How video editing softwares change the FPS from ~29.87 FPS to 30 FPS. Where do the extra frames come from?
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If you simply need to play back at little higher rate, you can simply speed up the video — a 1% speed up is going to be imperceptible.
However, for video editing, it’s a tad bit more complicated. You might be taking video of different frame rates and combining them to make a video of yet another rate.
In this case, the software simply maps a frame to a time (maybe as millisecond resolution) in the timeline. When you go to any point in the timeline, it figures what frame is displayed at that specific time and uses that. The frame rate is pretty much ignored.
For example, imagine two videos: A with a frame rate of 3 fps, and B that’s 4 fps, and we are going to add them to a project that will be 10 fps. Each frame in A is 333.33 ms and in B 250 ms. So in video A frame 0 is 0-333.33 ms, 1 is 333.34-666.66 ms, etc. For video B, frame 0 is 0 to 250 ms, 1 is 250.01 to 500ms, etc. For the 10 fps timeline, it has frames at 0, 100, 200, 300, 400, 500, 600 ms … etc. We can simply figure out which frame is displayed at a given time. If we write it (Time, Frame A, Frame B): (0, 0, 0), (100, 0, 0), (200, 0, 0), (300, 0, 1), (400, 1, 1), (500, 1, 1), (600, 1, 2), … and so on.
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