ActionScript 3.0

AS3: BitmapData Foibles

Today I ran into what looks like an odd oversight in Adobe’s otherwise excellent BitmapData class. Alternatively, it could be an oversight in my otherwise excellent critical thinking skills. Neither is unheard of, but either way, someone is missing something somewhere.

Here’s the scenario: I had loaded an image via a Loader, and I wanted to capture a chunk out of the middle of it. For the sake of this example, let’s say I wanted a 200×200 square, starting at (50, 50) in the source image. So, foolishly and naîvely, I used this code:

	// create the BitmapData and draw() the contents of sourceImage into it
	var bmpData:BitmapData = new BitmapData( 200, 200, true, 0x00000000 );
	bmpData.draw( sourceImage, null, null, null, new Rectangle( 50, 50, 200, 200 ), true );
 
	var bmp:Bitmap = new Bitmap( bmpData, PixelSnapping.ALWAYS, true );
 
	// add to display list

So imagine my surprise when I got this:

This was definitely not what I was expecting, but apparently it is the intended functionality of BitmapData.draw(). I expected this image to line up with the upper left corner of the Bitmap container but as you can see, it doesn’t. It does indeed start copying pixels at (50, 50) on the source image, but when added to a Bitmap, it begins drawing those pixels at (50, 50) as well. That means that the whole thing is shifted 50 pixels along both axes, leaving empty space at the top and left while cutting off pixels at the right and bottom. This, to me, seems like a Bad Thing.

I’ve played with this for a couple hours now and I can’t find a way to do this without resorting to what I think most people would term a “workaround.” This is why I think that either Adobe just left out something, or (more likely), that I am just not grasping how this whole BitmapData/Bitmap thing works.

Anyway, this is my workaround. In my traversals of the AS help files, I noticed that there is a method that allows you to specify both things that I need: coordinates of a set of pixels on the source image, and coordinates for where to put said pixels on the target image. That method is BitmapData.copyPixels. However, it only works between two instances of BitmapData.

So, I created two instances of BitmapData. To make it easier to use, I placed it in a static class called BitmapGrabber.

public static function grab( source:DisplayObject, rect:Rectangle, smoothing:Boolean ): BitmapData
{
 
	if( !source is IBitmapDrawable )
	{
		throw new Error( "Cannot create BitmapData.  Source must implement IBitmapDrawable" );
	}
 
	var bmpData1:BitmapData = new BitmapData( source.width, source.height, true, 0x00000000 );
	var bmpData2:BitmapData = new BitmapData( rect.width, rect.height, true, 0x00000000 );
 
	bmpData1.draw( source, null, null, null, null, smoothing );
	bmpData2.copyPixels( bmpData1, rect, new Point( 0, 0 ) );
 
	bmpData1.dispose();
 
	return bmpData2;
}

And this is me using the class:

// create the BitmapData and draw() the contents of sourceImage into it
var bmpData:BitmapData = BitmapGrabber.grab( sourceImage, new Rectangle( 100, 100, 100, 100 ), true );
var bmp:Bitmap = new Bitmap( bmpData, "auto", true );
 
// add to display list

And this is what I get:

It’s a ridiculously simple solution. However, I feel so strongly that BitmapData should have this capability (without a workaround), that I can’t shake the feeling that it must have this capability and I am just missing it. But, until some member of the Flash community tells me a better way to accomplish this, this is the class I’ll be using in my code.

Tags: Actionscript 3.0, Bitmap, BitmapData