Moore Threads: unwanted SSAO drawn on tiles (without read-only depth buffer)

[illwieckz]

The glitch disappears with r_readonlyDepthBuffer 2.

Maybe it's not a bug since it disappears with r_readonlyDepthBuffer 2 but with default values SSAO is computed on tiles filling the whole screen. I don't know what's those tiles are, if they are our own tiles, or some driver's internal tiles.

The glitch is not visible on static scenes, but appears when moving forward, and disappears when slowing down and stopping. So maybe it's also related to motion blur.

Moore Threads is a Chinese vendor of gaming GPUs similar to AMD/Intel/Nvidia targeted at the gaming PC market, with drivers for Linux and Windows.

This was reproduced on MTT S80:

• https://www.techpowerup.com/gpu-specs/mtt-s80.c4259
• https://en.mthreads.com/product/S80

Bug or not, this thread is used to document the issue.

[illwieckz]

GPU: MTT S80
CPU: AMD Ryzen 3 3200G (amd64)
System: Ubuntu 24.04.3
Kernel: Linux 6.8.0-100-generic
Kernel graphics driver: musa 3.1.0 (mtgpu)
OpenGL graphics driver: musa 3.1.0 (mtgpu)

Excerpt from gfxinfo:

GL_VENDOR: Moore Threads
GL_RENDERER: MTT S80
GL_VERSION: 3.2 build 20250122_master-227-gdf7bc68e
GL_SHADING_LANGUAGE_VERSION: 4.30

[slipher]

Having read the article you found, we can surmise that the artifacts happen in places where the SSAO shader does edge detection on pixels that belong to two different tiles. The depth buffer reads are probably coherent when reading from pixels that belong to the same tile as the one being rendered, but out of sync when reading from an adjacent tile. If the out-of-sync reads are getting data from a previous frame, that would explain why it mostly happens while moving: if you do not move and the scene does not change, the stale data is the same as the current frame's data.