Zool: Taking It to the N Dimension - The Game Maker's Companion: Game Development - page 54

obj_slope (grey) —Slopes are just steeper ramps that are made of ice, and therefore
slippery. Again, you'll find the actual slopes that are placed on the level (and
have the sloped collision masks) in the Slopes group.
obj_solid (no sprite) —This is a parent object that is used for all objects that are
considered to be solid parts of the landscape.
8. Notice that all the objects in the Ramps group have obj_ramp set as their parent and all
the objects in the Slopes group have obj_slope set as their parent. You can think of this
kind of parent relationship as meaning “a kind of.” So all the ramps are “a kind of”
obj_ramp and all the slopes are “a kind of” obj_slope .
9. There are more “a kind of” relationships set up for the other types of collision objects.
The obj_slope object (which represents icy slopes) has obj_ramp set as its parent
because slopes are “a kind of” slippery ramp. Both obj_ramp and obj_ledge have
obj_platform as their parent because they are “a kind of” platform. Finally, both
obj_platform and obj_wall have obj_solid as their parent because they are “a kind of”
solid object. Figure 3-5 shows the complete hierarchy of parent relationships used for
the landscape objects. Take a good look at this and make sure that the parent
relationships make sense to you before continuing.
10. Crucially, this makes it possible to consider collisions with objects that are “a kind of”
something. For example, a collision with obj_solid now includes every kind of
landscape object, and a collision with obj_platform would include everything apart
from walls.
Figure 3-5. The hierarchy of “is a kind of” relationships as defined by the parent settings of each object