Writing a to a global, single channel alpha (A8) texture via URP renderer feature

[shell gorge]

Hey all,

I'm looking for a way to create a mask based on the alpha output of a selection of renderers (no material override) in a scene.

I'm able to get that result but currently I'm writing a 4 channel texture only to use its alpha channel, and I'd like to only write a single channel. The only single channel color format available is R8.

Would appreciate help with this!

[last storm]

R8 should be the correct format. Format is unrelated to what data you put in the channel.

[shell gorge]

What I'm not sure how to do is, how do I save the alpha output of cmd.DrawRendererList into the red channel of the texture?

Currently I only get the red channel, which doesn't have the data I'm after. I can't find any way to switch up the channels before storing it in the R8 texture.

[last storm]

The r8 texture only has one channel.
Your shades would need to write to the render target r channel. If you're doing that and it doesn't contain the correct data then there must b some issue with your implementation.
Anyways, it's hard to say anything without more context.

[shell gorge]

It's a pretty simple setup.
I'm using a renderer feature that renders a set of objects (using their original shaders) and outputs the result. I store the result in a global texture and then composite it in a separate pass.

I'm only using the alpha channel, so the RGB channels are better skipped. But if I store the texture in an R8 format, I get the red channel written there, not the alpha.

I'm not sure how to get the alpha channel to write into the R8 channel.

Or rather, I'm wondering if there's a way to do that within the renderer feature rather than modifying the shaders themselves.

[last storm]

As I said earlier, there's no difference between red or alpha channels if you only have one channel, so that is not a problem.

It's just a channel that stores 8 bits of data. The texture doesn't care how you use that data.

So I don't really understand what the issue you're having is.

[shell gorge]

My problem is not about the difference in data type, it's the ability to write the alpha channel into the red channel.

The behavior is that when I set the render target, then render, if I use the R8 format, I get the data from the source's red channel written to my render target (which is R8).

I want to somehow instruct the process to store the alpha channel from the source, to the red channel in the target.

So right now it's doing source.r->target.r, but I want it to do source.a->target.r.

[last storm]

Well, that's controlled by the pixel shader.

You'll need to make sure that your pixel shaders in the pass output to the render target r channel.

[shell gorge]

Right, so that was the question, is there a way to switch the channels up before I save the global texture, in the renderer feature.

[last storm]

No. Only the shader.