Objective

• Depth texture downsampling in https://github.com/bevyengine/bevy/blob/main/crates/bevy_core_pipeline/src/experimental/mip_generation/downsample_depth.wgsl uses textureLoad on a depth texture.
• This unfortunately does not seem to be supported on GL platforms, see: Unable to sample depth texture gfx-rs/wgpu#4524. While this could be theoretically worked around, it seems fairly difficult - and the out of the box experience for bevy with the default plugins means that users have to manually disable GPU preprocessing if using the GL backend.
• Fixes Cannot run any example in 0.16.0 #18932

Solution

• There appears to be a flag already for this: DownlevelFlags::DEPTH_TEXTURE_AND_BUFFER_COPIES which is not set for the correct platforms (WebGL/GLES). It was proposed (at least I believe?) Unable to sample depth texture gfx-rs/wgpu#4524 (comment) to expand the definition of this downlevel flag to include this "textureLoad on depth texture" invalid usage. I've posted my support there, but for now it seems useful to unblock GL.

Testing

• At least on my machine, and with additional patches surrounding Wayland display management and WGPU 26, this limits me to GpuPreprocessingMode::PreprocessingOnly which runs just fine. I will attempt to test on additional platforms soon but would appreciate any help here (I've got a Vivante, PowerVR TI GPU).