While Flex uses bilinear resizing method when scaling images (event for Bitmap.smoothing() method), you won’t be able to get nice results when downscaling more than 2 times. Inspired by ImageResizer class I have also created SmoothImage Class. This object extends mx.Image and uses ImageResizer.bilinearIterative() class in order to generate smoother resized results.

In following example mx.Image, SmoothImage and sk.yoz.image.SmoothImage used to demonstrate resizing original 1024x679px image.

sk.yoz.image.SmoothImage class:

package sk.yoz.image
{
    import flash.display.Bitmap;
    import flash.display.BitmapData;
    import flash.geom.Matrix;
    import flash.geom.Point;

    import mx.controls.Image;

    import sk.yoz.math.ResizeMath;

    public class SmoothImage extends Image
    {
        protected var _resizeFunction:String = "bilinearIterative";
        protected var _iterationMultiplier:Number = 2;
        protected var resizedBitmap:Bitmap;

        public function SmoothImage()
        {
            super();
        }

        [Bindable(event="resizeFunctionChanged")]
        [Inspectable(category="Other", defaultValue="bilinearIterative",
            enumeration="bilinear,bilinearIterative")]
        public function set resizeFunction(value:String):void
        {
            if(value == resizeFunction)
                return;
            _resizeFunction = value;
            resizeBitmap();
        }

        public function get resizeFunction():String
        {
            return _resizeFunction;
        }

        [Bindable(event="iterationMultiplierChanged")]
        public function set iterationMultiplier(value:Number):void
        {
            if(value == iterationMultiplier)
                return;

            _iterationMultiplier = value;
            resizeBitmap();
        }

        public function get iterationMultiplier():Number
        {
            return _iterationMultiplier;
        }

        override protected function updateDisplayList(unscaledWidth:Number,
            unscaledHeight:Number):void
        {
            super.updateDisplayList(unscaledWidth, unscaledHeight);
            resizeBitmap();
        }

        protected function resizeBitmap(... rest):void
        {
            if(!width || !height || !content)
                return;

            if(resizedBitmap && resizedBitmap.parent)
                resizedBitmap.parent.removeChild(resizedBitmap);

            resizedBitmap = new Bitmap(resizedBitmapData);
            addChild(resizedBitmap);
            content.visible = false;
        }

        protected function get resizeMethod():String
        {
            return maintainAspectRatio ? ResizeMath.METHOD_LETTERBOX
                : ResizeMath.METHOD_RAW;
        }

        protected function get resizedBitmapData():BitmapData
        {
            var bitmapData:BitmapData;

            if(content is Bitmap)
            {
                bitmapData = Bitmap(content).bitmapData
            }
            else
            {
                bitmapData = new BitmapData(content.width / content.scaleX,
                    content.height / content.scaleY, true, 0);
                bitmapData.draw(content);
            }

            var dimensions:Point = ResizeMath.newDimensions(
                new Point(bitmapData.width, bitmapData.height),
                new Point(width, height), resizeMethod, true);

            return ImageResizer[resizeFunction](bitmapData, dimensions.x,
                dimensions.y, resizeMethod, true, iterationMultiplier);
        }
    }
}

Update (Jan 27, 2010): in sk.yoz.image.SmoothImage

Update (Jun 14, 2010): in sk.yoz.image.SmoothImage