Table of Contents Glide Programming Guide

A Higher Level Alpha Combine Function

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4A Higher Level Texture Combine Function

3A Higher Level Alpha Combine Function

guAlphaSource() is a higher level interface to the Voodoo Graphics alpha combine unit than grAlphaCombine(), which was presented in Chapter Chapter 6. .
void guAlphaSource( GrAlphaSourceMode_t mode )

The alpha combine unit has two configurable inputs and one output. The output of the alpha combine unit gets fed into the alpha testing and blending units. The selection of the a_local input is important because it is used in the color combine unit.

The following table describes how a_local and output alpha are computed based on the mode.

Table Glide Utilities .2 Alpha combine unit modes.

mode	output	a_local
GR_ALPHASOURCE_CC_ALPHA	constant color a^¨	constant color a^¨
GR_ALPHASOURCE_ITERATED_ALPHA	iterated vertex a	iterated vertex a
GR_ALPHASOURCE_TEXTURE_ALPHA	texture a^ª	none
GR_ALPHASOURCE_TEXTURE_ALPHA_TIMES_ITERATED_ALPHA	texture a iterated a*^ª	iterated vertex a

^¨Constant color a is the value passed to grConstantColorValue().
^ªIf texture has no alpha component, texture a is 255.

4A Higher Level Texture Combine Function

Configuring the Glide texture pipeline consists of setting up a mipmap source and configuring the texture combine function on each TMU. Mipmap sources are established by downloading mipmaps and naming them as the current texel source.
Chapter 9 presented a collection of Glide functions that configure the texture combine units; Chapter 10 talked about managing texture memory and downloading mipmaps. Most of the memory management details were left to the application.
The Glide Utilities Library includes some higher level routines that configure the texture combine unit on a functional level and that provides increased memory management functionality.
guTexCombineFunction() specifies the function used when combining textures on tmu with incoming textures from the neighboring TMU. Texture combining operations allow for interesting effects such as detail and projected texturing as well as the trilinear filtering of LOD blending.
void guTexCombineFunction( GrChipID_t tmu, GrTextureCombineFnc_t function )

The following table describes the available texture combine functions and their effects. c_local represents the color components generated by indexing and filtering from the mipmap stored on tmu, and c_other represents the incoming color components from the neighboring TMU. Typically, the texture combine function on the neighboring TMU operates in GR_TEXTURECOMBINE_DECAL (just pass the texel through) mode.

Table Glide Utilities .3 Texture combine functions.

texture combine function	result	effect
GR_TEXTURECOMBINE_ZERO	0	0x00 per component
GR_TEXTURECOMBINE_DECAL	c_local	decal texture
GR_TEXTURECOMBINE_OTHER	c_other	pass through
GR_TEXTURECOMBINE_ADD	c_other + c_local	additive texture
GR_TEXTURECOMBINE_MULTIPLY	c_other · c_local	modulated texture
GR_TEXTURECOMBINE_SUBTRACT	c_other – c_local	subtractive texture
GR_TEXTURECOMBINE_DETAIL	blend (c_other, c_local)	composite textures with composite on selected TMU
GR_TEXTURECOMBINE_DETAIL_OTHER	blend (c_other, c_local)	composite textures with composite on neighboring TMU
GR_TEXTURECOMBINE_TRILINEAR_ODD	blend (c_other, c_local)	LOD blended textures with odd levels on selected TMU
GR_TEXTURECOMBINE_TRILINEAR_EVEN	blend (c_other, c_local)	LOD blended textures with even levels on selected TMU
GR_TEXTURECOMBINE_ONE	255	0xFF per component

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