DX12 with DirectComposition transparency not working

[amrbashir]

Description

Using DX12 backend to render into a DirectComposition Visual, alpha channel doesn't affect the render.

Repro steps

The repro can be found here, https://github.com/amrbashir/wgpu-dcomp-repro/blob/master/src/main.rs

It is adapted from a wgpu basic example and windows-rs DirectComposition example https://github.com/microsoft/windows-rs/blob/master/crates/samples/windows/dcomp/src/main.rs

Expected vs observed behavior

I expected to be able to create semi-transparent colors using DirectComposition + wpgu but instead, I only get Opaque composite alpha mode.

Extra materials

This is DirectComposition FillRectangle

This is wgpu rendering to same visual, using the same color and alpha

Platform
Information about your OS, version of wgpu, your tech stack, etc.
OS: Windows 11
wpgu: 24
wgpu-backend: DX12
windows-rs: 0.58

[cwfitzgerald]

This should be a fairly simple fix - this is caused by us just never testing transparency with a DComp swapchain.

• https://github.com/gfx-rs/wgpu/blob/trunk/wgpu-hal/src/auxil/dxgi/conv.rs#L284 needs to be modified to actually properly map modes.
• https://github.com/gfx-rs/wgpu/blob/trunk/wgpu-hal/src/dx12/adapter.rs#L866 needs to present different lists if it's targetting a normal hwnd swapchain, or a DComp one.

Would you be able to submit a PR for this?

[amrbashir]

yeah sure I can take a crack at it, thanks for pointing out where I should look