Cautionary Notes Flash

Flash retains existing .JPEG compression levels on any imported .JPEG image, but, unless specified otherwise in the Library, it reapplies .JPEG compression when the movie is published or exported. Recompressing an image that has already been compressed usually leads to serious image degradation, due to the introduction of further compression artifacts. When importing .JPEGs (and other bitmaps), you’ll note that Use document default quality in the Library is checked by default. This is a feature, not an annoyance. That’s because (a) Flash has a relatively generic .JPEG compression engine, which is easily surpassed by both Fireworks and Photoshop, and because (b) as mentioned previously, recompressing a .JPEG is routinely disastrous to image quality.

Apply compression settings to each individual bitmap in the Flash Library to determine the quality that you need before you use the general .JPEG settings in the Export Movie or Publish Settings dialog.

Bitmap shift
There is a known problem in Flash that’s referred to as bitmap shift, which means that colors may shift slightly from one instance to another of the same image. This has been attributed to several reasons. Some developers have reported that turning off compression has, at times, eliminated problems with bitmap shift. Another reported method for eliminating bitmap shift is to make the image a symbol, and then assign it an alpha of 99 percent. Yet the clearest explanation, and related fix, are as follows: Flash renders a bitmap while animating or transforming it, and then rerenders the bitmap as a static image when the motion or transformation ceases.

Often, the two don’t quite match. From this perspective, the optimal solution is to set the final bitmap’s scaling to 99 percent. The advantage of this solution (aside from the fact that it works) is that it’s less processor intensive, because any alpha adjustment burdens the processor with computations.

Cross-browser consistency
We’ve received more than a few queries about image formats and Flash’s capability to transcend issues of browser inconsistency, so here’s the answer. Many image formats, such as .PNG, are not supported across all browsers. When you import such an image format into Flash and publish or export to the .SWF format, you have accomplished browser independence because the .SWF is browser independent and because the image has been encapsulated within the .SWF format. (The image is not being sent to the browser in the imported format and then magically empowered to display.) Conversely, if you export any .FLA to .PNG or to any other format that’s subject to cross-browser inconsistency, browser independence is lost.


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