Render

Encode the final video with format, resolution, FPS, and subtitle burn-in options.

The .render() stage configures the final video encoding. It controls the output format, resolution, frame rate, and subtitle burn-in styling.

Basic usage

await Recast
  .from('./traces')
  .parse()
  .render({ format: 'mp4', resolution: '1080p' })
  .toFile('demo.mp4')

Configuration options

Option	Type	Default	Description
`format`	`string`	`'mp4'`	Output format
`resolution`	`string`	`'1080p'`	Output resolution: `'720p'`, `'1080p'`, `'4k'`
`fps`	`number`	`30`	Output frame rate
`burnSubtitles`	`boolean`	`false`	Burn subtitles into the video
`subtitleStyle`	`object`	default ffmpeg	Subtitle appearance configuration

Output with `.toFile()`

After configuring the render, call .toFile() to execute the pipeline and write the result:

.render({ format: 'mp4', resolution: '1080p' })
.toFile('demo.mp4')

Subtitle burn-in

Set burnSubtitles: true to render subtitles directly into the video frames. Without subtitleStyle, ffmpeg's default SRT rendering is used.

Subtitle style options

Option	Type	Default	Description
`fontFamily`	`string`	`'Arial'`	Any system font
`fontSize`	`number`	`48`	Font size in pixels (relative to 1080p)
`primaryColor`	`string`	`'#FFFFFF'`	Text color (hex)
`backgroundColor`	`string`	`'#000000'`	Box background color (hex)
`backgroundOpacity`	`number`	`0.75`	Box opacity (0.0 transparent - 1.0 opaque)
`padding`	`number`	`20`	Box padding in pixels
`bold`	`boolean`	`false`	Bold text
`position`	`string`	`'bottom'`	`'bottom'` or `'top'`
`marginVertical`	`number`	`50`	Distance from edge in pixels
`marginHorizontal`	`number`	`100`	Side margins (text wraps within)
`wrapStyle`	`string`	`'smart'`	`'smart'`, `'endOfLine'`, `'none'`
`chunkOptions`	`object`	—	Punctuation-based text splitting

Full subtitle styling example

.render({
  format: 'mp4',
  resolution: '1080p',
  fps: 60,
  burnSubtitles: true,
  subtitleStyle: {
    fontFamily: 'Arial',
    fontSize: 48,
    primaryColor: '#1a1a1a',
    backgroundColor: '#FFFFFF',
    backgroundOpacity: 0.75,
    padding: 20,
    bold: true,
    position: 'bottom',
    marginVertical: 50,
    marginHorizontal: 100,
    wrapStyle: 'smart',
    chunkOptions: {
      maxCharsPerLine: 55,
      minCharsPerChunk: 15,
    },
  },
})

Chunk options

Long subtitle text is split into shorter single-line entries. Time is distributed proportionally by character count.

Option	Type	Default	Description
`maxCharsPerLine`	`number`	`55`	Split text when it exceeds this length
`minCharsPerChunk`	`number`	`15`	Merge fragments shorter than this

Splitting priority:

Sentence boundaries (. ! ?)
Clause boundaries (, ; :) if still too long

CLI equivalent

# Basic render
npx playwright-recast -i ./traces -o demo.mp4

# With subtitle burn-in
npx playwright-recast -i ./traces --srt narration.srt --burn-subs -o demo.mp4

Tips

Use 720p during development for faster rendering. Switch to 1080p or 4k for final output.
Without burnSubtitles, subtitle files (SRT/VTT) are still generated alongside the video for use in video players that support external subtitles.
Subtitle font size is relative to 1080p. At other resolutions, the size scales proportionally.
The smart wrap style balances line lengths for a centered, even appearance.

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