The SWF export features in Adobe After Effects 5 make it easy to add video content to your SWFs. The following five tips will help you get the best results with the lowest file size.

Frame Rates and Posterize Time

Reducing the frame rate of video in a SWF is essential to minimizing file size. To reduce 30fps video to a minimal Web frame rate (say, 9fps), drop the video into an After Effects composition and set the comp frame rate to 9fps. Unfortunately, because Flash is frame-based and not time-based, if you import that 9fps SWF into an 18fps Flash movie, it will run twice as fast as you intended (each imported frame will be adjacent to the next one), meaning you'll have to redistribute each frame in Flash by hand.

To get around this issue, you can try After Effects' Posterize Time filter. Posterize Time reduces the frame rate of the footage to which it's been applied, effectively giving the footage a different frame rate than the composition that contains it (something not normally possible in SWFs). So in our example, you would set the composition to 18fps and apply a Posterize Time setting of 9fps to the imported video. Unfortunately After Effects will export each frame twice, but it will have the effect of giving the correct timing in Flash. To optimize the file you can remove every other keyframe by hand (a little tedious, but faster and more reliable than redistributing frames by hand). One note of caution: Posterize Time will rasterize any content to which it's applied, so don't use it on vectors or text animation.

Alpha Channels in Video

Video processed with After Effects and sent to SWF will have an 8-bit alpha channel if necessary. That means that you can rotate video on a 3D plane, for example, and the individual frames will have enough transparency information to blend smoothly with other elements when you import the SWF into Flash. Be aware that when using 3D, After Effects rasterizes the frame, not just the area where the video exists on that frame. It therefore makes sense to apply any 3D effects in a comp that's sized down to just what the animation demands.

Seperating Vector and Bitmap Content

You may want to combine video and animated objects in After Effects but keep them separate for further work in Flash. Luckily, After Effects will export only what's visible. To export separate SWF, enable the solo key for the layers you want in each SWF, export each combination as SWF, and import the SWFs together in Flash. For example, it may make sense to keep Illustrator layers together and prevent them from being rasterized with video underneath them. By soloing the Illustrator layers and exporting them together as one SWF, then soloing the video and exporting it separately, you can keep each in its most efficient form for SWF.

Nested Compositions

Remember that nested compositions are rasterized when exported to SWF, regardless of contents.

Frame Rates

Finally, don't be afraid of dropping video frame rates well below what you'd otherwise consider the minimum possible, and consider scaling or stretching video inside Flash. Juxtaposing short bursts of low-res video with smooth type animation, for example, can provide some interesting results. See the work of Hillman Curtis for some creative examples of using limited image sequences efficiently.