Bevy input lag vs egui for GUIs

[DKolter]

Yesterday I tried to port a tile editor GUI from egui to bevy using the bevy egui plugin as a replacement, because I hoped to get a performance boost. The GPU usage is significantly less with bevy, as the editor objects aren't recreated every frame, but when I drag tiles across the screen, there is a pretty large input lag, that isn't pleasant to look at and gives the user the impression that the app is slow and unresponsive. In the video below i removed the gui part to make sure it doesn't cause the problem. I have found a pull request (#6503) that is merged into main, but switching to that branch doesn't fix the problem. Is there any way this can be reduced or fixed completely or did I miss something about the main branch?

app.2023-02-10.11-07-22.mp4

[nicopap]

There is several things you can do to reduce lag:

• Use bevy framepace It's basically a free latency reduction
• Order your systems carefully so that the system responsible for moving the piece runs after the system to select the piece etc. This is very important.

Setting component values

Another source of latency is inserting/removing components. If you are updating a component's value, you should be using a Query<&mut MyComponent> instead of commands.insert(MyComponent).

Commands are applied at the end of a stage, so if you have a system that sets MyComponent, and another that read its value in the same stage, the reading system will only be able to read the value of MyComponent from last frame.

System ordering

For example, your systems may look like

app
  .add_system(set_velocity_from_mouse_delta)
  .add_system(update_transform);

This runs, but if set_velocity_from_mouse_delta runs after update_transform. At any given frame, the input of the last frame will be used for the transform.

You need to make sure set_velocity_from_mouse_delta runs before update_transform, this way:

app
  .add_system(set_velocity_from_mouse_delta.before(update_transform))
  .add_system(update_transform);

[DKolter]

Ohhh, thank you very much, i see that there is a lot of room for improvement in my code now. The system ordering is a point i have completely missed, and i am using the commands.insert function every frame for updating the mouse last position. I will look into framepace as well, thank you very much for all these tips and the quick help 💯