Rotating JPEGs in .NET with minimal loss of quality

Question

I am attempting to support rotating JPEG images from ASP.NET MVC (in 90 degree increments). I am attempting to use System.Drawing (GDI+), however I am running into issues.

I tried using Image.RotateFlip which is able to rotate the image but causes a loss of quality. Even with an encoder quality of 100, there are still visible artifacts on the rotated image that weren't on the original image nor do they show up when I rotate it using other programs (Gimp, etc.).

using (Image image = Image.FromFile("C:\\source.jpg")) {
    ImageFormat sourceFormat = image.RawFormat;
    image.RotateFlip(RotateFlipType.Rotate90FlipNone);
    EncoderParameters encoderParams = null;
    try {
        if (sourceFormat == ImageFormat.Jpeg) {
            encoderParams = new EncoderParameters(1);
            encoderParams.Param[0] = new EncoderParameter(Encoder.Quality, 100L);
        }
        image.Save("C:\\target.jpg", GetEncoder(sourceFormat), encoderParams);
    } finally {
        if (encoderParams != null)
            encoderParams.Dispose();
    }
}

I found an article on transforming a JPEG without loss of information. Using Encoder.Transformation appears to be an option from .NET - however I cannot get it to cause any of my JPEG test images to rotate at all, whether or not the dimensions are a multiple of 16.

using (Image image = Image.FromFile("C:\\source.jpg")) {
    ImageFormat sourceFormat = image.RawFormat;
    EncoderParameters encoderParams = null;
    try {
        if (sourceFormat == ImageFormat.Jpeg) {
            encoderParams = new EncoderParameters(1);
            encoderParams.Param[0] = new EncoderParameter(Encoder.Transformation, 
                (long)EncoderValue.TransformRotate90);
        }
        image.Save("C:\\target.jpg", GetEncoder(sourceFormat), encoderParams);
    } finally {
        if (encoderParams != null)
            encoderParams.Dispose();
    }
}

Does anyone know how to successfully rotate a JPEG in .NET in 90 degree increments with minimal or no loss of quality using either of the above methods or another method? Thanks.

Also, here's my implementation of GetEncoder:

private ImageCodecInfo GetEncoder(ImageFormat format) {
    foreach (var info in ImageCodecInfo.GetImageEncoders())
        if (info.FormatID == format.Guid)
            return info;
    return null;
}

Edit:

I updated the above code to better match my actual code. The bug was in the following line:

if (sourceFormat == ImageFormat.Jpeg) {

It should have been:

if (sourceFormat.Guid == ImageFormat.Jpeg.Guid) {

Answer 1

Thanks for confirming that my posted code worked. This helped me isolate my problem. I feel stupid now. My actual code had a check for image format before setting encoderParams - but it had a bug:

if (sourceFormat == ImageFormat.Jpeg) {
    // set encoderParams here

I discovered the above conditional was always false so encoderParams wasn't being set. The fix was simple:

if (sourceFormat.Guid == ImageFormat.Jpeg.Guid) {

Answer 2

With any method that decompresses the image, rotates it and compresses it, you will get a loss of quality.

The JPEG format compresses color information in squares of 2x2 pixels by getting an average color to represent all four pixels, so if your image width and height are divisible by two you will lose less quality as most of the information removed in the compression is information that was interpolated in the decompression.

Likewise the brightness information is compressed in squares of 8x8 pixels, so if your width and hight are divisable by eight, the grid will align after rotating the image and you will lose less actual information.

To make a lossless rotation you have to use a completely different method, reading the JPEG file and rearranging and rotating each square of information so that it forms the rotated image without decompressing and recompressing it.