="syntax-symbol">light-cut-off</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
</pre></div>
<p>The macro <code>define-shader-type</code> closely resembles the familiar
<code>define-record-type</code> from SRFI-9, but with one notable
difference: Each struct field contains type information. The type
must be one of several primitive types (documented below) or another
shader type in the case of a nested structure.
</p>
<p>It is important to note that the names of the shader type fields
<em>must</em> match the names of the struct members in the GLSL code,
otherwise Chickadee will be unable to perform the proper translation.
</p>
<p>As of this writing, this interface is new and experimental. It
remains to be seen if this model is robust enough for all use-cases.
</p>
<p>Primitive data types:
</p>
<dl>
<dt id="index-bool">Variable: <strong>bool</strong></dt>
<dd><p>Either <code>#t</code> or <code>#f</code>.
</p></dd></dl>
<dl>
<dt id="index-int">Variable: <strong>int</strong></dt>
<dd><p>An integer.
</p></dd></dl>
<dl>
<dt id="index-unsigned_002dint">Variable: <strong>unsigned-int</strong></dt>
<dd><p>An unsigned integer.
</p></dd></dl>
<dl>
<dt id="index-float">Variable: <strong>float</strong></dt>
<dd><p>A floating point number.
</p></dd></dl>
<dl>
<dt id="index-float_002dvec2">Variable: <strong>float-vec2</strong></dt>
<dd><p>A 2D vector (see <a href="Vectors.html">Vectors</a>.)
</p></dd></dl>
<dl>
<dt id="index-float_002dvec3">Variable: <strong>float-vec3</strong></dt>
<dd><p>A 3D vector (see <a href="Vectors.html">Vectors</a>.)
</p></dd></dl>
<dl>
<dt id="index-float_002dvec4">Variable: <strong>float-vec4</strong></dt>
<dd><p>A color (see <a href="Colors.html">Colors</a>) or rectangle (see <a href="Rectangles.html">Rectangles</a>.)
</p></dd></dl>
<dl>
<dt id="index-mat3">Variable: <strong>mat3</strong></dt>
<dd><p>A 3x3 matrix (see <a href="Matrices.html">Matrices</a>.)
</p></dd></dl>
<dl>
<dt id="index-mat4">Variable: <strong>mat4</strong></dt>
<dd><p>A 4x4 matrix (see <a href="Matrices.html">Matrices</a>.)
</p></dd></dl>
<dl>
<dt id="index-sampler_002d2d">Variable: <strong>sampler-2d</strong></dt>
<dd><p>A texture (see <a href="Textures.html">Textures</a>.)
</p></dd></dl>
<dl>
<dt id="index-sampler_002dcube">Variable: <strong>sampler-cube</strong></dt>
<dd><p>A cube map (see <a href="Textures.html">Textures</a>.)
</p></dd></dl>
<dl>
<dt id="index-local_002dfield">Variable: <strong>local-field</strong></dt>
<dd><p>A special type that means that the data is for the client-side
(Scheme-side) only and should not be sent to the GPU. Any object may
be stored in a local field.
</p></dd></dl>
<dl>
<dt id="index-define_002dshader_002dtype">Syntax: <strong>define-shader-type</strong> <em><name> constructor predicate (field-type field-name [field-getter] [field-setter]) …</em></dt>
<dd>
<p>Define a new shader data type called <var><name></var>.
</p>
<p>Instances of this data type are created by calling the
<var>constructor</var> procedure. This procedure maps each field to a
keyword argument. A shader data type with the fields <code>foo</code>,
<code>bar</code>, and <code>baz</code> would have a constructor that accepts the
keyword arguments <code>#:foo</code>, <code>#:bar</code>, and <code>#:baz</code>.
</p>
<p>A procedure named <var>predicate</var> will test if an object is a
<var><name></var> shader data type.
</p>
<p>Fields follow the format <code>(field-type field-name [field-getter]
[field-setter])</code>. <var>field-type</var> and <var>field-name</var> are required
for each field, but <var>field-getter</var> and <var>field-setter</var> are
optional.
</p>
</dd></dl>
<dl>
<dt id="index-shader_002ddata_002dtype_003f">Procedure: <strong>shader-data-type?</strong> <em>obj</em></dt>
<dd><p>Return <code>#t</code> if <var>obj</var> is a shader data type object.
</p></dd></dl>
<hr />
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