video/.claude/skills/remotion-best-practices/rules/calculate-metadata.md

name, description, metadata

name	description	metadata
calculate-metadata	Dynamically set composition duration, dimensions, and props	tags calculateMetadata, duration, dimensions, props, dynamic

Using calculateMetadata

Use calculateMetadata on a <Composition> to dynamically set duration, dimensions, and transform props before rendering.

<Composition
  id="MyComp"
  component={MyComponent}
  durationInFrames={300}
  fps={30}
  width={1920}
  height={1080}
  defaultProps={{ videoSrc: "https://remotion.media/video.mp4" }}
  calculateMetadata={calculateMetadata}
/>

Setting duration based on a video

Use the getVideoDuration and getVideoDimensions skills to get the video duration and dimensions:

import { CalculateMetadataFunction } from "remotion";
import { getVideoDuration } from "./get-video-duration";

const calculateMetadata: CalculateMetadataFunction<Props> = async ({
  props,
}) => {
  const durationInSeconds = await getVideoDuration(props.videoSrc);

  return {
    durationInFrames: Math.ceil(durationInSeconds * 30),
  };
};

Matching dimensions of a video

Use the getVideoDimensions skill to get the video dimensions:

import { CalculateMetadataFunction } from "remotion";
import { getVideoDuration } from "./get-video-duration";
import { getVideoDimensions } from "./get-video-dimensions";

const calculateMetadata: CalculateMetadataFunction<Props> = async ({
  props,
}) => {
  const dimensions = await getVideoDimensions(props.videoSrc);

  return {
    width: dimensions.width,
    height: dimensions.height,
  };
};

Setting duration based on multiple videos

const calculateMetadata: CalculateMetadataFunction<Props> = async ({
  props,
}) => {
  const metadataPromises = props.videos.map((video) =>
    getVideoDuration(video.src),
  );
  const allMetadata = await Promise.all(metadataPromises);

  const totalDuration = allMetadata.reduce(
    (sum, durationInSeconds) => sum + durationInSeconds,
    0,
  );

  return {
    durationInFrames: Math.ceil(totalDuration * 30),
  };
};

Setting a default outName

Set the default output filename based on props:

const calculateMetadata: CalculateMetadataFunction<Props> = async ({
  props,
}) => {
  return {
    defaultOutName: `video-${props.id}.mp4`,
  };
};

Transforming props

Fetch data or transform props before rendering:

const calculateMetadata: CalculateMetadataFunction<Props> = async ({
  props,
  abortSignal,
}) => {
  const response = await fetch(props.dataUrl, { signal: abortSignal });
  const data = await response.json();

  return {
    props: {
      ...props,
      fetchedData: data,
    },
  };
};

The abortSignal cancels stale requests when props change in the Studio.

Return value

All fields are optional. Returned values override the <Composition> props:

• durationInFrames: Number of frames
• width: Composition width in pixels
• height: Composition height in pixels
• fps: Frames per second
• props: Transformed props passed to the component
• defaultOutName: Default output filename
• defaultCodec: Default codec for rendering