Calculating Position of Rectangle Corners

[Guest]

Hello again everyone - I'm in the middle of attempting to write my own collision detection algorithm because the built in 'hitTestObject' function seems to be incredibly inaccurate (it's registering 'hits' when the objects are still a considerable distance from each other).

I need to be able to calculate the position of each of the 4 corners of a rectangle, based on its rotation. Obviously one of the corners will simply be the rectangle's x and y coordinates. I have made a little test program to make sure I am calculating the positions of the corners accurately - it works when the rectangle is rotated to 0, 90, 180 and 270 degrees, but produces strange results for any other angles...

I have attached the program at the bottom, but here is the code:

package
{
    import flash.display.MovieClip;
    import flash.geom.Point;
   
    public class CornerFinder extends MovieClip
    {
        //vars
        var myRect:MovieClip;
       
        //constructor
        public function CornerFinder():void
        {
            myRect = new MyRectangle();
            myRect.x = stage.stageWidth/2;
            myRect.y = stage.stageHeight/2;
            myRect.rotationZ = 270;
            addChild(myRect);
           
            AddCornerMarkers();
        }
       
        private function AddCornerMarkers():void
        {
            //The rotation of the rectangle in radians
            var rectRotation = myRect.rotationZ*(Math.PI/180);
           
            var corners:Array = new Array();
           
            corners[0] = new Point(myRect.x, myRect.y);
            corners[1] = new Point( (myRect.x + myRect.width * Math.cos(-rectRotation)), (myRect.y + myRect.height*Math.sin((rectRotation) )) );
            corners[2] = new Point( (myRect.x + myRect.width * Math.sin(-rectRotation)), (myRect.y + myRect.height*Math.cos((rectRotation) )) );
            corners[3] = new Point(corners[2].x + (corners[1].x - corners[0].x) , corners[2].y + (corners[1].y - corners[0].y));
           
            for(var i:uint = 0; i < corners.length; i++)
            {
                var marker:MovieClip = new CornerMarker();
                marker.x = corners.x - marker.width/2;
                marker.y = corners.y - marker.height/2;
                trace("Adding marker at: " + marker.x + ", " + marker.y);
                addChild(marker);
            }
        }
    }   
}

MyRectangle, as you may have guessed is a simple rectangular symbol that is in the library of my fla file. The CornerMarker is just a red circle symbol.

Can anyone see what the problem is? I thought if it worked for the 'main' angles (e.g. 0, 90, 180, etc) it would work for everything in between, but apparently not...

[kglad]

if you want an accurate hittest you should be using the bitmapdata's hittest.

[Guest]

Would this not be rather slow? I need to perform a large number of collision detection checks every update so I'm trying to get a balance between accuracy and preformance - at the moment hitTestObject is simply too innacurate, but I think using bitmap data could cause a major performance hit...

[Guest]

So no one has any idea why my code isn't working? I had assumed that I'd just made a simple error in my maths - it's been a while since I've had to do anything like this haha

If anyone has any other possible solutions then please let me know, failing that, how do I get the bitmap data from a MovieClip?

[kglad]

you have some basic problems.

1.  assign myRect's rotationZ to zero, then compute your corners, then reassign the rotationZ to myRect

2.  your calculations are wrong.  here's corners[1] to get you started:

corners[1] = new Point( (myRect.x + myRect.width * Math.cos(rectRotation)), (myRect.y + myRect.width*Math.sin((rectRotation) )) );

3.  to create a bitmapdata object from a displayobject, use the draw() method of the bitmapdata class.

[Guest]

Forgive me if I'm misunderstanding you here, but if I assign the rotationZ to 0 first then change it back the locations of the CornerMarkers will not change - they'll just be sitting where they were before the rotation... It's calculating it with the rotation that's being the major problem here, if I could do it without the rotation corner[1] would simply be (corner[0].x + width, corner[0].y), etc

Also if I change my formulas to use the one you suggest it only works for 0 and 180 degrees, and not for 90 or 270 anymore... that's why I changed it in the first place I think lol Originally I had the same as you suggest.

I'm considering changing the rotated rectangles to circles at this rate - that way I can avoid having to rotate them at all

[kglad]

you misunderstand.

try:

package
{
    import flash.display.MovieClip;
    import flash.geom.Point;
   
    public class CornerFinder extends MovieClip
    {
        //vars
        var myRect:MovieClip;
       
        //constructor
        public function CornerFinder():void
        {
            myRect = new MyRectangle();
            myRect.x = stage.stageWidth/2;
            myRect.y = stage.stageHeight/2;
            myRect.rotationZ = 270;
            addChild(myRect);
           
            AddCornerMarkers();
        }
       
        private function AddCornerMarkers():void
        {
            //The rotation of the rectangle in radians
            var rectRotation = myRect.rotationZ*(Math.PI/180);
           
            var corners:Array = new Array();

myRect.rotationZ=0;

           
            corners[0] = new Point(myRect.x, myRect.y);
corners[1] = new Point( (myRect.x + myRect.width * Math.cos(rectRotation)), (myRect.y + myRect.width*Math.sin((rectRotation) )) );
           // corners[2] = new Point( (myRect.x + myRect.width * Math.sin(-rectRotation)), (myRect.y + myRect.height*Math.cos((rectRotation) )) );
          //  corners[3] = new Point(corners[2].x + (corners[1].x - corners[0].x) , corners[2].y + (corners[1].y - corners[0].y));

myRect.rotationZ=rectRotation*180/Math.PI;
           
            for(var i:uint = 0; i < corners.length; i++)
            {
                var marker:MovieClip = new CornerMarker();
                marker.x = corners.x - marker.width/2;
                marker.y = corners.y - marker.height/2;
                trace("Adding marker at: " + marker.x + ", " + marker.y);
                addChild(marker);
            }
        }
    }   
}

[bmeutzner]

This could be accomplished by using the transform matrix for your rectangle.  Try the following:

var matrixClone:Matrix = rectangleItem.transform.matrix.clone();

matrixClone.rotate(degreesOfRotation * (Math.PI / 180));

Then using the original boundary points of your rectangle, use the transformPoint() method of the rotated matrix:

ie.

var newTopLeft:Point = matrixClone.transformPoint(originalTopLeft.x, originalTopLeft.y);

var newTopRight:Point = matrixClone.transformPoint(originalTopLeft.x + rectangleItem.width, originalTopLeft.y);

...

Do the transformPoint call on each of the corners to return the new point location based on the rotation.

Realize this is a very late reply to the thread, but hopefully this helps out folks in the future.