The new ClearPass leads to flickering when fps is less than 60

[folke]

Bevy version

Latest git

Operating system & version

Fedora

What you did

cargo run --example 3d_scene_pipelined

What you expected to happen

The scene should render without flickering.

What actually happened

Sometimes the scene stays blank. Other times, it flickers between the correct frame and a blank frame.

This seems to be caused by the changes in #3209

Additional information

The scene runs at 40fps on my laptop. Most time seems to be spent in light clustering:

• update_clusters: 5ms
• assign_lights_to_clusters: 9ms
• prepare_clusters: 3ms

Improving the performance of the clustering might fix this issue, but the real issue seems to be that clear happens every frame, while new frames might not be ready yet, leading to the blank frames.

If I go to a commit right before the ClearPass, then the scene renders as expected.

[mockersf]

I tried reproducing by adding thread::sleep in a user system or in system assign_lights_to_clusters, lowered FPS to 15 but no flickering issues

[folke]

The shadow pass node seems to take 4ms and runs after Clear and before Main, so maybe that's why I'm seeing flickering? And that's why it worked before?

Try adding a sleep in the shadow pass.

[folke]

The pbr_pipelined example normally runs as it should, but if I add a thread sleep of 16ms inside ShadowPassNode, then I get a black screen (in windowed mode). Whenever I move the mouse, I get flickering frames coming through.

What's weird though is that when I maximize the screen, things seem to work as expected.

Edit: still flickering on fullscreen, but much less than in windowed mode.

I have these issues on Linux both for X11 and Wayland.

[mockersf]

Can't reproduce on my mac with a sleep inside ShadowPassNode.

It may be a linux issue, or on how your card driver handles the command. Could you add the info from your gpu if someone can reproduce?

[folke]

This is on a Dell Xps 9310

AdapterInfo {
  name: "Intel(R) Xe Graphics (TGL GT2)",
  vendor: 32902,
  device: 39497,
  device_type: IntegratedGpu,
  backend: Vulkan
}

It might be a graphics card issue, but if I disable the ClearPass and do the clear as before inside MainPass3d, then it works again without flickering.

[folke]

I can reproduce the issue on my end with the following minimal example. The screen flickers very rapidly between a grey frame and a black window.

use bevy::{
    prelude::{App, Commands},
    render2::camera::PerspectiveCameraBundle,
    utils::Duration,
    PipelinedDefaultPlugins,
};

fn main() {
    App::new()
        .add_plugins(PipelinedDefaultPlugins)
        .add_startup_system(setup)
        .add_system(sleep)
        .run();
}

fn setup(mut commands: Commands) {
    commands.spawn_bundle(PerspectiveCameraBundle::default());
}

fn sleep() {
    std::thread::sleep(Duration::from_millis(20));
}

[zamazan4ik]

Do we need to test it on additional environments? I have:

• Windows 10, Nvidia videocard, proprietary driver
• Fedora 35, Tiger Lake Intel videocard
• Fedora 35, Ivy bridge Intel videocard
• Fedora 35, Nvidia videocard, proprietary driver

[folke]

Did a lot more testing and I think I'm hitting three different issues, but they give similar results:

• on Wayland, my minimal example above shows the issue. On X11 however, it works as expected. On Wayland the issue also disappears if I maximize the window. Might be fixed by an upstream wgpu PR (GLES Wayland: properly resize on configure (#2094) gfx-rs/wgpu#2284)
• on X11 I still have flickering for the 3d_scene_pipelined. That is casused by the 4ms delay between clear, shadow and mainpass. If I disable the Shadow code, the 3d scene no longer flickers. I can also trigger the issue by adding a tiny sleep inside shadow with the minimal example. This used to be fine before the ClearPass changes.
• on both Wayland and X11 in one of my projects I get a lot of flickering (which is the reason I started looking into this). But that seeems to be caused by Call to Surface::get_current_texture() seems to delay by one frame gfx-rs/wgpu#2269. I added debug prints for the duration and flickering only happens when it's over 16ms. The other two issues above are unrelated to this.

[folke]

@zamazan4ik would be great if you can confirm seeing the same behavior on your Fedora. (I'm testing on Fedora 35 with a Tiger Lake integrated GPU) Might also be good to check on Windows for sure.