mask_index
Returns: Real
When you define an object in GameMaker: Studio you can
assign it a mask to be used for collisions rather than the one that
corresponds to the defined sprite. This variable can be used to
find the sprite_index of that mask (or it will return -1 if no
sprite has been assigned) or to set the mask for an instance to the
chosen sprite. Setting the mask index means that you can have, for
example, a sprite for the instance with an irregular shape, yet
give it a circular collision mask that is gotten from a different
sprite.
mask_index = spr_Round;
The above code sets the mask of the instance to that of the sprite "spr_Round".