Render bevy images including render targets

[StarArawn]

We should be able to render bevy images using bevy vello and these should also include render targets.

Some problems:

1. Bevy images can be formats not supported by vello or formats difficult to display like HDR textures.
2. Current vello API is very rough and requires render.override_image.

Similar Vello Issue:
linebender/vello#664

[StarArawn]

Possible Solution 1:

Vello can add a better API for supporting image rendering. Perhaps rendering any wgpu::Texture?

Pros:

• Easy to use.

Cons:

• Supporting weird texture formats will be difficult. HDR targets need to be resolved to srgba. Maybe we just let bevy do this for us?
• Wont work on the CPU renderer or other non-wgpu renderers.

Possible Solution 2:

We provide a way to handle image assets loaded in on disk, but for render targets we convert the standard bevy Bgra8UnormSrgb to Rgba8Unorm in a special bevy vello render target API. Use the render.override_image api?

Pros:

• Wont need changes with vello.

Cons:

• Kinda clunky to use as there would be separate API for render targets.

[StarArawn]

The solution I came up with in my bevy UI library is:

1. Treat images and render targets as two different things
2. Convert bevy render targets to Rgba8Unorm and override vello images.

You can see this in action here:
https://github.com/StarArawn/woodpecker_ui/blob/4a7d89835871778143c47ac60818f535282839cd/src/convert_render_target.rs

Treating them separately works well as conversion between images is expensive as it requires a pass-per image. This could be avoided though if we had better vello API for handling direct image.

[simbleau]

Just so I understand, you want bevy_vello to support rendering raster images already in GPU-land, tagged as render targets in bevy?

I'd like to see a PR for this behavior and how it would fit in our API. From my understanding, vello's API doesn't support using GPU-resident images.

[StarArawn]

Just so I understand, you want bevy_vello to support rendering raster images already in GPU-land, tagged as render targets in bevy?

I'd like to see a PR for this behavior and how it would fit in our API. From my understanding, vello's API doesn't support using GPU-resident images.

Sorry for the late reply, ideally vello's api would allow us to pass in a wgpu texture directly, but that isn't currently possible. My solution was to convert the render target into a vello friendly texture. Then I could use vello's existing API's for using custom textures: renderer.override_image.

The downside is that you need a pass to convert the texture into a format that is vello friendly.