
Unreal Engine – Rendering at Different Frame Rates
In the Movie Render Queue in Unreal Engine there is an option in the output settings to render in a different frame rate. This at first seems, fairly simple, but it is quite nauced with many different results possible.
Rendering at Different Frame Rates
Rendering at different frame rates basically increases or decreases the number of frames per second. This may increase the overall length of the shot, but this depends if the ‘playback frame rate’ is the same as the ‘rendered footage.’ For example, if footage at 24fps is rendered at 120fps, but then played back at 24fps then the playback will be smooth and in slow motion, and play at a longer duration then the original clip.
See the explanation below:
If you have a clip of 3.5 seconds and you choose to render this out at different framerates you will get the results below:
- Render the footage at 12 fps = 42 total frames.
- Render the footage at 24 fps = 84 frames.
- Render the footage at 120 fps = 420 frames
So, obviously, you have different total frames when rendering at higher or lower frame rates. But will this increase the playback time? This depends.
If you play back consistently at 24 fps then yes this will definitely effect the playback times and either increase or decrease the time it takes to play the clip. For example if you have 420 frames and play this back at 24 fps then it will take 17.5 seconds. Although the original shot was only 3.5 seconds! This is simply because the render has created additional frames.
So to play all of the rendered frames at 24 fps per second it needs more time to fit all the frames in.
If you play footage rendered at 120 fps which comes to 420 frames and you play that back at 3.5 seconds (the original time) the footage will play silly quick! And will go from slow motion to super fast motion!
Consistent Clip Duration | 3.5 Second Clip (Playing Back at 24fps) |
---|---|
Rendered At Frame Rates | Total Frames and Playback Time |
12fps | 42 total frames Playback at 24fps = 1.75 seconds |
24fps | 84 total frames Playback at 24fps = 3.5 seconds |
120fps | 420 total frames Playback at 24fps = 17.5 seconds |
Why render at Different Frame Rates?
There are different reasons for rendering at different frame rates, and some of these reasons, may just be an artistic decision.
If you render at a high frame rate, and play back at 24fps you will get a very smooth and crisp playback. This though will be in slow motion as the same action is occuring, but many more frames have been rendered.
It’s a bit like flicking through a sequence of cards and the only difference between 12fps and 120fps is that 12fps only covers a few phases of the characters movement, where as 120fps covers every little detail!
However, at the same time you will lose motion blur and motion blur is one of the elements that gives video a cinematic look. However, you do get excellent slow motion sequences with higher frame rates, so you may wish to render your action sequences at higher frame rates, whilst the rest of your footage is at the standard 24fps and the overall playback speed would still be 24fps.
Therefore two key elements for changing the speed, motion blur and smoothness of your video are:
- Playback Frame Rate (for cinema this is usally 24fps).
- Rendered Frames to play back (this is what ever frame rate the video was rendered at).