Compaq 300 manual ∙ Blending using a destination alpha value

Operation (Compaq Tru64 UNIX)

Allocating 16 bits per texel allows more textures to be stored in the texture cache and may provide slightly higher rasterization rates. On the other hand, using 32 bits per texel provides better texture resolution in just the same way that a color frame buffer with 8 bits per color component has better color resolution than a frame buffer with 4 bits per color component.

∙The rasterization rate for PowerStorm 300/350 PCI graphics controllers is twice as fast for bilinear texture filtering as for trilinear texture filtering.

∙Software developers should use OpenGL Version 1.1 texture objects when an application uses more than one texture.

7.3.2.1.2Alpha Buffers

PowerStorm 300/350 PCI graphics controllers on the Tru64 UNIX platform do not provide an alpha buffer (i.e., they have zero alpha planes). Few applications actually need an alpha buffer. The two operations that depend on the values stored in the alpha buffer are:

∙Storing alpha values so that they can be read later

The application needs an alpha buffer if it must store alpha values for later retrieval by the glReadPixels function with one of the following values:

–GL_ALPHA

–GL_RGBA

–GL_LUMINANCE_ALPHA

∙Blending using a destination alpha value

The application needs an alpha buffer if it calls the glBlendFunc function with one of the following values:

–DST_ALPHA

–ONE_MINUS_DST_ALPHA

–ONE_MINUS_DST_COLOR

–SRC_ALPHA_SATURATE

In conformance with OpenGL, when an alpha buffer is not present, glReadPixels returns

1.0for all alpha values, and OpenGL uses 1.0 for all destination alpha values when computing blending factors.

Note that all source-alpha related aspects of the rasterization and per-fragment portions of the OpenGL pipeline (blending, alpha test, alpha related texturing functionality) are supported by PowerStorm 300/350 PCI graphics controllers on the Tru64 UNIX platform.

7–10PowerStorm 300 AGP and 300/350 PCI Graphics Controllers