.NET C# library for lossless Exif rewriting? [closed]

Question

I have found various code and libraries for editing Exif.

But they are only lossless when the image width and height is multiple of 16.

I am looking for a library (or even a way to do it myself) to edit just the Exif portion in a JPEG file (or add Exif data if it doesn't exist yet), leaving the other data unmodified. Isn't that possible?

So far I could only locate the Exif portion (starts with 0xFFE1) but I don't understand how to read the data.

Answer 1

Here are the specifications for the Exif interchange format, if you plan to code your own library for editing tags.

http://www.exif.org/specifications.html

Here's a library written in Perl that meets your needs that you may be able to learn from:

http://www.sno.phy.queensu.ca/~phil/exiftool/

Here's a decent .NET library for Exif evaluation from The Code Project:

http://www.codeproject.com/KB/graphics/exiftagcol.aspx

Answer 2

You can do this without any external lib:

// Create image.
Image image1 = Image.FromFile("c:\\Photo1.jpg");

// Get a PropertyItem from image1. Because PropertyItem does not
// have public constructor, you first need to get existing PropertyItem
PropertyItem propItem = image1.GetPropertyItem(20624);

// Change the ID of the PropertyItem.
propItem.Id = 20625;

// Set the new PropertyItem for image1.
image1.SetPropertyItem(propItem);

// Save the image.
image1.Save("c:\\Photo1.jpg", ImageFormat.Jpg);

List of all possible PropertyItem ids (including exif) you can found here.

Update: Agreed, this method will re-encode image on save. But I have remembered another method, in WinXP SP2 and later there is new imaging components added - WIC, and you can use them to lossless write metadate - How-to: Re-encode a JPEG Image with Metadata.

Answer 3

exiv2net library (a .NET wrapper on top of exiv2) may be what you're looking for.

Answer 4

I wrote a small test where I compress one file many times to see the quality degradation and you can see it in the third-fourth compression, which is very bad.

But luckily, if you always use same QualityLevel with JpegBitmapEncoder there is no degradation.

In this example I rewrite keywords 100x in metadata and the quality seems not to change.

private void LosslessJpegTest() {
  var original = "d:\\!test\\TestInTest\\20150205_123011.jpg";
  var copy = original;
  const BitmapCreateOptions createOptions = BitmapCreateOptions.PreservePixelFormat | BitmapCreateOptions.IgnoreColorProfile;

  for (int i = 0; i < 100; i++) {
    using (Stream originalFileStream = File.Open(copy, FileMode.Open, FileAccess.Read)) {
      BitmapDecoder decoder = BitmapDecoder.Create(originalFileStream, createOptions, BitmapCacheOption.None);

      if (decoder.CodecInfo == null || !decoder.CodecInfo.FileExtensions.Contains("jpg") || decoder.Frames[0] == null)
        continue;

      BitmapMetadata metadata = decoder.Frames[0].Metadata == null
        ? new BitmapMetadata("jpg")
        : decoder.Frames[0].Metadata.Clone() as BitmapMetadata;

      if (metadata == null) continue;

      var keywords = metadata.Keywords == null ? new List<string>() : new List<string>(metadata.Keywords);
      keywords.Add($"Keyword {i:000}");
      metadata.Keywords = new ReadOnlyCollection<string>(keywords);

      JpegBitmapEncoder encoder = new JpegBitmapEncoder {QualityLevel = 80};
      encoder.Frames.Add(BitmapFrame.Create(decoder.Frames[0], decoder.Frames[0].Thumbnail, metadata,
        decoder.Frames[0].ColorContexts));

      copy = original.Replace(".", $"_{i:000}.");

      using (Stream newFileStream = File.Open(copy, FileMode.Create, FileAccess.ReadWrite)) {
        encoder.Save(newFileStream);
      }
    }
  }
}