[Bug 34052] The Witcher: Character models are sometimes invisible

[wine-bugs at winehq.org]

https://bugs.winehq.org/show_bug.cgi?id=34052

--- Comment #62 from Arthur Huillet <arthur.huillet at free.fr> ---
Reproducing here my reply from
https://devtalk.nvidia.com/default/topic/773623/linux/character-models-are-sometimes-invisible-in-the-witcher-game/

---

We have investigated this issue and found out the cause of the problem.
An OpenGL vertex shader (generated by Wine based on a D3D shader in the
Witcher) is created with the following uniform:
uniform vec4 vs_c[256];

The shader then dereferences vs_c[x], with "x" a dynamically computed value
that sometimes exceeds 255. Reading outside of the bounds of a uniform array in
GLSL is undefined behavior, which means that the implementation is allowed to
do whatever it likes. On NVIDIA GPUs, the value returned will sometimes be 0.0,
and sometimes different.
Since this value is used to warp the bones of the characters (to animate them),
having an unpredictable value can result in some vertices being offscreen and
even some geometry disappearing entirely.

The problem is not a NVIDIA bug, since the application is asking the GPU to do
something that is undefined in the specification. We have been working with
Wine on the investigation of this issue, and have also communicated our
findings to the developers of the game.
A patch on the Wine bug tracker appears to work around the issue.

---


Arthur Huillet
NVIDIA Linux Graphics

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