Problem with accum buffer

In my program, I want to use the accumulation buffer. It’s the first time I use it, so I first tested it with the code :
and display the result.
The display is OK when I comment these two lines, but when I use them, all is stretched, as if all lines displayed were the same as the center line. What does this mean ?
Thanks for tour answers.

Make sure you request a pixel format that has an accumulation buffer. Could be that you don’t have one.

I tried tu use glGet, with argument GL_ACCUM_ALPHA_BITS, GL_ACCUM_RED_BITS…
All calls return a value of 16, so it seems that the pixel format has an accumulation buffer.

Nobody has an answer to my question ?

…nobody uses the accumulation buffer since it’s almost never hardware accelerated (which is a shame IMO)

I believe it’s implemented by reading back an on-card buffer to system memory, and then operated on in software, and finally written back to the gfx card. (=rendering done in hardware, but accum buffer ops in software => slow!) [correct me if I’m wrong]

It’s ok, it was finally a problem of drivers. I put the last ATI drivers and all is correctly displayed, without changing a line in my program.