Rogues’ Rendezvous: Vertically Sliced - The Game Maker's Companion: Game Development - page 298

As skeletons will be able to pursue both Flynn and Archie, we are using a variable
called target_obj to store which of the two is currently being chased. The variables
target_hidden and target_dist will be used as part of the line-of-sight tests, and
pursuing is self-explanatory. Both gravity and speed are initialized here in case the
skeleton comes back into this state after a fall, and the final four variables are the start
and end positions of the line-of-sight test. These last four could easily be temporary
variables elsewhere, but putting them here will allow us to draw the lines on screen,
too. You'll see that in operation a bit later.
2. Reopen the Step event of obj_skeleton_land and insert these lines at the start of the
code (after the first curly bracket):
1: x1 = x+(facing*20);
2: y1 = y-48;
3: x2 = target_obj.x;
4: y2 = target_obj.y;
5:
6: target_dist = point_distance(x1, y1, x2, y2);
7: target_hidden = collision_line(x1, y1, x2, y2, obj_solid, true, true) ||
8: collision_line(x1, y1, x2, y2, obj_enemy, true, false);
9:
10: if( target_obj.x > x )
11: target_dirn = FACE_RIGHT;
12: else
13: target_dirn = FACE_LEFT;
Lines 1-4 set up the start and end positions of the line-of-sight test. This begins just in
front of the skeletons eyes and ends at the origin of the target object (the center of the
sprite for both Flynn and Archie). Line 6 then calculates the distance between these
two points, as the skeletons will have a limited viewing distance.
Line 7 performs a collision test by checking if there are any collisions with
obj_solid between the start and end points. The last parameter (set to true )
determines whether the calling object (the skeleton instance) should be disregarded
from collisions in the test. It makes no difference in this case, as obj_skeleton_land is
not a kind of obj_solid so such a collision would not be included anyway.
Line 8 performs a second collision test on the same line, but this time for
obj_enemy and with the final parameter set to false . As obj_skeleton_land is a kind of
obj_enemy , it will be included in the collision test this time. This means that it will
detect a collision with itself if the target is behind the skeleton, as the line will pass
through the skeletons head. If either this collision test or the previous one returns true,
then target_hidden will also be true, as the results are combined with a logical or
operator (“ || ”).
Finally, lines 10-13 determine the direction of the target based on its x position
relative to the skeleton.