Sprites - GameMaker Essentials - page 114

This is where the editing of the collision mask starts. You can see two checkboxes, one
labeled Precise collision checking and the other labeled Separate collision masks . Se-
parate collision masks can only be used when Precise collision checking is enabled.
With Precise collision checking turned on, GameMaker will use the actual sprite as a col-
lision mask. This makes collisions as precise as possible with the actual shape of the sprite
being used. By default, this feature does not take alpha and transparency into account.
Any pixel that has more than zero alpha on it will be included in the collision mask.
Below the checkboxes is a button labeled Modify Mask ; clicking on this will show the
mask properties window.