bc8a18ebc3
Setting up layer combine functions and blend modes is very awkward to do programatically. This adds a parser for string based descriptions which are more consise and readable. E.g. a material layer combine function could now be given as: "RGBA = ADD (TEXTURE[A], PREVIOUS[RGB])" or "RGB = REPLACE (PREVIOUS)" "A = MODULATE (PREVIOUS, TEXTURE)" The simple syntax and grammar are only designed to expose standard fixed function hardware, more advanced combining must be done with shaders. This includes standalone documentation of blend strings covering the aspects that are common to blending and texture combining, and adds documentation with examples specific to the new cogl_material_set_blend() and cogl_material_layer_set_combine() functions. Note: The hope is to remove the now redundant bits of the material API before 1.0
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_COMBINE_ALPHA, 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_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_SRC1_RGB, GL_TEXTURE);
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glTexEnvi (GL_TEXTURE_ENV, GL_OPERAND1_RGB, 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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