G200 & OpenGL problem

[zroy]

Now we are developing our application using OpenGL on Window NT 4.0. We know that G200, G400 don't support OpenGL acceleration on NT4.0, we don't expect that it can accelerate the OpenGL rendering. But we do want to get the correct result image rendered.
Now we meet some problem as follow:
(1) using glReadPixel, we can't get the alpha value(the value is always 255 even though the area is transparent), when we use other card like ELSA Sernergy II or 3DLabs Oxygen GVX1, they all can get the alpha value correctly. Are there any setting should we set?
(2) texture mapping is very slow.

[Serge K]

> We know that G200, G400 don't support OpenGL acceleration on NT4.0, we don't expect that it can accelerate the OpenGL rendering.

This mean, that you are dealing with MS software-only OpenGL implementation.
And yes, texture mapping IS very slow in this case.

About alpha value:
If color buffer has no alpha channel OpenGL uses 1.0 (=255) as Destination_Alpha.
Are you sure, that you have alpha channel in color buffer? If you work in 16bpp (HiColor) - you have only RGB.

You can check it:

PIXELFORMATDESCRIPTOR pixelFormatDescriptor;
HDC hDC;
int nPixelFormat;

// current pixel format index
iPixelFormat = GetPixelFormat(hDC);

// description of pixel format
DescribePixelFormat(hDC, nPixelFormat, sizeof(PIXELFORMATDESCRIPTOR), &pixelFormatDescriptor);

if(!pixelFormatDescriptor.cAlphaBits)
{
// No Alpha channel
}