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POWhat quality level does Image.Save() use for jpeg files?
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copied!<p>I just got a real surprise when I loaded a jpg file and turned around and saved it with a quality of 100 and the size was almost 4x the original. To further investigate I open and saved without explicitly setting the quality and the file size was exactly the same. I figured this was because nothing changed so it's just writing the exact same bits back to a file. To test this assumption I drew a big fat line diagonally across the image and saved again without setting quality (this time I expected the file to jump up because it would be "dirty") but it decreased ~10Kb!</p> <p>At this point I really don't understand what is happening when I simply call Image.Save() w/out specifying a compression quality. How is the file size so close (after the image is modified) to the original size when no quality is set yet when I set quality to 100 (basically no compression) the file size is several times larger than the original?</p> <p>I've read the documentation on Image.Save() and it's lacking any detail about what is happening behind the scenes. I've googled every which way I can think of but I can't find any additional information that would explain what I'm seeing. I have been working for 31 hours straight so maybe I'm missing something obvious ;0)</p> <p>All of this has come about while I implement some library methods to save images to a database. I've overloaded our "SaveImage" method to allow explicitly setting a quality and during my testing I came across the odd (to me) results explained above. Any light you can shed will be appreciated.</p> <p>Here is some code that will illustrate what I'm experiencing:</p> <pre><code>string filename = @"C:\temp\image testing\hh.jpg"; string destPath = @"C:\temp\image testing\"; using(Image image = Image.FromFile(filename)) { ImageCodecInfo codecInfo = ImageUtils.GetEncoderInfo(ImageFormat.Jpeg); // Set the quality EncoderParameters parameters = new EncoderParameters(1); // Quality: 10 parameters.Param[0] = new EncoderParameter( System.Drawing.Imaging.Encoder.Quality, 10L); image.Save(destPath + "10.jpg", codecInfo, parameters); // Quality: 75 parameters.Param[0] = new EncoderParameter( System.Drawing.Imaging.Encoder.Quality, 75L); image.Save(destPath + "75.jpg", codecInfo, parameters); // Quality: 100 parameters.Param[0] = new EncoderParameter( System.Drawing.Imaging.Encoder.Quality, 100L); image.Save(destPath + "100.jpg", codecInfo, parameters); // default image.Save(destPath + "default.jpg", ImageFormat.Jpeg); // Big line across image using (Graphics g = Graphics.FromImage(image)) { using(Pen pen = new Pen(Color.Red, 50F)) { g.DrawLine(pen, 0, 0, image.Width, image.Height); } } image.Save(destPath + "big red line.jpg", ImageFormat.Jpeg); } public static ImageCodecInfo GetEncoderInfo(ImageFormat format) { return ImageCodecInfo.GetImageEncoders().ToList().Find(delegate(ImageCodecInfo codec) { return codec.FormatID == format.Guid; }); } </code></pre>

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