[Bug 83510] Graphical glitches in Unreal Engine 4

[bugzilla-dae...@freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=83510

Marek Olšák  changed:

   What|Removed |Added

 Status|NEW |RESOLVED
 Resolution|--- |FIXED

--- Comment #16 from Marek Olšák  ---
(In reply to Henri Verbeet from comment #11)
> (In reply to Marek Olšák from comment #10)
> > However, Tonga and later hardware doesn't have the legacy dx9 opcodes for
> > MUL, MAD, RCP, RSQ, and LOG and they must be emulated with a comparison (or
> > V_CLASS_F32 that returns whether it's NaN/+-Inf/Denorm/+-0/etc) +
> > conditional assignment. It's better to do the conditional assignment after
> > every RCP than after every MUL.
> 
> Interesting, I wonder how that works out for Direct3D 9 applications. I
> don't suppose there's a global switch instead like (at least some) Intel and
> NVIDIA hardware has?

Yes, there is. See the commit.

Fixed by Mesa commit 2150db4d5daad3781876254d2b440367afd756cd. Closing.

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[Bug 83510] Graphical glitches in Unreal Engine 4

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https://bugs.freedesktop.org/show_bug.cgi?id=83510

--- Comment #15 from Daniel Scharrer  ---
FWIW, Shooter Game, Realistic Rendering Demo and Light Room Interior Day Demo
all look fine to me now with a Radeon HD 7950 and recent Mesa and LLVM
snapshots.

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[Bug 83510] Graphical glitches in Unreal Engine 4

[bugzilla-dae...@freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=83510

--- Comment #14 from Clément Guérin  ---
(In reply to Michel Dänzer from comment #13)
> Make sure you use the current versions of the demos. I was seeing the
> darkness in the first version of the Realistic Rendering demo, but it's
> fixed with the current version, and Shooter Game looks fine as well.

It's great if latest versions of Unreal Engine don't have this issue, but it
also means it's not exposing a potential driver bug anymore. This could show up
later :(

If anyone else can confirm it's fixed, I think we can close this bug report.

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[Bug 83510] Graphical glitches in Unreal Engine 4

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--- Comment #13 from Michel Dänzer  ---
(In reply to Clément Guérin from comment #12)
> Low lighting glitch is still here. "can be seen in the Realistic Rendering
> demo or the Shooter Game demo with the Sanctuary map"

Make sure you use the current versions of the demos. I was seeing the darkness
in the first version of the Realistic Rendering demo, but it's fixed with the
current version, and Shooter Game looks fine as well.

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[Bug 83510] Graphical glitches in Unreal Engine 4

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--- Comment #12 from Clément Guérin  ---
I just upgraded mesa-git with the NaN fix.

Black pixel and motion blur glitches are gone. Nice work!

Low lighting glitch is still here. "can be seen in the Realistic Rendering demo
or the Shooter Game demo with the Sanctuary map"

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[Bug 83510] Graphical glitches in Unreal Engine 4

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https://bugs.freedesktop.org/show_bug.cgi?id=83510

--- Comment #11 from Henri Verbeet  ---
(In reply to Marek Olšák from comment #10)
> However, Tonga and later hardware doesn't have the legacy dx9 opcodes for
> MUL, MAD, RCP, RSQ, and LOG and they must be emulated with a comparison (or
> V_CLASS_F32 that returns whether it's NaN/+-Inf/Denorm/+-0/etc) +
> conditional assignment. It's better to do the conditional assignment after
> every RCP than after every MUL.

Interesting, I wonder how that works out for Direct3D 9 applications. I don't
suppose there's a global switch instead like (at least some) Intel and NVIDIA
hardware has?

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[Bug 83510] Graphical glitches in Unreal Engine 4

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--- Comment #10 from Marek Olšák  ---
(In reply to Henri Verbeet from comment #9)
> (In reply to Marek Olšák from comment #8)
> > (In reply to Christoph Haag from comment #7)
> > > I think in the new "Lightroom Interior" the blackness glitch is visible 
> > > in a
> > > more interesting way:
> > > https://www.youtube.com/watch?v=R9N-srsF_Fg
> > 
> > I think I've tracked down what's causing the blackness in all Unreal Engine
> > demos: Division by zero.
> > 
> I didn't debug this of course, but it seems suspicious that this would
> depend on division be zero returning zero instead of infinity. I don't think
> any other driver for current hardware does that with GLSL. Are you sure
> there's not a subsequent multiplication by zero that depends on INF * 0
> returning 0 instead of NAN?

