Here is my very little test od custom collision mask of physics behavior
Left mouse button - add square
Roght mouse button - add "diamond"
R - erease squares and "diamonds"
You fail at drawing diamonds
But good example otherwise Remember when you make the vector you can use the control key and the mouse wheel to zoom in to the layout editor, and get better control of clicking!
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I found a bug! Delete some stuff you spawned and spawn new diamonds : they will collide with the non-existent stuff
Doh! Forgot to free the box2d body. Fixed for next version.