[Mesa-dev] I need some help about OGL extensions (Was Re: About merging pipe-video to master)

[Christian König]

Am Mittwoch, den 13.07.2011, 18:29 +0200 schrieb Christoph Bumiller:
> > As the name of this (well documented) hardware flag already implies: It
> > just disabled clamping the output of the fragment shader BEFORE blending
> > to a normalized buffer.
> >From ARB_color_buffer_float:
> 
> 3. 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.
> 
> So you want to clamp the colour only before blending but NOT after ?
No, just the other way around. I want to DISABLE clamping before
blending, clamping after the blending equation would occur anyway
because of format conversion. One example:

My fragment shader outputs the value -0.5, the color buffer contains the
value 0.75 and the blending function is ADD.

For a float buffer (with clamping disabled) I get the expected value of
0.25, but for an unsigned normalized buffer I still get 0.75, because
the input value -0.5 gets clamped to [0,1] BEFORE blending occurs.

My expectation was that disabling fragment_color_clamp also disables the
clamping before the blending equation occurs, because it doesn't make so
much sense to me to disable the output clamping, and clamping the value
than anyway because of blending. But that's not the way
"ARB_color_buffer_float" defines it, and also how piglit tests it.

I have a working workaround, but it is something around 20% slower than
doing it directly with the blender and I would really like to avoid that
overhead.

Regards,
Christian.