Alpha channel affectation

Hi,

I need to affect, change or clear the Alpha componant.
I know it works cause I can see the result on the screen with BLEND but why the glReadPixels always give me a value equal to 255 !?!?

		Vinnie

what format do you use in the glreadpixel function ???

I’m using the format GL_RGBA

Looks like similar problem :
glReadPixels on GeForce2 .

> I know it works cause I can see the result on the screen with BLEND

??? What do you mean?
Alpha blending works without alpha channel in the color buffer (if you don’t use _DST_ALPHA).

>> I know it works cause I can see the result >>on the screen with BLEND

>??? What do you mean?

I made an operation with glblendfunc using the alpha componant and it works.
I just can’t read something different than 255 with glReadPixels

I’m on a SGI o2
Vinnie

Does your pixel format include a destination alpha buffer? If not, then obviously, the alpha is not being read back because it doesn’t exist.