Ok I know exactly that this is. Hook up ActorPositionWS to the “Position” input on the function.
The issue here is that even when the source mesh is very small, it is evaluating the Position for each vertex instead of for a single position for all verts. So the material was blending between different angles when the verts crossed the angle threshold at different times.
As far as not matching perfectly, there will always be a little bit of error when you are between two frames. This method also cannot account for perspective stretching when you get very close to the imposters.
A good test to see how closely they match is to try and line up something like the bottom base element to be flat, and see how close the imposter is. Keep in mind that the imposter is basically evaluated for the view angle to the center of the imposter, it can’t do change the perspective of just the bottom piece. So the bottom piece will appear flat when your view is flat relative to the center of the imposter. Make sense?
If you look carefully in my example shot above, you can see how the imposters closest to the camera on the far right have a bit of a ‘skewed’ appearance since a real 3D mesh would have been affected by perspective which isn’t possible.