Camera spins unevenly at <=100 FPS rendering speed

[roym899]

Once the displayed CPU time reaches ~= 10ms, the camera spins at uneven speed for me.

uneven_spin.mp4

Rerun Version

rerun_py 0.12.0 [rustc 1.74.0 (79e9716c9 2023-11-13), LLVM 17.0.4] x86_64-unknown-linux-gnu release-0.12.0 afa112e, built 2024-01-09T17:56:39Z

Desktop (please complete the following information):

• Ubuntu 22.04

[roym899]

Maybe also adding here that the same scene didn't have this issue back in 0.8 (or at least much less) and that the spinning speed has increased noticeably since then.

080.mp4

Same for 0.9. Faster rendering (2x faster than now), and consistent spinning speed.

[Wumpf]

@emilk not sure this is a "quick issue". The code hat does the spinning looks like this:

            orbit_eye.rotate(egui::vec2(
                -response.ctx.input(|i| i.stable_dt).at_most(0.1) * 150.0,
                0.0,
            ));

so if high framerates cause issues here it implies that stable_dt isn't working properly

[roym899]

I tried out some more versions and it seems like this has changed from 0.11 -> 0.12.

Each frame takes ~2x as long, the rotation speed is higher, and the speed seems to be inconsistent.

[Wumpf]

@roym899 and me had a look on his laptop where it's reportedly not as bad as on the desktop - we'll check on puffin traces of the desktop machine later.

It seems that we slightly regressed performance in 0.12 it's hard to pinpoint where exactly though. However, (likely due to multithreading!) we have much more varying frame time lengths and I think this doesn't bode well with stable_dt. The visual effect is in fact the camera spin seems to speed up and slow down at random, giving a jittered impression.
@emilk how is stable_dt measured and is that even what we should be using here? Feels like we should just use a regular frame dt.

[emilk]

The docs explains it pretty well: https://docs.rs/egui/latest/egui/struct.InputState.html#structfield.stable_dt

Feel free to try out some other dt

[emilk]

Ooh, it seems stable_dt is broken in egui since the big multi-viewport refactor! Will fix :)

• stable_dt is broken emilk/egui#3830

[emilk]

• Fix coming in Fix stable_dt emilk/egui#3832