Two-part question on slow motion


#1

Two-part question on slow motion quality.

  1. If input is 60p and export speed is 0.500x, does Shotcut use only existing frames, simply running them at ½ speed? Would this give better quality than other export speeds?

  2. What is the best slow motion setting for input 60p, output 24p with motion at 0.4x original?

I have done both of these:
Export 1: speed 0.400x, output 60fps
Result: slow motion, appears to be 24fps.
Export 2: speed 0.400x, output 24fps
Result: slow motion, appears to be 24fps.
Which is better for quality?


Video mode - blank clips in timeline
#2

Shotcut drops or duplicates frames to achieve the desired speed. If you set speed to 0.5x, Shotcut will duplicate every frame to make it run slow. If you set the speed to 2.0x, Shotcut will drop every other frame to make it run faster.

Framerate conversion occurs separately from this process. But the idea is the same: duplicate or drop frames to achieve the desired rate.

When both speed and framerate is being changed at the same time, these processes coordinate to find the best ratio of drop/duplicate.

Any time you have a process that drops/duplicates frames, you have a risk of a studdery motion effect because you may not be able to display all frames with the exact temporal spacing. For example, in order to achieve your desired result, Shotcut may have to duplicate frames in some pattern like: 2, 3, 2, 3, 2, 3, 2, 3. Due do that, you will always have a reduced studder effect if you have a higher frame rate on the output. So I would expect 60fps output to appear “smoother” than 24fps.


#3

Thanks for the explanation and suggestion on quality.

I’m still confused (not a technical person :confused:) about one thing:

If I start with 60p and want 1/2 speed (slow motion) output, why do any frames need to be dropped or added? Each second of 60p video has 60 frames; if I use those same 60 frames over a duration of two seconds, I would have 1/2 speed (same action in twice the time).

Mathematically it would seem that Shotcut could simply output at 30p, using all the frames. Wouldn’t this yield 1/2 speed, using all existing frames and not adding any?


#4

Yes. That is mathematically correct, and you can do that with Shotcut. Just set your Video Mode (Settings->Video Mode) to 30p and Shotcut will do exactly that.

The reason that isn’t the default behavior is because in the most common use cases, the user has some target output video format that they want to use and their project will be a mix of regular speed and modified speed clips. So Shotcut will always convert to the Video Mode rate.


#5

This is good news! Keeps things simple and maintains quality.
Thanks for all your help and answers to my questions.
Shotcut is an incredible program and has allowed me to salvage tilted videos and put together projects that would have been difficult to impossible with Avidemux. (I still love Avidemux, but… :wink: )