Bounding box doesn't rotate when the sprite rotates

Question

Working on a simple Java 2D game. A tank is controlled by the user. It can be rotated in any angle, and it moves in the direction of that angle.

I'm making a bounding box for the tank, so later I can add collision detection. Currently, I have the box following the sprite wherever it goes, but it doesn't rotate when the sprite rotates. I tried to add code to make the box rotate with the sprite, but adding this code simply makes the bounding box go crazy. It doesn't appear as a box anymore, more like a line just flickering and moving around.

(Obviously the bounding box should be invisible, but for now I'm drawing it on the screen to check for problems).

xcoo[] and ycoo[] are both of type double and hold four values each. These values are the coordinates of the four vertices of the bounding box (a rectangle-shaped Path2D).

(Please note that an actual bounding box is only created when it needs to be drawn on the screen, or in collision detection which I haven't written yet. Most of the time, it's just coordinates.)

The Tank's move() method is called every cycle of the game-loop.

The move() method updates the location of the sprite, and the location of the bounding box. This works fine. But as I said, trying to rotate the bounding box in that method (aka manipulate it's vertices using the rotation matrix), makes it go

Here is the Tank's move() method:

public void move(){

    // Updates the location of the sprite. (works)
    x += dx;
    y += dy;

    bbCenterX += dx;
    bbCenterY += dy;

    // Updates the location of the bounding box. (works)    
     for(int i=0;i<4;i++){
        xcoo[i] += dx;
        ycoo[i] += dy;
    } 

    // Should rotate the bounding box (aka, manipulate it's coordinates
    // so when it is created with these coordinates, it's rotated).
    // (Doesn't work).
    for(int z=0;z<4;z++){

        xcoo[z] = xcoo[z] - bbCenterX;
        ycoo[z] = ycoo[z] - bbCenterY;

        xcoo[z] = ((xcoo[z] * Math.cos(angle)) - (ycoo[z] * Math.sin(angle)));
        ycoo[z] = ((xcoo[z] * Math.sin(angle)) + (ycoo[z] * Math.cos(angle)));

        xcoo[z] = xcoo[z] + bbCenterX;
        ycoo[z] = ycoo[z] + bbCenterY;

    }

}

What's wrong with my rotation? I suspect that that's the part of the code where the problem is, but if you think differently, please tell me to post more code.

Thanks a lot

Reminds me of a story my commercial artist uncle told me ca 1970. He was working on a TV commercial (for trucks) with some of the earliest computer graphics. The geeks first had the truck wheels rolling onto the screen, then the truck body descended onto the wheels. Then the wheels rolled away, leaving (oops!) the truck body standing there. They got it right on the second take. — Hot Licks

OldCurmudgeon OldCurmudgeon · Accepted Answer · 2014-01-06T23:32:32

Answering the title of your question - Bounding box doesn't rotate when the sprite rotates

In computer graphics the most common implementation of a bounding box is the Axis-aligned minimum bounding box which obviously will not rotate when you rotate your sprite because it is axis aligned.

I am not aware of what implementation the Java 2D package you are using implements but it certainly sound like this and not an Arbitrarily oriented minimum bounding box as you hoped.

Perhaps the rendering of your bounding box somehow assumes that it is axis-aligned and therefore fails to render when you present it with a box that is not axis-aligned.

Bounding box doesn't rotate when the sprite rotates

1 Answers