Problems with PBOs and frame grabbing

I wrote code for grabbing some screenshots using a PBO. it works well - but only with certain window sizes. 800x600 works fine. With 961x713, the PBO contains only black pixels. 1268 x 942 works fine again.

What is going on here? Are there any size restrictions I am unaware of? Already tried 962x714, it gives me a black shot too, so the restriction doesnt seem to be even-sizes-only.

Did you try to adjust pack alignment (glPixelStorei call)? Post some code if this hint doesnt help.

Hi there.

The specs mention glPixelStore, so my guess is the problem is 4 byte (OpenGL’s default) padding.
800 % 4 = 0 and 1268 % 4 = 0 !

Try calling glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
before your PBO operations.

Pixel Buffer Objects:


Btw, the other glPixelStore options might let you download only a subrectangle of the texture, in case you need such functionality too.

Ooops yooyo was faster…
Hope it helps anyway :slight_smile:

Just a quick clarification…From what I understood, you need to call glPixelStore once, before you start playing arround with your TexImage calls or PBO calls. You do NOT need to call it everytime you bind a texture or a buffer, right?

So, I’m guessing if this only affects one particular area of your code…it’s probably best to have something like:

glPixelStorei(GL_UNPACK_ALIGNMENT, 1);

lots of code here that does PBO operations and
glBindTexture, glTexSubImage2D, etc.

// set it back to it’s default value so you don’t mess other things up.
glPixelStorei(GL_UNPACK_ALIGNMENT, 4);

Am I right in assuming this? (or do I need to call it more often?).


Yes, you are right.
According to the specs, the GL_UNPACK_ALIGNMENT flag is initialized to 4.

But as said before, you might want to change the GL_UNPACK_ROW_LENGTH, GL_UNPACK_SKIP_ROWS, GL_UNPACK_SKIP_PIXELS values to only download/upload a subrectangle of a given texture/image.

Therefore, i would advise you to call:
glPushClientAttrib(GL_CLIENT_PIXEL_STORE_BIT); // Save state
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
// Maybe some other glPixelStorei calls
glPopClientAttrib(); // Back to the previous state

Rather than
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glPixelStorei(GL_UNPACK_ALIGNMENT, 4);

sure thing…pushing and popping is much better…
But still, do I need to call all the glPixelStore stuff ONCE, or do I need to call them everytime I bind a texture/buffer?

Yes, you can call it once.

It is a (client) state, therefore the ‘value’ remains as it is unless another glPixelStore call on the same flag is made.

If you’re sure you only want 1 unpack/pack alignment, then you can call glPixelStore once for all before any operation affected by the related states.