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Texture Combiners

In Chapter 6, "More on Colors and Materials," you learned how to use the blending equation to control the way color fragments were blended together when multiple layers of geometry were drawn in the color buffer (typically back to front). OpenGL's texture combiners allow the same sort of control (only better) for the way multiple texture fragments are combined. By default, you can simply choose one of the texture environment modes (GL_DECAL, GL_REPLACE, GL_MODULATE, or GL_ADD) for each texture unit, and the results of each texture application are then added to the next texture unit. These texture environments were covered in Chapter 8.

Texture combiners add a new texture environment, GL_COMBINE, that allows you to explicitly set the way texture fragments from each texture unit are combined. To use texture combiners, you call glTexEnv in the following manner:

glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);

Texture combiners are controlled entirely through the glTexEnv function. Next, you need to select which texture combiner function you want to use. The combiner function selector, which can be either GL_COMBINE_RGB or GL_COMBINE_ALPHA, becomes the second argument to the glTexEnv function. The third argument becomes the texture environment function that you want to employ (for either RGB or alpha values). These functions are listed in Table 9.4. For example, to select the GL_REPLACE combiner for RGB values, you would call the following function:

glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_REPLACE);

This combiner does little more than duplicate the normal GL_REPLACE texture environment.

Table 9.4. Texture Combiner Functions

Constant

Function

GL_REPLACE

Arg0

GL_MODULATE

Arg0 * Arg1

GL_ADD

Arg0 + Arg1

GL_ADD_SIGNED

Arg0 + Arg1 - 0.5

GL_INTERPOLATE

(Arg0 * Arg2) + (Arg1 * (1 - Arg2))

GL_SUBTRACT

Arg0 - Arg1

GL_DOT3_RGB/GL_DOT3_RGBA

4*((Arg0r - 0.5) * (Arg1r - 0.5) + (Arg0g - 0.5) * (Arg1g - 0.5) + (Arg0b - 0.5) * (Arg1b - 0.5))

The values of Arg0 - Arg2 are from source and operand values set with more calls to glTexEnv. The values GL_SOURCEx_RGB and GL_SOURCEx_ALPHA are used to specify the RGB or alpha combiner function arguments, where x is 0, 1, or 2. The values for these sources are given in Table 9.5.

Table 9.5. Texture Combiner Sources

Constant

Description

GL_TEXTURE

The texture bound to the current active texture unit

GL_TEXTUREx

The texture bound to texture unit x

GL_CONSTANT

The color (or alpha) value set by the texture environment variable GL_TEXTURE_ENV_COLOR

GL_PRIMARY_COLOR

The color (or alpha) value coming from the original geometry fragment

GL_PREVIOUS

The color (or alpha) value resulting from the previous texture unit's texture environment

For example, to select the texture from texture unit 0 for Arg0, you would make the following function call:

glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_TEXTURE0);

You also have some additional control over what values are used from a given source for each argument. To set these operands, you use the constant GL_OPERANDx_RGB or GL_OPERANDx_ALPHA, where x is 0, 1, or 2. The valid operands and their meanings are given in Table 9.6.

Table 9.6. Texture Combiner Operands

Constant

Meaning

GL_SRC_COLOR

The color values from the source. This may not be used with GL_OPERANDx_ALPHA.

GL_ONE_MINUS_SRC_COLOR

One's complement (1-value) of the color values from the source. This may not be used with GL_OPERANDx_ALPHA.

GL_SRC_ALPHA

The alpha values of the source.

GL_ONE_MINUS_SRC_ALPHA

One's complement (1-value) of the alpha values from the source.

For example, if you have two textures loaded on the first two texture units, and you want to multiply the color values from both textures during the texture application, you would set it up as shown here:

// Tell OpenGL you want to use texture combiners
glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_COMBINE);

// Tell OpenGL which combiner you want to use (GL_MODULATE for RGB values)
glTexEnvi(GL_TEXTURE_ENV, GL_COMBINE_RGB, GL_MODULATE);

// Tell OpenGL to use texture unit 0's color values for Arg0
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_TEXTURE0);
glTexEnvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR);

// Tell OpenGL to use texture unit 1's color values for Arg1
glTexEnvi(GL_TEXTURE_ENV, GL_SOURCE0_RGB, GL_TEXTURE1);
glTexenvi(GL_TEXTURE_ENV, GL_OPERAND0_RGB, GL_SRC_COLOR);

Finally, with texture combiners, you can also specify a constant RGB or alpha scaling factor. The default parameters for these are as shown here:

glTexEnvf(GL_TEXTURE_ENV, GL_RGB_SCALE, 1.0f);
glTexEnvf(GL_TEXTURE_ENV, GL_ALPHA_SCALE, 1.0f);

Texture combiners add a lot of flexibility to legacy OpenGL implementations. For ultimate control over how texture layers can be combined, we will later turn to shaders.

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