glReadPixels gives only black...

[Nick Nougat]

Hi!
I want to make screenshots in my engine and I'm trying this via glReadPixels. The thing is, that when I call the function, my buffer only gets filled black pixels. I tried glReadBuffer with back and front, as well as calling the function at different rendering stages (i.e. after or before calling swapbuffers) - to no avail.
I use this:
BYTE *pData = new BYTE[m_cx * m_cy * 3];
glReadPixels(0, m_cy-1, m_cx, m_cy, GL_RGB, GL_UNSIGNED_BYTE, pData);

with m_cx and m_cy being the window sizes.

What am I doing wrong?
Thanks,
Nick

[SThomas]

try

glReadPixels(0, 0, m_cx, m_cy, GL_RGB, GL_UNSIGNED_BYTE, pData);

you should be reading from the front buffer.

[Nick Nougat]

Thanks! That did the trick.
Very strange - I had systematically tried all possible combinations, including your proposition, before posting but I still got a blank screen - I guess I messed something different up while testing. *shrug*
Anyway, it works now...

So the "lower left corner" in the red book is to be interpreted thus, that the lower left corner of the image is (0,0) - the origin of the picture - and, in effect, if I read any picture data from the framebuffer, I get it upside down?

Thanks
Nick

[MickeyMouse]

Probably you're running in software, not hardware.
Test the result of GL_RENDERER, "Generic..." means you're running in software.