Compositor's Corner Pin node does something odd to input image Using Corner Pin on an image has an irritating little quirk. It seems to distort the edges of shapes, even when applied in such a way that should cause no change. In the screengrab below, 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?
Using Subtract instead of Difference shows the image seems offset a bit, rather than blurred (2nd and 3rd screengrabs).
While I cannot be certain, I strongly suspect that you have been greeted by one of the many reasons that PNGs are a cancer; their alpha values are unassociated by design, making them an absolutely worthless default in a visual effects / CGI context.