Speed effects are different from other effects in that they alter the length of a clip. This puts speed effect into a separate effect category.
There are different challenges that arise with speed change.
- When slowing down, intermediate frames are needed. This is done e.g. by slowmoVideo using optical flow.
- When speeding up, motion blur may be desired to keep the image look natural
- Audio also changes. Is it simply stretched, whereby pitch changes as well, or should only the speed change (but not pitch) as done e.g. by Paulstretch?
Editing also poses challenges:
- It is impossible to edit inside the timeline due to the clip length change
- When allowing curves for speed change, the effect GUI becomes much more complex and does not fit into a widget anymore. slowmoVideo has an own UI just for that with a 2D canvas. The input video is on one axis and the output video on another axis, and a curve defines which frames the output video picks from the input video.