Base64-Bild komprimieren (warum Gzip meist scheitert)

Kurzes Beispiel mit Browser-APIs

async function compressBase64Image(dataUrl, maxWidth = 1600, quality = 0.82) {
  const [header, payload] = dataUrl.split(',', 2);
  const mime = header.match(/^data:([^;]+);base64$/)?.[1];
  if (!mime || !/^image\/(png|jpeg|webp)$/.test(mime)) throw new Error('Unsupported image');

  const binary = atob(payload.replace(/\s+/g, ''));
  const bytes = Uint8Array.from(binary, (char) => char.charCodeAt(0));
  const bitmap = await createImageBitmap(new Blob([bytes], { type: mime }));
  try {
    const scale = Math.min(1, maxWidth / bitmap.width);
    const canvas = document.createElement('canvas');
    canvas.width = Math.max(1, Math.round(bitmap.width * scale));
    canvas.height = Math.max(1, Math.round(bitmap.height * scale));
    const context = canvas.getContext('2d');
    if (!context) throw new Error('Canvas 2D unavailable');
    context.drawImage(bitmap, 0, 0, canvas.width, canvas.height);
    const blob = await new Promise((resolve) => canvas.toBlob(resolve, 'image/webp', quality));
    if (!blob) throw new Error('Encoding failed');
    const output = new Uint8Array(await blob.arrayBuffer());
    let outputBinary = '';
    for (let offset = 0; offset < output.length; offset += 0x8000) {
      outputBinary += String.fromCharCode(...output.subarray(offset, offset + 0x8000));
    }
    return `data:${blob.type};base64,${btoa(outputBinary)}`;
  } finally {
    bitmap.close();
  }
}