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<a name="cogl-Blend-Strings"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle"><a name="cogl-Blend-Strings.top_of_page"></a>Blend Strings</span></h2>
<p>Blend Strings — A simple syntax and grammar for describing blending and texture
combining functions.</p>
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<div class="refsect1">
<a name="cogl-Blend-Strings.description"></a><h2>Cogl Blend Strings</h2>
<p>
Describing GPU blending and texture combining states is rather awkward to do
in a consise but also readable fashion. Cogl helps by supporting
string based descriptions using a simple syntax.
</p>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="id-1.2.5.2.3.3"></a>Some examples</h5></div></div></div>
<p>Here is an example used for blending:</p>
<pre class="programlisting">
"RGBA = ADD (SRC_COLOR * (SRC_COLOR[A]), DST_COLOR * (1-SRC_COLOR[A]))"
</pre>
<p>In OpenGL terms this replaces glBlendFunc[Separate] and
glBlendEquation[Separate]</p>
<p>
Actually in this case it's more verbose than the GL equivalent:
</p>
<pre class="programlisting">
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
</pre>
<p>
But unless you are familiar with OpenGL or refer to its API documentation
you wouldn't know that the default function used by OpenGL is GL_FUNC_ADD
nor would you know that the above arguments determine what the source color
and destination color will be multiplied by before being adding.
</p>
<p>Here is an example used for texture combining:</p>
<pre class="programlisting">
"RGB = REPLACE (PREVIOUS)"
"A = MODULATE (PREVIOUS, TEXTURE)"
</pre>
<p>
In OpenGL terms this replaces glTexEnv, and the above example is equivalent
to this OpenGL code:
</p>
<pre class="programlisting">
glTexEnvi (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
glTexEnvi (GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_REPLACE);
glTexEnvi (GL_TEXTURE_ENV, GL_SRC0_RGB, GL_PREVIOUS);
glTexEnvi (GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR);
glTexEnvi (GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_MODULATE);
glTexEnvi (GL_TEXTURE_ENV, GL_SRC0_ALPHA, GL_PREVIOUS);
glTexEnvi (GL_TEXTURE_ENV, GL_OPERAND0_ALPHA, GL_SRC_COLOR);
glTexEnvi (GL_TEXTURE_ENV, GL_SRC1_ALPHA, GL_TEXTURE);
glTexEnvi (GL_TEXTURE_ENV, GL_OPERAND1_ALPHA, GL_SRC_COLOR);
</pre>
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<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="cogl-Blend-String-syntax"></a>Here's the syntax</h5></div></div></div>
<pre class="programlisting">
<statement>:
<channel-mask>=<function-name>(<arg-list>)
You can either use a single statement with an RGBA channel-mask or you can use
two statements; one with an A channel-mask and the other with an RGB
channel-mask.
<channel-mask>:
A or RGB or RGBA
<function-name>:
[A-Za-z_]*
<arg-list>:
<arg>,<arg>
or <arg>
or ""
I.e. functions may take 0 or more arguments
<arg>:
<color-source>
1 - <color-source> : Only intended for texture combining
<color-source> * ( <factor> ) : Only intended for blending
0 : Only intended for blending
See the blending or texture combining sections for further notes and examples.
<color-source>:
<source-name>[<channel-mask>]
<source-name>
See the blending or texture combining sections for the list of source-names
valid in each context.
If a channel mask is not given then the channel mask of the statement
is assumed instead.
<factor>:
0
1
<color-source>
1-<color-source>
SRC_ALPHA_SATURATE
</pre>
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