Operation (Windows NT)
5.3.3.2 Performance Considerations
PowerStorm 300/350 PCI graphics controllers for the Windows NT Alpha platform accelerate virtually all OpenGL Version 1.1 operations. However, some aspects of OpenGL use can affect performance as described in the following sections.
5.3.3.2.1 Texture Mapping
Several aspects of texture mapping can affect application performance. Some of these are under the control of the application user. Others are under the control of the application developer.
∙When an application defines textures to OpenGL it can explicitly specify the resolution (for example 16 or 32 bits per texel) with which textures are to be stored. Alternatively, the application can leave this decision to OpenGL. In this latter case the PowerStorm 300/350 OpenGL driver will, by default, use 16 bits per texel for RGB and RGBA textures. The application user can override this default behavior by setting the Texel 32 Bit Default selection in the Display Applet.
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 has more than one texture.
5.3.3.2.2Alpha Buffers
PowerStorm 300/350 PCI graphics controllers on the Windows NT Alpha platform do not provide an alpha buffer (i.e., it has 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 formats:
–GL_ALPHA
–GL_RGBA
–GL_LUMINANCE_ALPHA