OpenGL Programming Guide

Chapter 9Texture Mapping

Modulating and Blending

In all the examples so far in this chapter, the values in the texture map have been used directly as colors to be painted on the surface being rendered. You can also use the values in the texture map to modulate the color that the surface would be painted without texturing, or to blend the color in the texture map with the nontextured color of the surface. You choose one of these three texturing functions by supplying the appropriate arguments to glTexEnv*(). void glTexEnv{if}{v}(GLenum target, GLenum pname, TYPEparam);

Sets the current texturing function. target must be GL_TEXTURE_ENV. If pname is GL_TEXTURE_ENV_MODE, param can be GL_DECAL, GL_MODULATE, or GL_BLEND, to specify how texture values are to be combined with the color values of the fragment being processed. In decal mode and with a three-component texture, the texture's colors replace the fragment's colors. With either of the other modes or with a four-component texture, the final color is a combination of the texture's and the fragment's values. If pname is GL_TEXTURE_ENV_COLOR, param is an array of four floating-point values representing R, G, B, and A components. These values are used only if the GL_BLEND texture function has been specified as well.

The texturing function operates on selected components of the texture and the color values that would be used with no texturing. Recall that when you specify your texture map with glTexImage*d(), the third argument is the number of R, G, B, A components to be selected for each texel. A single selected component is interpreted as a luminance value (L); if there are two, the first is luminance, and the second is an alpha value (A). Three components form an RGB color triple (C), and four components provide an RGB triple and a value for alpha. Note that this selection is performed after the pixel-transfer function has been applied. Therefore, it makes sense, for example, to specify a texture with a GL_COLOR_INDEX image because the indices are converted to RGBA values by table lookup before they're used to form the texture image. These components are used by the texturing functions as shown in Table 9-2 .

Table 9-2 : Decal, Modulate, and Blend Functions

Components	Decal Mode	Modulate Mode	Blend Mode
1	undefined	C = LtCf, A = Af	C = (1-L t)Cf + LtCc, A = Af
2	undefined	C = LtCf, A = AtAf	C = (1-L t)Cf + LtCc, A = AtAf
3	C = Ct, A = Af	C = CtCf, A = Af	undefined
4	C = (1 t)Cf + AtCt, A = Af	C = CtCf, A = AtAf	undefined

In the table, a subscript of t indicates a texture value, f indicates a fragment value, c indicates the values assigned with GL_TEXTURE_ENV_COLOR, and no subscript indicates the final, computed value. Also in the table, multiplication of a color triple by a scalar means multiplying each of the R, G, and B components by the scalar; multiplying (or adding) two color triples means multiplying (or adding) each component of the second by the corresponding component of the first.

Decal mode makes sense only if the number of components is three or four (remember that texture mapping doesn't work in color-index mode). With three selected components, the color that would have been painted in the absence of any texture mapping (the fragment's color) is replaced by the texture color, and its alpha is unchanged. With four components, the fragment's color is blended with the texture color in a ratio determined by the texture alpha, and the fragment's alpha is unchanged. You use decal mode in situations where you want to apply an opaque texture to an object - if you were drawing a soup can with an opaque label, for example.

For modulation, the fragment's color is modulated by the contents of the texture map. For one or two components, the color values are multiplied by the same value, so the texture map modulates between the fragment's color (if the luminance is 1) to black (if it's 0). With three or four components, each of the incoming color components is multiplied by a corresponding (possibly different) value in the texture. If there's an alpha value (which there is for two or four components), it's multiplied by the fragment's alpha. You need to use modulation to create a texture that responds to lighting conditions; most of the texture-mapping examples in the color plates use modulation for this reason.

Blending mode makes sense only for one- or two-component textures. The luminance is used somewhat like an alpha value to blend the fragment's color with the color specified by GL_TEXTURE_ENV_COLOR. With two components, the fragment's alpha is also multiplied by the alpha in the texture. See "Sample Uses of Blending" for the billboarding example, which uses a blended texture.

OpenGL Programming Guide