How to set `videoTimescale` / the purpose of `frameRate`

[branks]

I was looking for how to set videoTimescale on an mp4 and it's led me to some confusion

In VideoTrackMetadata frameRate is the rate in hertz of the video

/**
 * Additional metadata for video tracks.
 * @group Output files
 * @public
 */
export type VideoTrackMetadata = BaseTrackMetadata & {
    ...
    /**
    * The expected video frame rate in hertz. If set, all timestamps and durations of this track will be snapped to
    * this frame rate. You should avoid adding more frames than the rate allows, as this will lead to multiple frames
    * with the same timestamp.
    */
    frameRate?: number;
};

But its later used in src/isobmff/isobmff-muxer.ts#getVideoTrackData to do:

// The frame rate set by the user may not be an integer. Since timescale is an integer, we'll approximate the
// frame time (inverse of frame rate) with a rational number, then use that approximation's denominator
// as the timescale.
const timescale = computeRationalApproximation(1 / (track.metadata.frameRate ?? 57600), 1e6).denominator;

So using 24 vs 57600 for frameRate on VideoTrackMetadata gives a very different values

computeRationalApproximation(1 / (24 ?? 57600), 1e6).denominator
24

vs

computeRationalApproximation(1 / (90000 ?? 57600), 1e6).denominator
90000

[Vanilagy]

What is the source of confusion? Both your 24 and 90000 examples make perfect sense!

[branks]

Maybe I'm missing something but I assumed the frame rate should be ~24/30 for MP4 but the standard timescale for an mp4 is 90k. So should I put the frame rate at 90k or 24? Because I want a 24fps video with 90k timescale and I can't do that here, I can have 24fps with 24 timescale or 90k fps with 90k timescale

Or am I misunderstanding frame rate?

[Vanilagy]

What's wrong with 24 FPS with a timescale of 24? The frameRate field is meant to specify the frame rate of the track, and the fact that it sets the MP4 timescale is an implementation detail. You can set the field to 90000 if you want and then supply 24 frames per second, that works perfectly fine!