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Frame rate

lastmod 2023-09-25created 2023-09-251 edits

Frame rate is the speed at which a display device shows the data of one screen; in Korean it is also rendered as "frame speed" (frame速度) or "frame ratio" (frame率).

Frames per second (秒當frame數), used with the same meaning, refers to the number of screens shown in one second; fps (or frames/second) is used as its Latin-alphabet abbreviation and unit.

In Japan, 18fps or 15fps video is sometimes produced to cut the budget when making animation. When produced for BD, the fps figure generally goes up and correction is also added. Broadcast TV and DVD often exceed 20fps by default.

Because display usually relies on the eye's afterimage, 30 or 24 or more times per second are needed.

Variable Frame Rate

Variable frame rate (Variable Frame Rate; VFR) is a very efficient recording method that saves the output at the optimal FPS value the recording target can achieve, reducing file size.

Constant Frame Rate

Constant frame rate (Constant Frame Rate; CFR) is a method that always saves the output at a fixed 60FPS (CFR).

How to fix VFR to CFR

To fix it at 30 frames: 

ffmpeg -i <input> -vsync cfr -r 30 <output>

How to check VFR/CFR

Just use ffprobe 

ffprobe -v quiet -print_format json -show_streams input

And check for the avg_frame_rate parameter of the desired video stream. If the value of this parameter is a fraction like this 

"avg_frame_rate": "1205285219/50270287"

with a denominator that is not 1 then the video file has a variable frame rate.

If the value is like this 

"avg_frame_rate": "25/1"

See also

  • FFmpeg
  • FFprobe
  • Screen refresh rate (refresh rate)
  • Frame error rate (FER)

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