[Bug 46304] New: Fog handling seems to be off in Prince of Persia

https://bugs.winehq.org/show_bug.cgi?id=46304

Bug ID: 46304
           Summary: Fog handling seems to be off in Prince of Persia
           Product: Wine
           Version: 3.21
          Hardware: x86
                OS: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: directx-d3d
          Assignee: [e-mail]
          Reporter: [e-mail]
      Distribution: ---

Created attachment 63028
  --> https://bugs.winehq.org/attachment.cgi?id=63028
apitrace dump of frame that contains shader:

Hello, I came across this by trying out Prince of Persia with virgl ad a driver
and comparing it to the host. Initially I thought it was a virgl bug, but it
seems to be proper wine where things go wrong:

Here are my findings:

When I run "wine PrinceOfPersia.exe" on the host (r600) the game is playable
but when run with the virgl driver there is not contrast in the images.
However, a apitrace created with virgl as driver and replayed on host also
renders wrong, where a apitrace recorded on the host and run via virgl renders
correctly (only op-side down, because virgl doesn't support
GL_ARB_clip_control, but disabling this extension when recording the apitrace
makes it possible to get exactly the same output like when run  on the r600
host.

The difference between the two runs in the apitrace is that when run on r600 I
get a vertex shader that does:

R6.w = (R6.w * vs_c[20].z);
R6.w = (vs_c[20].z + -R6.w);
vs_out[11].x = (vs_c[16].y + -R6.w);
vs_out[10].xyzw = (R8.xyzw);
setup_vs_output(vs_out);

The same shader created via virgl does:

R6.w = (vs_c[16].y + -R6.w);
vs_out[11].x = (R6.w * vs_c[20].z);
vs_out[10].xyzw = (R8.xyzw);
setup_vs_output(vs_out);
ffp_varying_fogcoord = gl_Position.z;
gl_Position.y = gl_Position.y * pos_fixup.y;

The value ffp_varying_fogcoord is not assigned a value with the r600 version. 
However, the following fragment shader uses this value in both cases:

float fog = (ffp_fog.end - ffp_varying_fogcoord) * ffp_fog.scale;
ps_out[0].xyz = mix(ffp_fog.color.xyz, ps_out[0].xyz, clamp(fog, 0.0, 1.0));

In the r600 version ffp_fog.end=0 and ffp_fog.scale=-1, and I assume that this
results in  clamp(fog, 0.0, 1.0) returning 1.0, and in the scene the fog effect
seems to be generated differently. In the virgl version the values are
ffp_fog.end=1 and ffp_fog.scale=1, and I can only assume that gl_Position too
large so that clamp(fog, 0.0, 1.0) returns very small values (or even 0) and
the scene gets rendered in the fog colour overlayed by some post processing
effects.

I have not yet investigated why wine creates different code path for the two
drivers, I guess that it depends on some extension(s) that may or may not be
available.

The initial but report for virglrenderer can be found here: 
https://gitlab.freedesktop.org/virgl/virglrenderer/issues/62