072 : Better collisions

// Return true if the 2 objects overlap.

// If the 2 objects overlap, return a [dx, dy, dir] array containing the x,y

// overlap and the direction of the collision (relative to obj1).

// Return null if they don't overlap. function collide(obj1, obj2) {

if ((obj1.x - obj1.origin_x + obj1.width) <= (obj2.x - obj2.origin_x))

return false;

if ((obj1.y - obj1.origin_y + obj1.height) <= (obj2.y - obj2.origin_y))

return false;

if ((obj2.x - obj2.origin_x + obj2.width) <= (obj1.x - obj1.origin_x))

return false;

if ((obj2.y - obj2.origin_y + obj2.height) <= (obj1.y - obj1.origin_y))

return false;

return true;

var o1_center_x = obj1.x - obj1.origin_x + (obj1.width / 2);

var o1_center_y = obj1.y - obj1.origin_y + (obj1.height / 2);

var o2_center_x = obj2.x - obj2.origin_x + (obj2.width / 2);

var o2_center_y = obj2.y - obj2.origin_y + (obj2.height / 2);

var dx = o1_center_x - o2_center_x;

var dy = o1_center_y - o2_center_y;

var half_width = (obj1.width + obj2.width) / 2;

var half_height = (obj1.height + obj2.height) / 2;

// No overlap.

if (half_width <= Math.abs(dx) || half_height <= Math.abs(dy))

return null;

var overlap_x = half_width - Math.abs(dx);

var overlap_y = half_height - Math.abs(dy);

if (overlap_x >= overlap_y) {

if (dy > 0)

return [0, -overlap_y, "top"];

else

return [0, overlap_y, "bottom"];

} else {

if (dx > 0)

return [-overlap_x, 0, "left"];

else

return [overlap_x, 0, "right"];

} }

It now returns an array instead of true/false. Fortunately, JavaScript considers a null array to be false and any valid array to be true.

run the code now and everything should still work like before, but now we have the information we need to make the platforms behave properly.