Blackmagic Forum

I've recorded some slow motion action videos, in order to use them in a project.
My goal was to record the action them in the slowest possible speed, so that I could speed them up in Resolve later, if needed.
Videos were recorded in 60fps, the same as the project, and when played, the displayed action is smooth.

I did it in order to avoid having to eventually apply a slow motion effect in Resolve, because I assumed I wouldn't have problems speeding up those videos later in Resolve if needed. In my knowledge, I would only have problems if I had to slow down the videos in Resolve, because it would cause the animation to lose its smoothness, forcing me to use some frame interpolation tech at the cost of some video artifacting.


So after recording, I started working in Resolve, and for my surprise, when increasing the speed of the slow motion video from 100% to 150%, it's not smooth, like if the video was skipping frames all the time.
However, if I set the speed to 200% (instead of 150%), it's smooth again.

Note I'm speaking about the final render. I'm aware playback in fast forward is costly for Resolve, but that's not the case here.

I'm a beginner in video editing, but I assume it might be some general video rule, although it doesn't make sense to me.

Why does this happens, assuming the original video has extra frames (due to being already recorded in slow motion) to allow the speed up?

alb3530 wrote:I assume it might be some general video rule

In order of quality of results.

1. Record at the frame rate you need for the shot. Make that decision when you film, not in post.

2. Change the frame rate in post.

Jim Simon wrote:

alb3530 wrote:I assume it might be some general video rule

In order of quality of results.

1. Record at the frame rate you need for the shot. Make that decision when you film, not in post.

2. Change the frame rate in post.

I know that would be the ideal way to do it, but due to the nature of the specific project, I couldn't escape changing the video speed in Resolve. I cannot give details about the project, but I would eventually have to change the speed of some clips in this project in Resolve.

It's a project requirement the source video to have 60 frames per second, so source footage features the slowest-possible motion action when played in 60 frames per second, in order to have a greater number of action frames to work with.

When in Resolve, if I add that footage to the project:

-When clip speed stays in 100%, the motion action is smooth;
-When clip speed is changed to 150%, the motion action is not smooth;
-When clip speed is changed to 200%, the motion action is smooth;



I'd just like to know the technical explanation why 100% and 200% cases are ok, but 150% isn't.

alb3530 wrote:It's a project requirement the source video to have 60 frames per second, so source footage features the slowest-possible motion action when played in 60 frames per second, in order to have a greater number of action frames to work with.

When in Resolve, if I add that footage to the project:

-When clip speed stays in 100%, the motion action is smooth;
-When clip speed is changed to 150%, the motion action is not smooth;
-When clip speed is changed to 200%, the motion action is smooth;

I'd just like to know the technical explanation why 100% and 200% cases are ok, but 150% isn't.

So your recording fps and project timeline fps are both 60 fps? That's 1:1 real time playback, not slow motion, though motion looks ”smooth”. Perhaps that's what you meant?

I think the default for Project settings - Master settings - Frame interpolation is "Nearest". This means that for:
Speed 200 %, Resolve can use every other frame: 1, 2, 3, 4, 5, 6, …
Speed 150 %, doesn't fit evenly: 1, 2, 3, 4, 5, 6, …

Try setting the interpolation to "frame blend" or "optical flow". There may still be artifacts, but should be less jumpy at least.

ttakala wrote:
I think the default for Project settings - Master settings - Frame interpolation is "Nearest". This means that for:
Speed 200 %, Resolve can use every other frame: 1, 2, 3, 4, 5, 6, …
Speed 150 %, doesn't fit evenly: 1, 2, 3, 4, 5, 6, …

Try setting the interpolation to "frame blend" or "optical flow". There may still be artifacts, but should be less jumpy at least.

I understood your explanation about 150%, thanks.

It was all I needed to know.

Thanks again!