encode static method

Implementation

static String encode(img.Image image) {
  final buffer = StringBuffer();

  // Start Sixel: ESC P q
  buffer.write('\x1bPq');

  // Set pixel aspect ratio (1:1)
  buffer.write('\"1;1');

  final width = image.width;
  final height = image.height;

  // Simple color mapping: Map each unique color to a Sixel color register.
  final palette = <int, int>{};
  var nextRegister = 0;

  // Sixel processes 6 vertical pixels at a time.
  for (var y = 0; y < height; y += 6) {
    // For each color register used in this 6-pixel row, we store the bitmasks.
    final rowMasks = <int, List<int>>{};

    for (var x = 0; x < width; x++) {
      for (var dy = 0; dy < 6; dy++) {
        if (y + dy >= height) break;

        final pixel = image.getPixel(x, y + dy);
        if (pixel.a == 0) continue; // Skip transparent

        final colorValue =
            (pixel.r.toInt() << 16) |
            (pixel.g.toInt() << 8) |
            pixel.b.toInt();

        if (!palette.containsKey(colorValue)) {
          if (nextRegister < 256) {
            palette[colorValue] = nextRegister;
            // Define color: #n;2;r;g;b (r,g,b are 0-100)
            final r = (pixel.r * 100 / 255).round();
            final g = (pixel.g * 100 / 255).round();
            final b = (pixel.b * 100 / 255).round();
            buffer.write('#$nextRegister;2;$r;$g;$b');
            nextRegister++;
          } else {
            // Fallback to closest or just use register 0
            palette[colorValue] = 0;
          }
        }

        final reg = palette[colorValue]!;
        rowMasks.putIfAbsent(reg, () => List<int>.filled(width, 0));
        rowMasks[reg]![x] |= (1 << dy);
      }
    }

    // Output each color's bitmasks for this row.
    var firstColor = true;
    for (final entry in rowMasks.entries) {
      final reg = entry.key;
      final masks = entry.value;

      if (!firstColor) {
        buffer.write('\$'); // Carriage return to start of row
      }
      buffer.write('#$reg');

      var lastMask = -1;
      var repeatCount = 0;

      for (var x = 0; x < width; x++) {
        final mask = masks[x];
        final char = String.fromCharCode(mask + 63);

        if (mask == lastMask) {
          repeatCount++;
        } else {
          if (repeatCount > 3) {
            buffer.write(
              '!$repeatCount${String.fromCharCode(lastMask + 63)}',
            );
          } else if (repeatCount > 0) {
            for (var i = 0; i < repeatCount; i++) {
              buffer.write(String.fromCharCode(lastMask + 63));
            }
          }
          buffer.write(char);
          lastMask = mask;
          repeatCount = 0;
        }
      }
      // Flush remaining repeats
      if (repeatCount > 3) {
        buffer.write('!$repeatCount${String.fromCharCode(lastMask + 63)}');
      } else if (repeatCount > 0) {
        for (var i = 0; i < repeatCount; i++) {
          buffer.write(String.fromCharCode(lastMask + 63));
        }
      }

      firstColor = false;
    }

    buffer.write('-'); // New line (6 pixels down)
  }

  // End Sixel: ESC \
  buffer.write('\x1b\\');

  return buffer.toString();
}