As you already discovered, the chop works at the opposite of what you need. You select a cross to remove from an image and the remaining areas are glued together in a new image.
I drew to pictures to better explain what I mean:
If you select a corner, you will observe exactly the behaviour you described (area color meanings are the same of the picture above):
To select a rectangular surface of an image, mainly to get sprites from a spritesheet, you need to use the pygame.Surface.subsurface method. It define a child image of your main one ready to be blitted on a new surface or used as is.
Warning Changes to the main surface or to the subsurface will be reflected on the other one.
thanks for the explanation!
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