Empowerment: Sliding Ninjas - The Game Maker's Companion: Game Development - page 74

Now take a look at Figure 4-4. At the start of this event, Zool is set back to his position before
the collision ( xprevious and yprevious —the blue cross), but then the Move to Contact action
moves Zool's x and y position to the collision point with obj_solid (red cross). The variable
vspeed provides Zool's vertical speed in pixels per step (orange arrow), so yprevious+vspeed gives
us the vertical position of Zool if he had never collided with obj_solid (black cross). Therefore, by
checking for instances of obj_solid at the position X= x , Y= yprevious+vspeed , we are seeing if
there is anything in the way if Zool continues to slide vertically (but not horizontally) after the
collision.
4. Immediately follow this with a Speed Vertical action ( move tab) that sets Vert. Speed
to 0 . This makes sure that Zool stops moving vertically if there is something in the way,
and leaves Zool's position at the collision point (red cross in Figure 4-4).
5. Follow that with an Else action ( control tab).
6. Then include a Set Variable action that sets Variable y to yprevious+vspeed . This will
move Zool's position to the sliding point (black cross in Figure 4-4) if there isn't
anything in the way and maintain Zool's vertical speed after the collision.
7. Now we repeat the process for horizontal movement. Include a Check Object action to
check for Object obj_solid at a position of X = xprevious+hspeed , Y = y . In a similar way,
we are checking if there is anything in the way if Zool continues to slide horizontally
(but not vertically) after the collision.
8. Immediately follow this with a Speed Horizontal action ( move tab) that sets the Horiz.
Speed to 0 .
9. Follow that with an Else action ( control tab).
10. And then follow that with a Set Variable action that sets Variable x to
xprevious+hspeed . This will now maintain Zool's horizontal movement if there isn't
anything in the way after the collision.
11. All of the new actions we've added actually need to take place before the check to see if
Zool is standing on solid ground, so we'll need to move those two actions to the end of
the list again. Click and drag the first Check Object action to the end of the actions list.
Now do the same for the (only) Change Instance action. You should now have 13
actions in the Collision event with obj_solid and it should look like Figure 4-5.