[Bug 93594] Flickering Shadows in The Talos Principle

bugzilla-daemon at freedesktop.org bugzilla-daemon at freedesktop.org
Fri Feb 19 10:17:18 UTC 2016


--- Comment #18 from Kai <kai at dev.carbon-project.org> ---
(In reply to Marek Olšák from comment #10)
> (In reply to Michel Dänzer from comment #9)
> > Note that AFAIK using things like derivatives in non-uniform control flow
> > isn't supported by GLSL. What's the original GLSL shader?
> True.
> The GLSL shader is using discard followed by fwidth. This is undefined
> behavior.
> Therefore, it's an application bug.
> The easy workaround would be to disable register allocation in st/mesa to
> get a quasi-SSA form and trivially move KILL_IF to the end of the shader if
> the app is detected to be Talos Principle.

I forwarded this information to Croteam and Dean Sekulic answered:
> Oh, just found out that discard instruction is treated as dynamic flow control
> for some reason... :/
> Will fix this!
> On 19-02-16 8:21, Dean Sekulic wrote:
>> I can't find any reference anywhere that doing fwidth() (or ddx/ddy for
>> that matter) after discard should be undefined behavior... :/
>> GPUs should handle that case completely fine.
>> Can you elaborate, please?

Looks like there might be an update for The Talos Principle which will address

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