Yesterday I was trying to create some custom MUI frames but despite my efforts I couldn't find the logic adopted,even by analizing the supplied ones... it's ok that each png contains three elements of the same size with respective alpha channel but the result is unpredictable. The result is always scaled or cropped or replicated in a way which is never the one I am expecting. To the point that even trying to create a simple rectangular frame, for testing purposes, doesn't give the expected result. In particular I can't figure out easily the purpose of the third picture and its alpha.

Is there any tutorial or text covering the topic out there ? Thanks.