How to convert transparent part of PNG to specific color

[lhkzhl]

why it doesn't work? also has black backgroundColor;

final Directory? downloadsDir = await getDownloadsDirectory();
if (downloadsDir == null) {
  return;
}
// // files.last.
var targetPath = '${downloadsDir.path}/testconvert.jpg';
var image = await img.decodeImageFile(files.last.path);
if (image != null) {
  final p = image.getBytes();
  for (var i = 0, len = p.length; i < len; i += 4) {
    if (p[i + 3] != 255) {
      // If the alpha is transparent
      p[i] = 255; // Set the color to white
      p[i + 1] = 255;
      p[i + 2] = 255;
      p[i + 3] = 255;
    }
  }
  img.writeFile(targetPath, img.encodeJpg(image));
}

this is my image this is the result