Metadata Handling

jpgx supports reading and preserving JPEG metadata including EXIF data, comments, and JFIF information. This guide covers working with metadata.

Types of Metadata

Type	Description	Use Case
EXIF	Camera settings, GPS, timestamps	Photography
JFIF	Basic image info	Standard JPEG
Comments	Arbitrary text	Notes, copyright
Adobe	Color profile info	Design work

Reading Metadata

EXIF Data

import { decode } from 'jpgx'

const image = decode(jpegData)

if (image.exifBuffer) {
  console.log('EXIF data found')
  console.log('Size:', image.exifBuffer.byteLength, 'bytes')

  // The raw EXIF buffer can be parsed with an EXIF library
  // or preserved when re-encoding
}

Comments

const image = decode(jpegData)

if (image.comments && image.comments.length > 0) {
  console.log('Comments:')
  image.comments.forEach((comment, index) => {
    console.log(`  [${index + 1}] ${comment}`)
  })
}

Writing Metadata

Adding Comments

import { encode } from 'jpgx'

const result = encode({
  width: 800,
  height: 600,
  data: imageData,
  comments: [
    'Created with jpgx',
    'Author: John Doe',
    'Date: 2025-01-15',
  ],
}, 85)

Preserving EXIF

When re-encoding, preserve original EXIF data:

import { encode, decode } from 'jpgx'

// Decode original
const original = decode(originalJpegData)

// Process the image...
const processedData = processImage(original.data)

// Re-encode with preserved EXIF
const result = encode({
  width: original.width,
  height: original.height,
  data: processedData,
  exifBuffer: original.exifBuffer, // Preserve EXIF
  comments: original.comments,      // Preserve comments
}, 85)

EXIF Data Structure

The EXIF buffer contains raw EXIF data. Common fields include:

Field	Description
Make	Camera manufacturer
Model	Camera model
DateTime	Capture timestamp
ExposureTime	Shutter speed
FNumber	Aperture
ISO	Sensitivity
FocalLength	Lens focal length
GPSLatitude	GPS coordinates
GPSLongitude	GPS coordinates
Orientation	Image rotation

Parsing EXIF

jpgx returns raw EXIF bytes. Use a library like exif-js to parse:

// Example with a hypothetical EXIF parser
import { decode } from 'jpgx'
import { parseExif } from 'exif-parser-library'

const image = decode(jpegData)

if (image.exifBuffer) {
  const exif = parseExif(image.exifBuffer)

  console.log('Camera:', exif.Make, exif.Model)
  console.log('Date:', exif.DateTime)
  console.log('Settings:', `f/${exif.FNumber}`, `ISO ${exif.ISO}`)

  if (exif.GPSLatitude && exif.GPSLongitude) {
    console.log('Location:', exif.GPSLatitude, exif.GPSLongitude)
  }
}

Common Workflows

Strip All Metadata

For privacy or smaller files:

const original = decode(jpegData)

// Re-encode without metadata
const stripped = encode({
  width: original.width,
  height: original.height,
  data: original.data,
  // Don't include exifBuffer or comments
}, 85)

Transfer Metadata

Copy metadata from one image to another:

const source = decode(sourceJpegData)
const target = decode(targetJpegData)

// Re-encode target with source's metadata
const result = encode({
  width: target.width,
  height: target.height,
  data: target.data,
  exifBuffer: source.exifBuffer,
  comments: source.comments,
}, 85)

Add Copyright

Add copyright info to images:

const image = decode(jpegData)

const result = encode({
  width: image.width,
  height: image.height,
  data: image.data,
  exifBuffer: image.exifBuffer,
  comments: [
    ...(image.comments || []),
    `Copyright ${new Date().getFullYear()} Your Name`,
    'All rights reserved',
  ],
}, 85)

JFIF Information

The decoder extracts JFIF header information:

const image = decode(jpegData)

// JFIF data is used internally for:
// - Version information
// - Density units (DPI)
// - Pixel aspect ratio
// - Thumbnail data (if present)

Adobe Marker

For images with Adobe color profiles:

const image = decode(jpegData)

// Adobe marker data is used for:
// - Color transform information
// - CMYK color handling

Batch Processing

Process multiple images while preserving metadata:

async function processImages(paths: string[]) {
  for (const path of paths) {
    const data = await Bun.file(path).arrayBuffer()
    const image = decode(new Uint8Array(data))

    // Process image...
    const processed = applyFilter(image.data)

    // Save with preserved metadata
    const result = encode({
      width: image.width,
      height: image.height,
      data: processed,
      exifBuffer: image.exifBuffer,
      comments: image.comments,
    }, 85)

    await Bun.write(path.replace('.jpg', '_processed.jpg'), result.data)
  }
}

Privacy Considerations

EXIF data can contain sensitive information:

• GPS coordinates (location)
• Date/time (when photo was taken)
• Device information
• Thumbnail of original image

Always strip metadata before sharing publicly if needed:

function stripForPublic(jpegData: Uint8Array): Uint8Array {
  const image = decode(jpegData)

  return encode({
    width: image.width,
    height: image.height,
    data: image.data,
    // No exifBuffer
    // No comments (or add generic copyright only)
    comments: ['Public domain'],
  }, 85).data
}

Next Steps

• Configuration - Full configuration options
• JPEG Encoding - Complete encoding guide
• Performance - Optimize processing