// Create the bitmap
var imgData:BitmapData = new BitmapData(500, 500, true, 0x00000000);
var bmp:Bitmap = new Bitmap(imgData);
// Invoke lock( )
imgData.lock( );
// Set pixel color-values
var color:uint;
for (var i:int = 0; i < imgData.height ; i++) {
    for (var j:int = 0; j < imgData.width; j++) {
        color = Math.floor(Math.random( )*0xFFFFFFFF);
        imgData.setPixel32(j, i, color);
    }
}
// Invoke unlock( )
imgData.lock( );

Using lock() and unLock() saves about 20% when doing bulk pixel updates.

BitmapData Set Each Pixel To Random Color

ActionScript 3