Collision Detection of Sprites in Actionscript 3.0 Flash

Question

I am making an achtung die kurve-like game in AS3.0. So far I\'ve done the movements of the 4 different players, and it works alright.

I am now to make collision de

Answer 1

Try this function if you want a Pixel Perfect Collision Detection with efficient CPU usage:

trace("Collided: " + (areaOfCollision(mc1, mc2) != null));
trace("Where: " + areaOfCollision(mc1, mc2));

function areaOfCollision(object1:DisplayObject, object2:DisplayObject, tolerance:int = 255):Rectangle {
    if (object1.hitTestObject(object2)) {
        var limits1:Rectangle = object1.getBounds(object1.parent);
        var limits2:Rectangle = object2.getBounds(object2.parent);
        var limits:Rectangle = limits1.intersection(limits2);
        limits.x = Math.floor(limits.x);
        limits.y = Math.floor(limits.y);
        limits.width = Math.ceil(limits.width);
        limits.height = Math.ceil(limits.height);
        if (limits.width < 1 || limits.height < 1) return null;

        var image:BitmapData = new BitmapData(limits.width, limits.height, false);
        var matrix:Matrix = object1.transform.concatenatedMatrix;
        matrix.translate(-limits.left, -limits.top);
        image.draw(object1, matrix, new ColorTransform(1, 1, 1, 1, 255, -255, -255, tolerance));
        matrix = object2.transform.concatenatedMatrix;
        matrix.translate(-limits.left, -limits.top);
        image.draw(object2, matrix, new ColorTransform(1, 1, 1, 1, 255, 255, 255, tolerance), BlendMode.DIFFERENCE);

        var intersection:Rectangle = image.getColorBoundsRect(0xFFFFFFFF, 0xFF00FFFF);
        if (intersection.width == 0) return null;
        intersection.offset(limits.left, limits.top);
        return intersection;
    }
    return null;
}

After a successful preliminary hitTestObject(), this function backgroundly takes a snapshot from the shapes of both objects painted with different colors each, then overlays them intersecting the colors on a new one, returning the Rectangle of the resulting shape. So cool.

To learn more about Pixel Perfect Collision Detection you can google Collision Detection followed by one of these names: "The ActionScript Man", "Troy Gilbert", "Boulevart (wim)", "Grant Skinner (gSkinner)" or "Senocular". Those guys are awesome AS3 references by the way.

Answer 2

Looking at the screenshots of that game, I think the best model would be to describe each worm as a chain of circles. Then divide the world/level in a grid with cells somewhat larger than the circle radii.

The collision check would then be:

1. clear grid
2. place each circle into the 1 or more grid cells it falls in
3. iterate over all cells, for each cell:
   • for each pair of circles (partially) in this cell, check if they intersect. If they do; collision. Note that this may result in more than 1 collision occurrence between circle A and B, so you'd also need to check that to avoid duplicates.

Step 1 and 2 can be optimized by not clearing the grid, and instead of step 2, updating each circle's cell after it moves. If you size your cells like 5x the size of a circle, a circle can stay in the same cell for a few frames, avoiding excessive add/remove operations.

I'm doing something similar in a project of mine right now, except with space ships! My grid cells are currently 256x256 (too big for your project I think) and my units have radii of about 20.

Answer 3

The problem you discribe is a very common problem for collission detection because the object has a set width and height and therefor defines a rectangle as the object.

There is a solution however to make a colission detection system on pixel level I have found this on the official site and this made me able to make collission detection for bitmaps on pixel level.

http://help.adobe.com/en_US/ActionScript/3.0_ProgrammingAS3/WS5b3ccc516d4fbf351e63e3d118a9b90204-7d55.html

hope it helps you out in the same way.