Detect VR and eye in shader

Is there a way to tell (in a shader) which VR eye is being rendered?

 

Goodness, that sounds like it should be easy but it doesn't seem to be.

My only idea (that doesn't involve keeping track of a ton of material references) is to have 2 cameras targeted to your two separate eyes. In both eyes, render a fullscreen quad first with "blend zero one" and have each one set a different stencil value. Set up 2 passes on all your shaders that care about which eye you're using, each pass set to only render with the stencil value for the specific eye you want to see it. This may be GPU intensive, as each eye will have to run both passes and discard the unwanted pass per-pixel, so I'd limit the number of objects you use this technique on.

 

So I found this:
http://forum.unity3d.com/threads/possible-to-detect-which-vr-eye-in-shader.390772/

But I can't get it to work. I use that if statement and render red if left, green if right, but all I get is green.

 

Yeah, that seems way too heavy. I'd rather wait for Unity to implement an eye index in that case. I see they have one already actually, but it's only available if single stereo pass rendering is enabled.

 

I suppose one way to do this would be to create two cameras, and set eye target for each one. Then, OnPreRender, set a global shader variable telling the shaders which one of them is rendering. But that seems so long winded.

 

If you're using 5.4 and single pass stereo I believe there is now an index you can work off of. Not sure about non-single pass.

If you've already downloaded the built-in shader source files for Unity 5.4 open up UnityCG.cginc and look for UnityClipSpacePos or something like that. I can't remember the exact name. It has a special form for single pass stereo that gets the object to world transform from an array with an index for the left and right eye.

edit: That's not going to be helpful as the actual stereo index is hidden in a #define elsewhere such that by the time you get to that function (which is actually UnityObjectToClipPos) the unit_ObjectToWorld matrix has already been assigned the from the matrix array.

The value you're really after is:
unity_StereoEyeIndex