Question view direction in blit pass

Hello. I am trying to calculate the view direction in a blit pass for a image effect; but I am having some trouble calculating the direction each pixel is looking/pointing. The result I need is a normalized float3.

I tried quite a few different attempts but I guess my matrix math is too weak. The closest I've gotten is

float3 viewdir = mul(_CameraToWorldMatrix ,float3((.5-o.uv.x)*_AspectRatio,.5-o.uv.y,1));

Where I set _CameraToWorldMatrix and _AspectRatio in c#.
All help is greatly appreciated!

 

https://github.com/keijiro/DepthInverseProjection

 

Thank you bgolus. I reverse engineered keijiro's project and extracted the tasty bit.

For future reference:
vertex program

v2f vert (appdata v)

{

    v2f o;

    o.uv = v.uv;

    float3 pos = float3(v.uv*2.-1.,1);

    o.vertex = pos.xyzz;

    o.ray = mul(unity_CameraInvProjection, pos.xyzz ).xyz;

    o.ray.x *= -1;

    o.ray = mul(unity_CameraToWorld, o.ray.xyz);

    return o;

}

and then just

float3 viewDir = normalize(i.ray);

in the fragment program and you will have the viewdir.