Question

I have a byte array that contains a jpeg image. I was just wondering if it is possible to reduce its size?

Edit: Ok. I acknowledged my mistake. Then my question is how can I reduce the quality of the image comes from a byte array.

Answer 1

Please understand that there is no free lunch. Decreasing the size of a JPEG image by increasing the compression will also decrease the quality of the image. However, that said, you can reduce the size of a JPEG image using the Image class. This code assumes that inputBytes contains the original image.

var jpegQuality = 50; 
Image image; 
using (var inputStream = new MemoryStream(inputBytes)) {
  image = Image.FromStream(inputStream);
  var jpegEncoder = ImageCodecInfo.GetImageDecoders() 
    .First(c => c.FormatID == ImageFormat.Jpeg.Guid); 
  var encoderParameters = new EncoderParameters(1); 
  encoderParameters.Param[0] = new EncoderParameter(Encoder.Quality, jpegQuality); 
  Byte[] outputBytes; 
  using (var outputStream = new MemoryStream()) { 
    image.Save(outputStream, jpegEncoder, encoderParameters);
    outputBytes = outputStream.ToArray(); 
  } 
}

Now outputBytes contains a recompressed version of the image using a different JPEG quality.

By decreasing the jpegQuality (should be in the range 0-100) you can increase the compression at the cost of lower image quality. See the Encoder.Quality field for more information.

Here is an example where you can see how jpegQuality affects the image quality. It is the same photo compressed using 20, 50 and 80 as the value of jpegQuality. Sizes are 4.99, 8.28 and 12.9 KB.

Notice how the text becomes “smudged” even when the quality is high. This is why you should avoid using JPEG for images with uniformly colored areas (images/diagrams/charts created on a computer). Use PNG instead. For photos JPEG is very suitable if you do not lower the quality too much.