Color buffer changes even after disable writing

OpenGL OpenGL: Advanced Coding
1

Hi,

I have very unusual (to me) problem with color buffer.
I have 32 bit color buffer and I clear it and set the forth byte to 1. Then I disable writing to it by
glColorMask( GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE );

Then I draw a simple geometry. So I do change the depth buffer but I don’t want to change the color buffer in any way.
After this operation I read back the color buffer and the forth byte in the color buffer where geometry changed the depth buffer has changed.

Is that the expected/correct behavior? I would expect that there will be no writing to the buffer if I explicitly disable that operation.

Just to clarify - this happens on MS implementation of OpenGL.

2

Doesn’t seem right to me, either:

glColorMask specifies whether the individual color components in the frame buffer can or cannot be written. If red is GL_FALSE, for example, no change is made to the red component of any pixel in any of the color buffers, regardless of the drawing operation attempted.

So, it sounds like a bug to me.
However, maybe you can work around it. Are you using blending? Fullscreen antialiasing?

3

Actually I’m using this 4rth byte to know which pixel in color buffer has changed during drawing.

The problem appears when I draw in the depth buffer just to create some effects latter on with shadows.

Unfortunately for me until now I didn’t find any workaround of this problem.

But it is strange that nobady found this before and force MS to fix it.

I suspect I cannot expect MS to fix it now.

4

AFAIK, MS OpenGL implementation doesn’t support framebuffer with alpha channel.

yooyo