Suggestion: have the editor display a non-square image as the preview.

When calculating angles, most of the examples divide the point xy by the resolution xy. This creates a nice point in the 0..1 range. When calculating angles from that, things will look good if the image is square, but once you have a rectangle, things look weird since you'll have an ellipse rather than a circle.

I was trying to make one of my transitions properly calculate the angle and rotate the input image, but the only way I could test if it was really working was by saving the shader and visiting /gallery to see the preview (differently from the editing preview, the gallery preview uses a rectangle).