Custom renderpass in CallbackTrait?

[tao-hengyi]

I have read the custom3d_wgpu example. I now want to extend this example by adding features such as shadows that require multiple render passes.

However, in the parameters of the paint method of CallbackTrait, there is only the main egui render pass.

fn paint<'a>(
    &'a self,
    info: PaintCallbackInfo,
    render_pass: &mut wgpu::RenderPass<'a>,
    callback_resources: &'a CallbackResources,
);

In this case, how can I implement multiple render passes?

[9SMTM6]

I'm neither a webgpu nor egui expert. But can't you simply render this using the full wgpu in like another thread, and then render the final result in egui?

A decent bit more effort, but i dont see why that would not work, and it'd be arguably more portable.

[mkeeter]

I have gotten this working, and figured I'd post details (since this is the top search result for "egui custom render pass").

In my case, I needed to render images with a depth buffer attached. The solution is basically what @9SMTM6 suggests: render to a texture, then draw that texture in egui. However, you don't need to go off-thread; it's possible to do this all using egui_wgpu::CallbackTrait.

In CallbackTrait::prepare, you're given a &mut wgpu::CommandEncoder. You can use this command encoder to start your own render pass, with whatever settings you want (e.g. depth culling). Importantly, this render pass should use a texture (which you create!) as its color attachment. egui will run this render pass before the render pass which draws the UI.

Then, in CallbackTrait::paint, you blit the resulting texture to the screen as a full-screen quad.

You'll need two (or more) pipelines: all of your fancy custom rendering gets done in your custom render pass with fancy pipelines, then the CallbackTrait::paint pipeline just draws the texture.

In my case, I needed to create both RGBA and depth textures, but your needs may vary. The RGBA texture should be configured with usage: TextureUsages::RENDER_ATTACHMENT | TextureUsages::TEXTURE_BINDING, so it can be both a render target and used in drawing.