[Mesa-dev] [PATCH] nir/algebraic: Remove imprecise flog2 optimizations

[Jason Ekstrand]

On Mon, Jun 20, 2016 at 11:08 AM, Matt Turner <mattst88 at gmail.com> wrote:

> On Sat, Jun 18, 2016 at 12:42 PM, Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> > While mathematically correct, these two optimizations result in an
> > expression with substantially lower precision than the original.  For any
> > positive finite floating-point value, log2(x) is well-defined and finite.
> > More precisely, it is in the range [-150, 150] so any sum of logarithms
> > log2(a) + log2(b) is also well-defined and finite as long as a and b are
> > both positive and finite.  However, if a and b are either very small or
> > very large, their product may get flushed to infinity or zero causing
> > log2(a * b) to be nowhere close to log2(a) + log2(b).
> >
> > This imprecision was causing incorrect rendering in Talos Principal
> because
> > part of its HDR rendering process involves doing 8 texture operations,
> > clamping the result to [0, 65000], taking a dot-product with a constant,
> > and then taking the log2.  This is done 6 or 8 times and summed to
> produce
> > the final result which is written to a red texture.  In cases where you
> > have a region of the screen that is very dark, it can end up getting a
> > result value of -inf which is not what is intended.
> >
> > Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96425
> > Cc: Matt Turner <mattst88 at gmail.com>
>
> Ah, that's too bad.
>

Yeah, I liked those too. :(


> Reviewed-by: Matt Turner <mattst88 at gmail.com>
>

Thanks!
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.freedesktop.org/archives/mesa-dev/attachments/20160620/b304fb97/attachment-0001.html>