Clamping in RGBA32F FBOs

(This is on MacOSX)

On my 8600M, it seems like the values in my floating-point FBOs
are being clamped (I want them unclamped), both in fixed and
shader pipelines. As verification, I read back the buffers and
checked their values and those that are meant to be >1.0 are
stuck at 1.0. (I do use RGBA32F internal textures)

This list entry also contradicts my experience, saying you can
get unclamped behaviour (which is the default for FP32) on a

As a side question, isn’t glClampColorARB supposed to be part of
OpenGL 2.0? Why isn’t it implemented on OSX then?

Lastly, I quote the specs from:

  1. How does the clamping control affect the blending equation?

RESOLVED: For fixed-point color buffers, the inputs and the
result of the blending equation are clamped. For
floating-point color buffers, no clamping occurs.


I’m struggling to get my FBO to return >1.0 values.
What do I have to do to make it work?


Hi again,
please don’t reply I got it working!


So what was the problem? It looked like you were doing everything right.

See my other thread here “Read 2D texture pixel values”.

Basically, after getting the FBO to work I mistakenly did,

glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, mouseX, mouseY, 1, 1, 0);

instead of

glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGB32F_ARB, mouseX, mouseY, 1, 1, 0);