Is it possible to keyframe Time Remapping?

[Steve Bentley]

The values of the keyframes are the original footage frames. So if you put a keyframe at frame 50 of the time line but make the keyframe’s value 100, you will have sped up the footage by a factor of 2.
Keep in mind though that the Time Remapping feature, while handy doesn’t “invent” the missing frames, it simply grabs the closest whole frame to the one the keyframes are asking for. Take the example above, instead of 100 use 105. So now on timeline frame 50 you will see the original frame 105, so far so good. But go back one frame in the time line. What frame should be there? – technically it should be frame 102.9. Well there is no 102.9. So AE will display frame 103. If you keep going back one frame at a time by the time you get to timeline frame 45, you will end up seeing source frame 94 twice. This can have a choppy effect that doesn’t look like slow motion, or in this case smooth sped up motion.
Precomposing your source footage that has been shot at a much higher frame rate than is needed can help. Then use time remapping on the precomp, but in the outer comp. Because there are more frames to choose from per second in the precomp than the frame that is needed, the time remap will have a better chance of plucking out the frame it needs if the frame rate on the source footage is very high.
But if you want a true slow-mo or smooth speed-up, you must use the built in time warp effect or a third party time warper like Twixter. These will render a new frame based on the pixels of the frames either side and “invent” frames that were never shot, but correctly position objects where they would have been if shot at that frame rate. Its never perfect (especially with objects suddenly appearing from out of frame) but its surprisingly good and again better and better if the source is shot at a higher frame rate than you need (even if you are going to speed up the clip in the end – the more frames these warpers have to work with the more data they have to make the missing frames.

[Sam Mattson]

Note that you can also switch your frame blending on or toggle it to pixel motion (the little film icon in your switches/modes). This won’t give you quite as good of results as Twixtor will, but in many cases it works for what you’re doing.