Captain's Log: Stardate 78280.5

The last couple of days I continued to work through the various bugs that I had noted with the new Filament renderer. In particular, what I was looking at was that on MacOS with the Metal backend, if I changed the render quality settings enough, eventually I could pretty reliably get it into a state where there were weird graphical glitches like the one below.

I was able to get other weird glitches as well, like everything becoming green-tinted. At first I thought there was some specific configuration that caused it, but eventually I realized that it was kind of random, and really difficult to pinpoint exactly how to reproduce it. Sometimes things would work fine until I moved the camera around a bit, and then it would go corrupt.

The first thing I tried was to strip down my renderer to the absolute basics, getting rid of any code that wasn't absolutely necessary to reproduce the issue. This was disconcerting, because I was able to remove nearly all the renderer code and still get glitches.

The next thing I tried was to reproduce the glitches in one of Filament's demo apps, the gltf_viewer. I tried and tried and nothing I did would cause the glitches.

I started looking for any difference between how the gltf_viewer and Anukari worked. The way we set up the CAMetalLayer is different, so I transplanted my CAMetalLayer code into gltf_viewer, and it worked just fine.

Next I started from the other direction: removing things from gltf_viewer one by one until something weird happened. It took quite a bit of trial and error, but I found that when I removed the code that configured the renderer to clear the pixels before a render pass, I could get weird corruption to happen.

This is pretty weird, since in a 3D renderer when you have a skybox, clearing the color channel is unnecessary since you'll be overwriting the entire thing with skybox pixels anyway. But I went back to Anukari and turned on clearing, and it completely fixed the problem. I don't know what the bug is in the Filament Metal backend exactly, but there's definitely something wrong with it.

Anyway, there are no more glitches in Anukari on MacOS, and I filed a bug for the Filament folks to see if they'd like to fix it: https://github.com/google/filament/issues/8229.