# Objective
Post processing effects cannot read and write to the same texture. Currently they must own their own intermediate texture and redundantly copy from that back to the main texture. This is very inefficient.
Additionally, working with ViewTarget is more complicated than it needs to be, especially when working with HDR textures.
## Solution
`ViewTarget` now stores two copies of the "main texture". It uses an atomic value to track which is currently the "main texture" (this interior mutability is necessary to accommodate read-only RenderGraph execution).
`ViewTarget` now has a `post_process_write` method, which will return a source and destination texture. Each call to this method will flip between the two copies of the "main texture".
```rust
let post_process = render_target.post_process_write();
let source_texture = post_process.source;
let destination_texture = post_process.destination;
```
The caller _must_ read from the source texture and write to the destination texture, as it is assumed that the destination texture will become the new "main texture".
For simplicity / understandability `ViewTarget` is now a flat type. "hdr-ness" is a property of the `TextureFormat`. The internals are fully private in the interest of providing simple / consistent apis. Developers can now easily access the main texture by calling `view_target.main_texture()`.
HDR ViewTargets no longer have an "ldr texture" with `TextureFormat::bevy_default`. They _only_ have their two "hdr" textures. This simplifies the mental model. All we have is the "currently active hdr texture" and the "other hdr texture", which we flip between for post processing effects.
The tonemapping node has been rephrased to use this "post processing pattern". The blit pass has been removed, and it now only runs a pass when HDR is enabled. Notably, both the input and output texture are assumed to be HDR. This means that tonemapping behaves just like any other "post processing effect". It could theoretically be moved anywhere in the "effect chain" and continue to work.
In general, I think these changes will make the lives of people making post processing effects much easier. And they better position us to start building higher level / more structured "post processing effect stacks".
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## Changelog
- `ViewTarget` now stores two copies of the "main texture". Calling `ViewTarget::post_process_write` will flip between copies of the main texture.
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