Preserving Bitmap Quality | Macromedia Flash8 Bible

When you choose to use bitmap images, remember that they won't scale as well as vector drawings in the authoring environment. Furthermore, bitmaps will become degraded if the viewer scales your final movie so that the bitmap is displayed larger than its original size. Here are a few points to consider that will help you maintain the quality of your presentation when using bitmaps:

Know your audience, and design for the largest screen (at the highest resolution) that your audience may have. Or, if you deviate from this, remember that audience members with optimal equipment will see a low-quality version of your work. If you're using ActionScript to load image assets, consider having low-res and high-res versions of the images available.
Measure your largest hypothetical image dimensions in pixels. One way to determine these dimensions is to use the Flash Info panel to read the size of a placed image or a placeholder shape. Another way is to take a screen capture of your mock-up, and then measure the intended image area in Photoshop.
Create or resize your bitmap image to the maximum hypothetical dimensions. If there are any rotations or skews required, you may have to do a test to see if the final result is cleaner when the transformation is done in your image-editing program or in the Flash authoring environment. The improved bitmap smoothing in Flash 8 delivers better results with modified bitmaps than any previous version of Flash.
Import images into Flash at the maximum required size, and then scale them down to fit into your layout.

The advantage of using this approach is that the movie can be scaled for larger monitors without causing the bitmap image to degrade. The disadvantage is that it requires sending the same large bitmap to all users. A more sophisticated solution is to use JavaScript to detect browser dimensions and then send the appropriately scaled bitmaps to each user. Other workaround solutions that may help preserve the quality of your final presentation without adding file size include the following:

Restrict scaling capability of your published movie. You can do this by using HTML options in the Publish Settings or using ActionScript.
Set the bitmap's compression to Lossless (GIF/PNG) if it is already optimized in GIF format or if you want to preserve an alpha channel or editable filter effects in a .png or a .psd file.
Trace the bitmap to convert it to a vector graphic (covered later in this chapter).
Never apply double JPEG compression to your images. If you have compressed and saved images in JPEG format outside of Flash, be certain to select the Use imported JPEG data check box when importing the images to Flash.

Before sizing and importing bitmaps, you need to consider how you will set the dimensions for the Flash movie (.swf) in the HTML tab of the Publish Settings dialog box. You also need to know whether the bitmap is to be scaled in a Motion tween. If the Flash movie scales beyond its original pixel width and height (or if the bitmap is scaled larger in a tween), then bitmap images will appear at a lower resolution with a consequent degradation of image quality.

If you're uncertain of the final size that you need for a bitmap in Flash, it may be best to import a temporary low-resolution version of the image — being careful to store your original high-resolution version where you can find it later. Whenever you need to place the bitmap, drag an instance of the symbol onto the Flash Stage. Then, during final production and testing, after you've determined the required pixel size for the maximum scale of the final bitmap, create and swap-in a higher-resolution image, as follows:

Double-click the icon of the original low-resolution bitmap in the Flash Library to access the bitmap's properties.
In the Bitmap Properties dialog box, click the Import button and select the new, higher-resolution version of the bitmap.

After import of the high-res image, all instances of the bitmap will update automatically, with the scaling, animation, and placement of the image maintained.