[Mesa-dev] [PATCH] glsl: always do sqrt(abs()) and inversesqrt(abs())

[Erik Faye-Lund]

On Wed, Jan 11, 2017 at 4:14 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> On 11.01.2017 13:17, Marek Olšák wrote:
>>
>> On Tue, Jan 10, 2017 at 6:48 PM, Jason Ekstrand <jason at jlekstrand.net>
>> wrote:
>>>
>>> I'll be honest, I'm not a fan... Given that D3D10 has one defined
>>> behavior,
>>> D3D9 has another, and GL doesn't specify, I don't really think we should
>>> be
>>> making a global change to all drivers to do the D3D9 behavior just to fix
>>> one app.  Sure, other apps probably have the same bug, but are we going
>>> to
>>> have apps that expect the D3D10 behavior that we've now explicitly made
>>> not
>>> work?
>>>
>>> If we're going to hack around an app bug, I would really rather see it
>>> behind a driconf option rather than a global change to driver behavior.
>>> Even better, it'd be cool if we could see the app get fixed. (Yes, I know
>>> that's not likely).
>>
>>
>> I think we are not in a position to refuse this workaround, or put
>> more precisely, to have a different behavior from everybody else. By
>> "we", I mean i965, radeonsi, svga. All closed drivers use abs. Many
>> Mesa drivers also use abs internally (r300, r600, nv30, nv50/nvc0).
>> This is not really a workaround for a specific application, even
>> though it's strongly motivated by that. It's a fix to align the few
>> remaining drivers with all others.
>>
>> We talked with the publisher about this a very long time ago. While I
>> don't remember the details (Nicolai?), I think they refused to fix it
>> because radeonsi appeared to be the only driver not doing abs.
>
>
> If I remember correctly, it wasn't so much a refusal as a lack of
> follow-through. They even had an option in their framework to add the
> abs(...) when translating shaders, but somehow didn't turn it on
> unconditionally for some reason...

VP even says so here:
https://github.com/virtual-programming/specops-linux/issues/20

They recommend against patching mesa to do abs, though.