Shenmue II: Flickering sky

[KreeseXD]

Title

https://xemu.app/titles/4d530032/#Shenmue-II

NTSC MS-050 v1.00

Bug Description

Sky seems to be flickering weird patterns. Tested this with xemu version 0.8.29 and didn't have the issue on that build. Issue came up on version 0.8.30. Issue is still present on xemu version 0.8.40

xemu 0.8.29 (sky looks normal)

Recording.2025-03-20.183936.mp4

xemu 0.8.30 (sky bugging out)

Recording.2025-03-20.184113.mp4

xemu 0.8.40
#988 (comment)

Expected Behavior

Sky should not be flickering, should look like how xemu 0.8.29 looked

xemu Version

0.8.29 (good sky)
0.8.30 (bad sky)
0.8.40 (bad sky)

System Information

OS: Windows 11
CPU: 13th Gen Intel(R) Core(TM) i9-13980HX
GPU: NVIDIA GeForce RTX 4080 Laptop GPU/PCIe/SSE2
GPU Driver: 4.0.0 NVIDIA 566.14

Additional Context

Issue seems to have come up on build 0.8.30

[coldhex]

The game uses z-buffering and draws the sky using two different meshes. The first one draws the blue sky background and the second draws the clouds on top. For both meshes the (programmable) vertex shader outputs the exact z-value 16777213 for all vertices and so (with LEQUAL depth comparison) clouds shouldn't flicker. However, the w-buffering feature multiplies by w (and OpenGL/Vulkan then divides by w, or multiplies by the reciprocal of w) in order to fix other problems, and that introduces floating point round-off errors to the 16777213 values. The visible effect is z-fighting.

This can be fixed by custom interpolation of z-values and writing to gl_FragDepth for z-buffering in the fragment shader (which is already done with w-buffering). Also then requires implementing glPolygonOffset functionality because the depth value from the graphics pipeline is overridden.

[Triticum0]

Also affect Motor Trend Presents Lotus Challenge https://xemu.app/titles/58490005/#Motor-Trend-Presents-Lotus-Challenge