Sub View only projects correctly when aspect ratio of size and full_size match

[m-edlund]

Bevy version

Main branch (c841dd9)

What you did

I attempted to use the newly added sub_camera_view API (#15537 ) to create a subview that overlays a shader effect onto a part of the full image. However this only works when the size and full_size of the sub_camera_view have the same aspect ratio.

I made an example to illustrate this effect in a fork

What went wrong

Regardless of the aspect ratios of size and full_size, the shader effect should cleanly overlap the image of the main camera.

Instead in both orthographic and perspective projection the projection of the sub view is skewed when the aspect ratio of size and full_size differ.

Additional information

Perspective (overlay working correctly when aspect ratio is the same):

Perspective (overlay is skewed when aspect ratios differ):

Analogous in orthographic projection.

To run the example yourself, check out the branch and run cargo run --example camera_sub_view.

Press P to switch projection and A to switch aspect ratios.

Likely cause

The most likely reason is that I made a mistake in the projection calculations here and here.

[m-edlund]

I will attempt to find the fix for this issue myself in the coming days. However I'm very much an amateur when it comes to projection calculations, so if anyone with more experience in the area would like to take a look, the help would be greatly appreciated.