mutter/doc/reference/cogl-2.0-experimental/blend-strings.xml

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<refentry id="cogl-Blend-Strings">
<refmeta>
<refentrytitle role="top_of_page" id="cogl-Blend-Strings.top_of_page">Blend Strings</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>COGL Library</refmiscinfo>
</refmeta>

<refnamediv>
<refname>Blend Strings</refname>
<refpurpose>A simple syntax and grammar for describing blending and texture
combining functions.</refpurpose>
</refnamediv>

<refsect1 id="cogl-Blend-Strings.description" role="desc">
<title>Cogl Blend Strings</title>
<para>
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.
</para>

<section>
<title>Some examples</title>

<para>Here is an example used for blending:</para>
<programlisting>
"RGBA = ADD (SRC_COLOR * (SRC_COLOR[A]), DST_COLOR * (1-SRC_COLOR[A]))"
</programlisting>
<para>In OpenGL terms this replaces glBlendFunc[Separate] and
glBlendEquation[Separate]</para>
<para>
Actually in this case it's more verbose than the GL equivalent:
</para>
<programlisting>
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
</programlisting>
<para>
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.
</para>

<para>Here is an example used for texture combining:</para>
<programlisting>
"RGB = REPLACE (PREVIOUS)"
"A = MODULATE (PREVIOUS, TEXTURE)"
</programlisting>
<para>
In OpenGL terms this replaces glTexEnv, and the above example is equivalent
to this OpenGL code:
</para>
<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);
</programlisting>

</section>

<section id="cogl-Blend-String-syntax">
<title>Here's the syntax</title>

<programlisting>
&lt;statement&gt;:
  &lt;channel-mask&gt;=&lt;function-name&gt;(&lt;arg-list&gt;)

  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.

&lt;channel-mask&gt;:
  A or RGB or RGBA

&lt;function-name&gt;:
  [A-Za-z_]*

&lt;arg-list&gt;:
  &lt;arg&gt;,&lt;arg&gt;
  or &lt;arg&gt;
  or ""

  I.e. functions may take 0 or more arguments

&lt;arg&gt;:
  &lt;color-source&gt;
  1 - &lt;color-source&gt;                : Only intended for texture combining
  &lt;color-source&gt; * ( &lt;factor&gt; )     : Only intended for blending
  0                                 : Only intended for blending

  See the blending or texture combining sections for further notes and examples.

&lt;color-source&gt;:
  &lt;source-name&gt;[&lt;channel-mask&gt;]
  &lt;source-name&gt;

  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.

&lt;factor&gt;:
  0
  1
  &lt;color-source&gt;
  1-&lt;color-source&gt;
  SRC_ALPHA_SATURATE
</programlisting>

</section>


</refsect1>


</refentry>