Recipe 8.8. Copying Pixels

Problem

You want to copy a rectangular area from one BitmapData to another.

Solution

Use the copyPixels( ) method of the BitmapData class.

Discussion

The theory behind the copyPixels( ) method is quite simple. It takes the pixel data from one bitmap and draws it onto another. In that sense, it is much like the draw( ) method. However, in the case of copyPixels( ), you get more control over how much of the bitmap is copied and where it is copied to. You do this by specifying a source rectangle and a destination point:

bitmap.copyPixels(sourceBmp, srcRect, destPoint);

The source rectangle is an instance of the flash.geom.Rectangle class. You use this to define a rectangular area of the original BitmapData. This is the only portion of that bitmap that is copied.

The destination point is an instance of flash.geom.Point. This specifies the x, y coordinate of the destination bitmap where you want to paste the copied pixels.

The following example shows how to copy several rectangular areas of a loaded bitmap onto another BitmapData:

package {     import flash.display.Sprite;     import flash.display.Bitmap;     import flash.display.BitmapData;     import flash.display.Loader;     import flash.net.URLRequest;     import flash.events.Event;     import flash.geom.Point;     import flash.geom.Rectangle;     public class AS3CB extends Sprite {         private var _bitmap:BitmapData;         private var _loader:Loader;         public function AS3CB(  ) {             _loader = new Loader(  );             _loader.contentLoaderInfo.addEventListener(Event.COMPLETE, onLoad);             _loader.load(new URLRequest("myphoto.jpg"));             _bitmap = new BitmapData(stage.stageWidth,                                  stage.stageHeight,                                  false, 0xffffffff);             var image:Bitmap = new Bitmap(_bitmap);             addChild(image);         }         public function onLoad(event:Event):void {             var loaderBmp:Bitmap = Bitmap(_loader.content);             var w:Number = loaderBmp.width / 10;             for(var i:int = 0; i < 10; i++) {                 _bitmap.copyPixels(loaderBmp.bitmapData,                                 new Rectangle(i * w, 0,                                               w, loaderBmp.height),                                 new Point(i * (w + 2), i));             }         }     } }

In the preceding code, the important part is highlighted in bold.

The defined rectangle copies progressive vertical strips from the loaded image, and the point places them in the new bitmap, spaced out by a couple of extra pixels.

There are a few more optional parameters. You can specify another BitmapData to use as an alpha channel, and you can choose to blend the alpha levels of the existing and new alpha bitmap. The syntax for this is:

bitmap.copyPixels(sourceBmp, srcRect, destPoint,                alphaBmp, alphaDestRect, blend);

This is a more advanced use of the metho. We won't cover it here in detail, but you are free to experiment with it on your own.

Problem

Solution

Discussion

See Also