SFERIC COLLISION SILVERLIGHT

06-29-2009 7:57 AM |

Hello everyone,
I am following a tutorial on collision on a hypothetical game of billiards in Silverlight.

The sub that controls the collision and therefore the effects of shocks, however, has a part that I can not understand.

The sub control a ball and another ball, the first ball is inside the class of the ball itself. Then check the collision between the ball belongs to the same class (this) and the other ball always passed as an object/class "Ball" in the parameter b of the sub.
The variables _x and _y are the x and y coordinates of the balls.
The variables _vx and _vy are nothing more than the x and y coordinates of the vector direction and then describe the movement of the ball (they are generated by cosine and sine of angle).

Said that the sub is as follows:

public bool DoCollide(Ball b)
2: {
3: // calculate some vectors
4: double dx = this._x - b._x;
5: double dy = this._y - b._y;
6: double distance2 = dx * dx + dy * dy;
9:
10: if (Math.Abs(dx) > this._d || Math.Abs(dy) > this._d)
11: return false;
12: if (distance2 > this._d2)
13: return false;

up to here it's clear.

13:  double dvx = this._vx - b._vx;
14: double dvy = this._vy - b._vy;
15: // make absolutely elastic collision
16: double mag = dvx * dx + dvy * dy;
17:  
18: // test that balls move towards each other
19: if (mag > 0)
20: return false;
21:  
22: mag /= distance2;
23:  
24: double delta_vx = dx * mag;
25: double delta_vy = dy * mag;
26:  
27: this._vx -= delta_vx;
28: this._vy -= delta_vy;
29:  
30: b._vx += delta_vx;
31: b._vy += delta_vy;
32:  
33: return true;
34: }