Bevy lets you use ScreenshotManager even if it won't work

[banool]

Bevy version

0.12.1

Relevant system information

If you cannot get Bevy to build or run on your machine, please include:

• the Rust version you're using (you can get this by running cargo --version)
  • cargo 1.75.0 (1d8b05cdd 2023-11-20)
• the operating system or browser used, including its version
  • MacOS 14.2.1 (M1 ARM)

What you did

The docs for take_screenshot say this:

The given callback will eventually be called on one of the AsyncComputeTaskPool threads.

When I call take_screenshot it returns happily but the callback is never called. Same issue with save_screenshot_to_disk.

I'm setting up my systems with DefaultPlugins.build().disable::<WinitPlugin>(). I'm guessing that whatever is meant to come along later and make the screenshot happen never happens because I don't have a window.

What went wrong

While this is related to taking a screenshot in headless mode (#7163, #22), my main concern is with the API. If it's true that take_screenshot will never work without a window, one of the following should be true:

• Calling screenshot_manager.take_screenshot should return an error immediately indicating that screenshots aren't possible bc x y z other plugins are missing.
• It is impossible to get mut screenshot_manager: ResMut<ScreenshotManager> in the first place, I suppose bc the plugin is removed from the default set automatically if there is now window. Or maybe just because there is no event loop.

Additional information

I understand that without WinitPlugin there is no event loop and the update only runs once, so I wonder if there is a way to force the callback to be called later. It seems that just dropping the App instance doesn't make it happen. I guess it depends on whether taking a screenshot is expected to work without a window or not, and therefore if it relates to the window or the event loop.

[rparrett]

Yeah, we could do better here. I think that either of your suggestions could be made to work. The second seems like it would be a bit involved. Another option might be to add a warning around here:

bevy/crates/bevy_render/src/view/window/mod.rs

Lines 189 to 191 in 4695b82

	if let Some(window) = extracted_windows.get_mut(&window) {
	window.screenshot_func = Some(screenshot_func);
	}

Out of curiosity, what are you using as take_screenshot's first parameter? Could the docs be made clearer that this needs a Window entity to work?

#22 is still open for a reason. Totally headless screenshots aren't possible yet. Even Bevy's CI uses a slightly janky setup of using a virtual framebuffer, initiating a screenshot, and just waiting a few seconds and hoping it was saved to the disk in time before exiting.

I don't believe there are any technical blockers. Just need a motivated person to make it work.

There are a few third party crates that also deal with screenshotting and one or more of them may be able to do this. I haven't looked in a while.

[banool]

Out of curiosity, what are you using as take_screenshot's first parameter? Could the docs be made clearer that this needs a Window entity to work?

I'm using main_window.single() from accepting this to the system: main_window: Query<Entity, With<PrimaryWindow>>. Surprisingly without Winit this still returns a window, so perhaps that's sort of the root cause issue here.

#22 is still open for a reason

Yeah for sure that's cool, I'm trying to do something similar for now too, or just run the API with access to a windowing system.

[rparrett]

I'm using main_window.single() from accepting this to the system: main_window: Query<Entity, With>. Surprisingly without Winit this still returns a Window, so perhaps that's sort of the root cause issue here.

Ah, right. Thanks. WindowPlugin creates the PrimaryWindow automatically, and it seems like a web of various systems in bevy_winit and bevy_render are responsible for creating/associating "actual windows" and surfaces.