The Corner Pin node seems to distort the edges of shapes, even when applied in such a way that should cause no change. In the screengrab attached, the background shows the difference between the input (the alpha layer of a png), and the same image fed through a Corner Pin node with no distortion. I would expect there to be no difference. Is there a reason why there is a difference, or is this a bug?
This happens with any image, with or without alpha, and not just when working with pngs. I just happened to notice it when dealing with alpha layers, because it tends to add a halo. (There are no Alpha Overs involved, so this is unrelated to premultiplying behavior.)
Other Distort nodes, like Translate, Tansform, Scale, Rotate, etc., don't create these edges when they have "identity" settings that you'd expect would have no change. This has only been observed with Corner Pin.