CODEWITHSUNDEEP
javavectorrotation
Alex
Alex

Reputation: 97

Why is my Rotation Matrix not working?

I have various entities in my game that have hit-boxes defined by Vector array's,

one of my entities rotates slowly to follow the player's movements so naturally I need the hit-box to rotate too.

My code results in warped and distorted lines as I draw each vertex to the screen. My entites are 64 x 64 in size so + 32 indicates the center of the object.

    public  void rotateVertices(Vector2[] vertices, double angle) {

    double cos = Math.cos(angle);
    double sin = Math.sin(angle);

    for(int i = 0; i < vertices.length; i++){
        Vector2 v = vertices[i];
        vertices[i].x = ((v.x - (x + 32)) * cos - (v.y - (y + 32)) * sin) + (x + 32);
        vertices[i].y = ((v.x - (x + 32)) * sin + (v.y - (y + 32)) * cos) + (y + 32);
    }

}

my constructor:

    public EnemyTriBall(Handler handler, float x, float y) {
    super(handler, x, y, 3, 6);

    vertices[0] = new Vector2(x + 25, y + 13);
    vertices[1] = new Vector2(x + 64, y + 33);
    vertices[2] = new Vector2(x + 25, y + 53);

}

Upvotes: 1

Views: 495

Answers (1)

lexicore
lexicore

Reputation: 43709

Here:

Vector2 v = vertices[i];

You don't make a copy of the old vector, you just reference vectices[i]. So when you do this:

vertices[i].x = ((v.x - (x + 32)) * cos - (v.y - (y + 32)) * sin) + (x + 32);

you modify x of of the original vector. Then in the next line:

vertices[i].y = ((v.x - (x + 32)) * sin + (v.y - (y + 32)) * cos) + (y + 32);

You use old v.y but new v.x which gives you weird results. The easiest fix would be to get x and y and use them instead:

    float vx = vertices[i].x;
    float vy = vertices[i].y;
    vertices[i].x = ((vx - (x + 32)) * cos - (vy - (y + 32)) * sin) + (x + 32);
    vertices[i].y = ((vx - (x + 32)) * sin + (vy - (y + 32)) * cos) + (y + 32);

There might be other issues, I did not check the formulas but I'd start with this one.

Upvotes: 1

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