Ch. 6 Collision Detection

Collision Class

For this class all we need to add a constructor to add values to the constants that we declared at the beginning of the code.


class Collision {
    constructor(game_obj1, game_obj2) {
      this.obj1 = game_obj1;
      this.obj2 = game_obj2;
      this.has_collided = false;
      this.direction = new Vector2d(0, 0); // Direction of penetration
      this.magnitude = Number.NEGATIVE_INFINITY; // Shortest distance of penetration
    }
  }

getSATCollision Function

Uses Separating Axis Test to get the direction and magnitude of any possible collision between given two objects. https://en.wikipedia.org/wiki/Hyperplane_separation_theorem Objects can have any convex shape. Circles are a special case. The next thing that we want to add are 2 functions, these are not in a class because we want the ability use these from anywhere in the code without having a constructor.

To create the function:


function getSATCollision(game_obj1, game_obj2) {
	// code
}

We will add some constants to this function that we will use in some of the if statements.


var obj1 = game_obj1;
var obj2 = game_obj2;
let temp_pos1 = obj1.pos.clone();
var d_origins = temp_pos1.subtract(obj2.pos).get_magnitude();
var collision = new Collision(game_obj1, game_obj2);
collision.has_collided = false;
var verts1 = game_obj1.get_verts();
var verts2 = game_obj2.get_verts();
var pos1 = game_obj1.pos.clone();
var pos2 = game_obj2.pos.clone();

This first if-statement is for optimization to make the game run smoothly, don’t edit it please.


if (d_origins > MAX_DIST_FOR_COLLISIONS * CELL_SIZE) {
      // Optimization: Skip further checks for distant objects
      return collision;
}

We will add a for loop that our code will run though all the if statement if we call this function.


for (var i = 0; i < verts1.length + verts2.length; i++) {
	// code
}

This will calculate next axis by taking the normal of a side of one object


if (i < verts1.length) {
        var vert = verts1[i];
        if (i < verts1.length-1) var next_vert = verts1[i+1];
        else var next_vert = verts1[0];
      }
      else {
        var vert = verts2[i - verts1.length];
        if (i < verts1.length+verts2.length-1) var next_vert = verts2[i+1 - verts1.length];
        else var next_vert = verts2[0];
      }
      var side = next_vert.clone().subtract(vert).get_unit_vector();
      var axis = side.get_right_normal();

This will get minimum and maximum projections on axis from center of obj1


var min_dist1 = verts1[0].get_dot_product(axis);
      var max_dist1 = min_dist1;
      for (var j = 1; j < verts1.length; j++) {
        var distance = verts1[j].get_dot_product(axis);
        if (distance < min_dist1) min_dist1 = distance;
        else if (distance > max_dist1) max_dist1 = distance;
      }

Get minimum and maximum projections on axis from center of obj2


var min_dist2 = verts2[0].get_dot_product(axis);
      var max_dist2 = min_dist2;
      for (var j = 1; j < verts2.length; j++) {
        var distance = verts2[j].get_dot_product(axis);
        if (distance < min_dist2) min_dist2 = distance;
        else if (distance > max_dist2) max_dist2 = distance;
      }

This will calculate the distance between objects and flip axis if necessary, and the gaps between objects projected along axis, and the negative gap means that there is a collision on this axis.


var d = new Vector2d(pos2.x - pos1.x, pos2.y - pos1.y).get_dot_product(axis);

      var gap1 = d - max_dist1 + min_dist2;
      var gap2 = - d - max_dist2 + min_dist1;
      if (gap1 >= -EPSILON || gap2 >= -EPSILON) {
        // No collision on this axis - these objects cannot be colliding!
        collision.has_collided = false;
        return collision;
      }
      if (gap1 > gap2 && gap1 > collision.magnitude) {
        collision.magnitude = gap1;
        collision.direction = axis;
      }
      if (gap2 > gap1 && gap2 > collision.magnitude) {
        collision.magnitude = gap2;
        collision.direction = axis.get_inverted();
      }
    }
    collision.has_collided = true;
    return collision;
  }

GetCollisions Function

Creating the GetCollisions function:


function GetCollisions(obj) {
	// code
}

The point of this function is to check collisions given gameobject and all other gameobjects.


var ign1 = obj.ignored_collision_objs;
    var collisions = [];
    for (obj_ind in GAME_OBJECTS) {
      var other_obj = GAME_OBJECTS[obj_ind];
      var ign2 = other_obj.ignored_collision_objs;
      if (ign1.indexOf(other_obj) === -1 && ign2.indexOf(obj) === -1) {
        var collision = getSATCollision(obj, other_obj);
        if (collision.has_collided === true) {
          collisions.push(collision);
        }
      }
    }
    return collisions;
  }

ALL OF CHAPTER SIX CODE https://pastebin.com/CBA5nEgR

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Ch. 5 GameObject subclasses

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7️⃣

Ch. 7 Main Functions