Yes, that's possible. However, some internal documents suggest that Catalyst
always uses the standard IEEE version of MUL. There are also alternative
opcodes for RSQ, RCP and LOG that don't generate NaNs and Infs, which can be
used instead. You can certainly choose which behavior you want on a
case-by-case basis. Sometimes you want 0 instead of NaN, other times you want 0
or FLT_MAX instead of Inf. The hardware has RSQ and RCP opcodes that do that.

However, Tonga and later hardware doesn't have the legacy dx9 opcodes for MUL,
MAD, RCP, RSQ, and LOG and they must be emulated with a comparison (or
V_CLASS_F32 that returns whether it's NaN/+-Inf/Denorm/+-0/etc) + conditional
assignment. It's better to do the conditional assignment after every RCP than
after every MUL.

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[Bug 83510] Graphical glitches in Unreal Engine 4

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--- Comment #9 from Henri Verbeet  ---
(In reply to Marek Olšák from comment #8)
> (In reply to Christoph Haag from comment #7)
> > I think in the new "Lightroom Interior" the blackness glitch is visible in a
> > more interesting way:
> > https://www.youtube.com/watch?v=R9N-srsF_Fg
> 
> I think I've tracked down what's causing the blackness in all Unreal Engine
> demos: Division by zero.
> 
I didn't debug this of course, but it seems suspicious that this would depend
on division be zero returning zero instead of infinity. I don't think any other
driver for current hardware does that with GLSL. Are you sure there's not a
subsequent multiplication by zero that depends on INF * 0 returning 0 instead
of NAN?

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[Bug 83510] Graphical glitches in Unreal Engine 4

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--- Comment #8 from Marek Olšák  ---
(In reply to Christoph Haag from comment #7)
> I think in the new "Lightroom Interior" the blackness glitch is visible in a
> more interesting way:
> https://www.youtube.com/watch?v=R9N-srsF_Fg

I think I've tracked down what's causing the blackness in all Unreal Engine
demos: Division by zero.

There are several options to fix this:

1) Use V_RCP_LEGACY_F32 instead of V_RCP_F32 (not available on Tonga and later
chips).

2) Emulate V_RCP_LEGACY_F32(x) using (x != 0 ? 1/x : 0). This can be done in
Mesa or LLVM. For LLVM, a new intrinsic rcp.legacy needs to be added.

3) Disable lowering DIV_TO_MUL_RCP in glsl-to-tgsi and see what happens. Not
sure if this will change anything, but at least LLVM doesn't need the lowering
and we might need to do the same workaround for TGSI_OPCODE_DIV.


I've implemented (2) in Mesa and it fixes the Lightroom Interior demo. I'm not
sure if that's the best solution.

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[Bug 83510] Graphical glitches in Unreal Engine 4

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--- Comment #7 from Christoph Haag  ---
I think in the new "Lightroom Interior" the blackness glitch is visible in a
more interesting way:
https://www.youtube.com/watch?v=R9N-srsF_Fg

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Christoph Haag  changed:

   What|Removed |Added

 CC||haagch at frickel.club

--- Comment #6 from Christoph Haag  ---
HD 7970M pitcairn here.

On an early unreal tournament pre-alpha build I had these artifacts very
extremely:
https://www.youtube.com/watch?v=mjbANgbIR6o

In a new build they are reduced, but it still happens:
https://www.youtube.com/watch?v=J4yT2cUXMcw

Here is another Video from another user with a HD 7790M:
https://www.youtube.com/watch?v=0-zlT93DdyU



When you are talking about motion blur on cameras, are you talking about the
glitch you can see in this video? https://www.youtube.com/watch?v=lkDLlCEBKPA
This happened a while ago and it does not happen anymore. I think at least for
me this is fixed.



But I think my Realistic Rendering demo is still too dark too.

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--- Comment #5 from Maciej  ---
Can confirm, same thing on Cape Verde (7770) with 3.16.1 and latest mesa on
Ubuntu 14.10.

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--- Comment #4 from Cl?ment Gu?rin  ---
Created attachment 105767
  --> https://bugs.freedesktop.org/attachment.cgi?id=105767&action=edit
Black pixel + low light glitch in Realistic Rendering demo

Should look like this: https://www.youtube.com/watch?v=3AAf_2vry8A

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--- Comment #3 from Cl?ment Gu?rin  ---
Created attachment 105766
  --> https://bugs.freedesktop.org/attachment.cgi?id=105766&action=edit
Black pixel glitch in Mobile Game demo

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--- Comment #2 from Cl?ment Gu?rin  ---
Created attachment 105765
  --> https://bugs.freedesktop.org/attachment.cgi?id=105765&action=edit
Black pixel glitch in Stylized demo

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--- Comment #1 from Cl?ment Gu?rin  ---
Created attachment 105764
  --> https://bugs.freedesktop.org/attachment.cgi?id=105764&action=edit
Motion blur glitch in Shooter Game

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