GL_ALPHA and Texture memory

OpenGL OpenGL: Advanced Coding
1

When I look up the definition for glTexImage2D in the
blue book( OpenGL Reference Manual ) the entry for GL_ALPHA
is as follows.

GL_ALPHA Each element is a single alpha component. It is converted to
floating point and assembled into an RGBA element by
attaching 0.0 for red, green, and blue. Each component is
then multiplied by the signed scale factor GL_c_SCALE,
added to the signed bias GL_c_BIAS, and clamped to the range
[0,1] (see glPixelTransfer).

According to this definition, there is no savings of texture memory
when compared with using GL_RGBA. Is this accurate?

I am especially interested if this is accurate on the Matrox G400 and
Nvidia geForce256 chip sets.

Any ideas?

Thanks,
Harley Blumenfeld

2

Harley,

NVIDIA hardware does save memory when using GL_ALPHA, GL_LUMINANCE, GL_INTENSITY, and GL_LUMINANCE_ALPHA internalFormat.

The description in the man page is about how a format parameter is converted to an RGBA. This may be necessary, for example, if the internalFormat is GL_RGBA but the format is GL_ALPHA.

Hope this helps…
Cass

[This message has been edited by cass (edited 07-05-2000).]