Operation
4–10 PowerStorm 300 Graphics Options Windows NT and Hardware Installation
4.3.3.2 Performance Considerations
The PowerStorm 300 device accelerates virtually all OpenGL Version 1.1 operations from
the rasterization stage through the remainder of the OpenGL pipeline. However, some
aspects of OpenGL use can affect performance as described in the following sections.
4.3.3.2.1 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 (e.g. 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 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 Application.
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 PowerStorm 300 rasterization rate 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.
4.3.3.2.2 Alpha Buffers
The PowerStorm 300 device does not provide an alpha buffer (it has 0 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
