some question for Blend Render Compositor layer( OvrOverlay ) with Opaque & Transparent model

if you  want render something infront of Compositor layer something like 3d mesh, the layer must be underlay.
the blend forumla is not the default one --  eye.Color * eye.Alpha + underlay.Color * (1 - eye.Alpha)

the blend formula  should be eye.Color + underlay.Color * (1 - eye.Alpha) or
                                              eye.Color * eye.Alpha + underlay.Color 

i forgot which one is correct, you could try both.

so, if you want blend right, the hole you render in unity should multi alpha in frag shader first when formula is
eye.Color + underlay.Color * (1 - eye.Alpha) 

the  Compositor layer kills performance in android device develop by unity, which using a lot of blit operations. you could add native plugin to save blit operation by tex getnativeptr & opengl code, which direct render into overlay

thank you very much, i can not blend right without your help!!!!

Sorry for the confusion and delay here. Our Underlay_Impostor shader is based on Unity's Standard Shader, which doesn't have the alpha behavior you are looking for. I would recommend using Oculus/Unlit Transparent Color for your your object.

vrdaveb said:

Each layer is blended on top of the previous ones using src.a * src.rgb + (1-src.a) * dst.rgb. 

is any update to explain the above phenomenon?

I'm truly grateful for your help. And the render order your listed above is very clear to let "underlay" render right.

but i think there are some bugs for "underlay" tech because the rules you mentioned contradict with render result using unity 2017.3.1 & sdk-1.25. 

  1."Each layer is blended on top of the previous ones using src.a * src.rgb + (1-src.a) * dst.rgb." 
first, there is a scene looks like  below:



the bug appears step by step.

1. 
underlay quad panel render first, texture like below   

2. 
blend eyebuffer with 1 result 




the render result shows that: 
even though the hole's alpha = 0, underlay still blend with eyebuffer( underlay render first, then the eyebuffer which contian a hole). Is it a bug for alpha = 0 and  still blend two textures color not just dest.rgb?  or unity sdk behaivor diff with oculus home app which develop from native framework?

you can find the scene from UnderlayExample.unitypackage at underlayquad.scene~~ change 
plane imposter material's color to confirm what i said.

Yes, Oculus Home has 4 layers, which are rendered in the following order:

1. Cubemap background
2. Cylinder panel (tile grid)
3. Cursor
4. Eye buffers (controller geometry)

Each layer is blended on top of the previous ones using src.a * src.rgb + (1-src.a) * dst.rgb. So we set a=0 in each layer to "poke a hole" for the previously-rendered layer(s) to show through. In Unity, you can use the "Underlay Impostor" shader to poke a hole for OVROverlay underlays. Sorry this isn't in the documentation yet, we're updating it. For performance reasons, we can't easily change the blending formula, but you can change the compositing order by activating your OVROverlay instances in a different order. "Underlay" layers simply composite before the eye buffers and "overlay" layers composite after them.