polskalokalnie/.claude/skills/remotion-best-practices/remotion-markup/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. Use it when metadata depends on input props, fetched data, or asset metadata. For static dimensions, duration, FPS, and initial props, inline the values on <Composition> instead.

<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 is added automatically
  };
};

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