Keep track of when a texture is first cleared

[JMS55]

Objective

• Custom render passes, or future passes in the engine (such as Meshlet rendering (initial feature) #10164) need a better way to know and indicate to the core passes whether the view color/depth/prepass attachments have been cleared or not yet this frame, to know if they should clear it themselves or load it.

Solution

• For all render targets (depth textures, shadow textures, prepass textures, main textures) use an atomic bool to track whether or not each texture has been cleared this frame. Abstracted away in the new ColorAttachment and DepthAttachment wrappers.

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Changelog

• Changed ViewTarget::get_color_attachment(), removed arguments.
• Changed ViewTarget::get_unsampled_color_attachment(), removed arguments.
• Removed Camera3d::clear_color.
• Removed Camera2d::clear_color.
• Added Camera::clear_color.
• Added ExtractedCamera::clear_color.
• Added ColorAttachment and DepthAttachment wrappers.
• Moved ClearColor and ClearColorConfig from bevy::core_pipeline::clear_color to bevy::render::camera.
• Core render passes now track when a texture is first bound as an attachment in order to decide whether to clear or load it.

Migration Guide

• Remove arguments to ViewTarget::get_color_attachment() and ViewTarget::get_unsampled_color_attachment().
• Configure clear color on Camera instead of on Camera3d and Camera2d.
• Moved ClearColor and ClearColorConfig from bevy::core_pipeline::clear_color to bevy::render::camera.
• ViewDepthTexture must now be created via the new() method

[JMS55]

I can also maybe use Relaxed atomic ordering.

[superdump]

I don’t have time to check right now but I think we need to be careful about removing all the prepass checks for whether to load or clear the depth buffer as they may not mean the same thing. For example, logically if something is the first write then we have to clear it and cannot load it. But if something is not the first write, perhaps then it still needs to be cleared in some cases. I don’t think so but I would also want to check to make sure.

[JMS55]

Oops this was supposed to be for milestone 0.13, fixed. There's no urgency on this PR.

[JMS55]

I also want to add flags for the prepass textures and any other render targets as well.

[JMS55]

TODO: Add support for the prepass outputs and the deferred gbuffer prepass outputs.

[alice-i-cecile]

I'm not super familiar with this part of the renderer. Can you say more about why we should / need to use atomics here?

[JMS55]

I'm not super familiar with this part of the renderer. Can you say more about why we should / need to use atomics here?

Keeping track of whether or not to clear the texture is decided in the render nodes, which don't allow mutable access to components, so an atomic is needed. Otherwise it would just be a regular boolean.

[robtfm]

i definitely think this should be done, but i'm not convinced about the way it is done here.

• i'm not super happy with a function called is_first_write modifying the internal state
• we have duplicated logic for working out what the clear color should be in each node that potentially needs to clear
• the functions is_first_write, is_first_motion_vectors_write and friends all do basically the same thing

the last two points make me think we're missing an abstraction: it would be nicer i think to have something like a TrackedTexture which wraps a texture and a clear op (and an atomic), so the clear op is initialized once at the point the texture is created. then the nodes can just call texture.load_op() and get the right op for the texture without needing to know how to determine it themselves.

what do you think?

[JMS55]

I'm in favor of some kind of ClearOnceTexture wrapper, agreed.

i'm not super happy with a function called is_first_write modifying the internal state

Yeah it's not the best naming, but I couldn't think of anything better. I suppose moving the info into the type name helps, and then we can call the method just get_load_op() or something.

[alice-i-cecile]

This generally looks fine, I understand the motivation, and this is reviewed by folks with expertise. Merging now.