There’s an interesting discussion going on on the OSFlash list about what was originally termed “double buffering”, but is more accurately “offscreen buffering”. This is when you create an instance of BitmapData, but don’t add it to the display list, and then do a whole bunch of drawing to it, and when you are done, drawing that bitmap onto another bitmap that IS on the display list. There was some discussion as to whether or not this would actually be quicker, so I did a quick test. I usually get into trouble when I do these timing tests, but I’m going to post it anyway.
Here we have three BitmapData instances. One is wrapped in a Bitmap object and put on the display list. The second is put in a Bitmap object, but not added to the display list. The third is just left alone. Then a million random pixels are drawn to each one. The results bear out not only that the off-display list version is quicker than the on-display list version, but it’s also slightly faster than the version that has the Bitmap object wrapping the Bitmap data, but not on the display list. I’m seeing on average about 13% difference between the high and low versions, which is a decent increase in speed.
[as]package {
import flash.display.Sprite;
import flash.utils.getTimer;
import flash.display.Bitmap;
import flash.display.BitmapData;
public class BlitTest extends Sprite
{
public function BlitTest()
{
var bmp1:BitmapData = new BitmapData(500, 500, true, 0);
var bmp2:BitmapData = new BitmapData(500, 500, true, 0);
var bmp3:BitmapData = new BitmapData(500, 500, true, 0);
var holder1:Bitmap = new Bitmap(bmp1);
addChild(holder1);
var holder2:Bitmap = new Bitmap(bmp2);
var start:int
var i:uint;
start = getTimer();
for(i = 0; i < 1000000; i++) { bmp1.setPixel(Math.random() * 500, Math.random() * 500, Math.random() * 0xffffff); } trace(getTimer() - start); start = getTimer(); for(i = 0; i < 1000000; i++) { bmp2.setPixel(Math.random() * 500, Math.random() * 500, Math.random() * 0xffffff); } trace(getTimer() - start); start = getTimer(); for(i = 0; i < 1000000; i++) { bmp3.setPixel(Math.random() * 500, Math.random() * 500, Math.random() * 0xffffff); } trace(getTimer() - start); } } } [/as]