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84a438be55
An example of what could be the equivalent of
"RBG = REPLACE(TEXTURE)
A = MODULATE(PREVIOUS,TEXTURE)"
using the ARB_texture_env_combine extension was given, but it seems that
a few typo were left:
* remove a spurius GL_COMBINE_ALPHA
* use the _ALPHA variant of SRCN and OPERANDN when setting up the
alpha combiner
130 lines
3.8 KiB
XML
130 lines
3.8 KiB
XML
<?xml version="1.0"?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
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<!ENTITY version SYSTEM "version.xml">
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]>
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<refentry id="cogl-Blend-Strings">
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<refmeta>
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<refentrytitle role="top_of_page" id="cogl-Blend-Strings.top_of_page">Material Blend Strings</refentrytitle>
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<manvolnum>3</manvolnum>
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<refmiscinfo>COGL Library</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>Material Blend Strings</refname>
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<refpurpose>A simple syntax and grammar for describing blending and texture
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combining functions.</refpurpose>
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</refnamediv>
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<refsect1 id="cogl-Blend-Strings.description" role="desc">
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<title>Cogl Blend Strings</title>
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<para>
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Describing GPU blending and texture combining states is rather awkward to do
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in a consise but also readable fashion. Cogl helps by supporting
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string based descriptions using a simple syntax.
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</para>
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<section>
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<title>Some examples</title>
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<para>Here is an example used for blending:</para>
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<programlisting>
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"RGBA = ADD (SRC_COLOR * (SRC_COLOR[A]), DST_COLOR * (1-SRC_COLOR[A]))"
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</programlisting>
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<para>In OpenGL terms this replaces glBlendFunc[Separate] and
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glBlendEquation[Separate]</para>
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<para>
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Actually in this case it's more verbose than the GL equivalent:
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</para>
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<programlisting>
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glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
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</programlisting>
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<para>
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But unless you are familiar with OpenGL or refer to its API documentation
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you wouldn't know that the default function used by OpenGL is GL_FUNC_ADD
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nor would you know that the above arguments determine what the source color
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and destination color will be multiplied by before being adding.
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</para>
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<para>Here is an example used for texture combining:</para>
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<programlisting>
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"RGB = REPLACE (PREVIOUS)"
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"A = MODULATE (PREVIOUS, TEXTURE)"
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</programlisting>
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<para>
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In OpenGL terms this replaces glTexEnv, and the above example is equivalent
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to this OpenGL code:
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</para>
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<programlisting>
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glTexEnvi (GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);
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glTexEnvi (GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_REPLACE);
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glTexEnvi (GL_TEXTURE_ENV, GL_SRC0_RGB, GL_PREVIOUS);
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glTexEnvi (GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR);
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glTexEnvi (GL_TEXTURE_ENV, GL_COMBINE_ALPHA, GL_MODULATE);
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glTexEnvi (GL_TEXTURE_ENV, GL_SRC0_ALPHA, GL_PREVIOUS);
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glTexEnvi (GL_TEXTURE_ENV, GL_OPERAND0_ALPHA, GL_SRC_COLOR);
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glTexEnvi (GL_TEXTURE_ENV, GL_SRC1_ALPHA, GL_TEXTURE);
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glTexEnvi (GL_TEXTURE_ENV, GL_OPERAND1_ALPHA, GL_SRC_COLOR);
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</programlisting>
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</section>
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<section id="cogl-Blend-String-syntax">
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<title>Here's the syntax</title>
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<programlisting>
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<statement>:
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<channel-mask>=<function-name>(<arg-list>)
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You can either use a single statement with an RGBA channel-mask or you can use
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two statements; one with an A channel-mask and the other with an RGB
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channel-mask.
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<channel-mask>:
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A or RGB or RGBA
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<function-name>:
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[A-Za-z_]*
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<arg-list>:
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<arg>,<arg>
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or <arg>
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or ""
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I.e. functions may take 0 or more arguments
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<arg>:
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<color-source>
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1 - <color-source> : Only intended for texture combining
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<color-source> * ( <factor> ) : Only intended for blending
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0 : Only intended for blending
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See the blending or texture combining sections for further notes and examples.
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<color-source>:
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<source-name>[<channel-mask>]
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<source-name>
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See the blending or texture combining sections for the list of source-names
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valid in each context.
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If a channel mask is not given then the channel mask of the statement
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is assumed instead.
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<factor>:
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0
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1
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<color-source>
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1-<color-source>
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SRC_ALPHA_SATURATE
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</programlisting>
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</section>
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</refsect1>
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</refentry>
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