Inverting rotation in 3D, to make an object always face the camera?

Question

i have lots of sprites arranged in 3D space, and their parent container has rotations applied. How do i reverse the sprites 3D rotation, that they always face the camera (Ac

Answer 1

I've solved it using Wikipedia, matrices and black magic. I choose to implement custom rotation instead of inverting the rotation of all objects. Heres the code if anyones interested:

package{
import flash.display.Sprite;
import flash.events.Event;

public class test extends Sprite{

private var canvas:Sprite = new Sprite();
private var sprites:Array = []
private var rotx:Number=0,roty:Number=0,rotz:Number=0;
private var mm:Matrix3 = new Matrix3();
public function test(){
    addChild(canvas);
    canvas.x = canvas.y = 230
    for (var i:int=0;i<30;i++){
        var sp:Sprite = new Sprite();
        canvas.addChild(sp);
        sp.graphics.beginFill(0xFF0000);
        sp.graphics.drawCircle(0,0,2);
        sp.x = Math.random()*200-100;
        sp.y = Math.random()*200-100;
        sp.z = Math.random()*200-100;
        sprites.push(sp);
        rotx=0.06; //from top to bottom
        //roty=0.1; //from right to left
        rotz=0.1; //clockwise
        mm.make3DTransformMatrix(rotx,roty,rotz);
    }
    addEventListener(Event.ENTER_FRAME,function():void{
        for (var i:int=0;i

and the matrix3 class:

public class Matrix3{

    private var da:Vector.; // rows

    public function make3DTransformMatrix(rotx:Number,roty:Number,rotz:Number):void{
        var cosx:Number = Math.cos(rotx);
        var cosy:Number = Math.cos(roty);
        var cosz:Number = Math.cos(rotz);
        var sinx:Number = Math.sin(rotx);
        var siny:Number = Math.sin(roty);
        var sinz:Number = Math.sin(rotz);
        da =    new [
            cosy*cosz, -cosx*sinz+sinx*siny*cosz,  sinx*sinz+cosx*siny*cosz,
            cosy*sinz, cosx*cosz+sinx*siny*sinz , -sinx*cosz+cosx*siny*sinz,
            -siny    ,         sinx*cosy        ,        cosx*cosy         ];
    }

    public function rotateByAngles(d:DisplayObject):void{
        var dx:Number,dy:Number,dz:Number;
        dx = da[0]*d.x+da[1]*d.y+da[2]*d.z;
        dy = da[3]*d.x+da[4]*d.y+da[5]*d.z;
        dz = da[6]*d.x+da[7]*d.y+da[8]*d.z;
        d.x = dx;
        d.y = dy;
        d.z = dz;
    }
}
}