How to use _CameraToWorld builtin value?

Hi All
Still struggling with my reconstruction of world space coords and I am wondering if I am doing something very wrong with the built in value _CameraToWorld. It says in the Unity 3.0 release notes that

Thing is I can't find the param _CameraToWorld in any of the .cginc files, (I can find other builtin values such as UNITY_MATRIX_MV etc) if I try to use the value I get an error unless I add the value myself to my shader ie float4x4 _CameraToWorld and it seems if I have to do that that if it is indeed a builtin value, me adding it to my shader is confusing the issue.

Can anyone tell me the correct way to make use of this value, do I need to add it myself to my shader? Is there an include Iam missing? Do I need to do a SetMatrix to set the value of this builtin value?
Thanks
Chris

 

Thanks Daniel, been playing and it is working, just getting confused that you have to add some params for builtin values and for others you dont. Still cant get my world space calc to move away from the corner

 

I tried to do this too. Unsucessfull too.
One thing what i realize it's UNITY_MATRIX_MV is useless, in post-process shaders it's allways identity. So i just get camera.cameraToWorldMatrix in script, and pass it in to shader.

 

Arr that's interesting, did notice something screwy with my ray values always seeming to be 0. Would be nice if someone from Unity could explain the state of the matrices when used from a Post Effect shader. Also I can't find any direction information on what kind of values get passed for the quad used to render the effect, in one of the cginc files it says

If the ray is already in the texcoord why is it being recalculated inside the shader code below that Iam took from a Unity shader. All seems very vague. And what is the purpose of the * float3(-1, -1 1)?

It would be nice if some clearer info could be given on this. I realise Iam very far from being a shader expert, expecially in Unity, but a little more info would be handy Or have I missed a section in the docs, guess I need to go double check.