Comments on: Using BitmapData.hitTest for Collision Detection

By: anon

anon — Mon, 15 Feb 2010 21:32:35 +0000

What if the two movieclips you wish to compare are composed of many nested clips, all of which could be at any arbitrary x/y value and any arbitrary scale? It seems like the matrix transformation trick works if only the first layer is off center or rotated, but if the clip is many layers deep and the transformations are completely random for each layer, it’s not working.

For instance, what if blueClip were like this:

blueClip x=20, y=50
+ blueClipChild x = 400, y = -20
+ blueClipChildChild x = -1309, y = 20
+ Shape (square, 30px/300px at x = 10, y = -280)

How would you solve for this?

By: Claire

Claire — Tue, 09 Feb 2010 14:15:08 +0000

Thanks very much for posting this Mike.

I’m pretty new to AS3 and I’ve spent weeks investigating the most effective way to implement hit testing in the platform game I’m developing.

I’ve not found a conclusion this comprehensive anywhere, and it all makes total sense.

By: Converting from a Matrix3D to Matrix in ActionScript 3 at Mike Chambers

Converting from a Matrix3D to Matrix in ActionScript 3 at Mike Chambers — Wed, 09 Sep 2009 17:48:09 +0000

[...] I made a post earlier showing how (with much, much help from Senocular), I was able to use Matrix to do hit tests using BitmapData.hitTest on DisplayObjects which have had transformations applied to them (in this case, [...]

By: Aaron

Aaron — Wed, 12 Aug 2009 19:55:36 +0000

Brad, use the second example Mike posted. It uses the transform matrix to counter any offset created from a registration point not in the top left, as well as counter any rotation or scaling. The part that does this is:

var blueOffset:Matrix = blueClip.transform.matrix;
blueOffset.tx = blueClip.x – blueRect.x;
blueOffset.ty = blueClip.y – blueRect.y;

blueClipBmpData.draw(blueClip, blueOffset);

And same for the redClip.

By: hannah

hannah — Wed, 12 Aug 2009 16:20:47 +0000

Thanks, this is just what I had been looking for. It works with my own objects that change position, too.

By: brad

brad — Wed, 29 Jul 2009 17:45:05 +0000

This works great with clips that are aligned top-left but when I used center aligned clips only the center point detects as collision. I thought “blueClip.x – blueClip.width/2 ” would be the correct point for a centered clip but it isn’t working as expected. Why is that?

By: SparK

SparK — Tue, 21 Jul 2009 03:36:37 +0000

var performanceBoost = new Refactor();

if(isCDKOpenSource){
CDK.read();
CDK.edit(performanceBoost);
CDK.publish();
}

By: Relative performance for collision detection techniques in ActionScript 3 at Mike Chambers

Fri, 26 Jun 2009 22:28:24 +0000

[...] you have read my blog any this week, you have probably noticed that I have been doing some basic research on collision detection [...]

By: Alan Shaw

Alan Shaw — Thu, 25 Jun 2009 16:58:11 +0000

Thanks for this valuable guide. It’s something that people have been spending time and effort to figure out for themselves since Flash 8 came out.

I do have to say that all this clearly represents the intent of those who designed the API, and therefore should have been incuded in the documentation from the start.

Here’s another example: the effect of the mergeAlpha parameter of copyPixels(). Moock states in EAS 3.0 that it uses the SCREEN blending mode, and gives the precise formula. Why don’t the docs mention this?

And of course the documentation on the DisplacementMapFilter — don’t get me started…

By: 25/06/2009 « Robertopriz’s Weblog

25/06/2009 « Robertopriz’s Weblog — Thu, 25 Jun 2009 13:13:55 +0000

[...] bitmapdata collision detection http://www.mikechambers.com/blog/2009/06/24/using-bitmapdata-hittest-for-collision-detection/ [...]