Exporting videos on iOS: understanding and setting frame duration property?

Question

In this tutorial on merging videos, the author sets the frame duration for the exported video to 30 FPS.

1) Instead of fixing the frame duration to 30 FPS, shouldn't the frame duration be tied to the frame duration of the videos getting merged?

2) When exporting videos, what are the pros/cons of using a different FPS for the exported video that differs from the source video(s)? Is this one way of speeding up export time at the expense of video quality? For instance, what if the source videos in the tutorial were captured at 24 FPS?

We need to export videos on iOS, sometimes merging multiple videos, sometimes exporting a single video captured from the camera device.

Answer 1

1) When possible, yes.

2) When source FPS and target FPS are the same, the target video will have better quality and (most possible) the export will be a faster process.

Think of it this way:

When we have e.g. 24FPS source and 24FPS target, the process is just getting a source frame and producing a target frame.

When we have e.g. 24FPS source and 30FPS target, the process will either be:

1. get 1st frame source -> 1st frame target,

   2nd frame source -> 2nd frame target,

   2nd frame source -> 3rd frame target,

   3rd frame source -> 4th frame target, and so on.

or

2. get 1st frame source -> 1st frame target

   blend 1st frame source with 2nd frame source -> 2nd frame target

   blend 2nd frame source with 3rd frame source -> 3rd frame target

   get 3rd frame source -> 4th frame target, and so on.

so to produce 30 frames from 24 frames for every second.

Method 1 may not suffer in processing speed, but method 2 does because of the blending involved.