XR_FB_passthrough without alpha blend environment blend mode

I have gotten a passthrough layer which I can see if it is the last layer in XrFrameEndInfo. But the Quest 3 only supports opaque environment blending mode. So how can I blend between my application layer and the passthrough layer so that I can see the passthrough?

I have already set the clear color alpha for the application layer to zero, but since the layers aren’t blending at all it doesn’t work.

The Quest runtime unfortunately does not implement the environment blend modes right: they have their own passthrough layer extension instead of using the built-in core environment blend mode feature.

Even in opaque environment blend mode, you should be able to have alpha blending between layers, which I think is how their extension is supposed to work. Opaque environment blend mode means that you can’t see the external environment. (Which is why video passthrough is supposed to be exposed as “alpha environment blend mode” - you can see the environment through alpha blending.) But in any environment blend mode, you should be able to do alpha blending between composition layers. You’ll probably want to look for quest-specific sample code.