GLUT_ALPHA mode very slow in windowed

OpenGL
1

Trying to make use of alpha buffer for dst blends, I added GLUT_ALPHA to window initialization.
ie:
glutInitDisplayMode( GLUT_ALPHA | GLUT_DEPTH | GLUT_RGB | GLUT_DOUBLE );

this however resulted into the application performing extremely slow. And eventually my querys for GL_Extensions all fail. had to bypass to make the app work at all, but it felt like software mode
used a geforce4 and w2k to test
GLUT 3.7.6

Anyone had similar experiences with ALPHA buffer ? or any idea what could be going wrong.
I tried other buffers (accum, stencil) and those worked fine

some more testing: ALPHA works well if running in fullscreen-gamemode, but not if running windowed.

thx for your time

2

okay could find the issue (window bpp was 16bit)
but that would also mean that its impossible to get an alpha buffer, in windowed mode, if user has it set to 16bits ?

3

Yes, this is a hardware issue. Your card only supports destination alpha in “32 BPP” mode (24 bits RGB, 8 bits Alpha). This is a common limitation for “older” cards (pre GF 4). You will see the same issue for fullscreen displays in 16 BPP too, of course.

What you are experiencing is that the HW driver fails (can’t support your requested buffer configuration), so Windows falls back to sofware rendering with the default MS renderer (hence the missing extensions and lower performance).

4

thanks for that info