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While playing around with some of the GDI+ classes to manipulate pictures from my digital camera, I found that if you want to add or modify an image description of an EXIF image file (EXIF = JPEG plus additional information) you can read and write the image description with the PropertyItem data structure. These PropertyItems are very useful to avoid the bit manipulating in the file structure of an exif file.
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But the problem is: when writing the image with the changed or new description the picture part becomes recompressed. You can notice this from the file size; you add information to the file and the file size decreases. When you repeat changing the description, the image become more and more poor, because jpg is a lossy compression. So how do you load and save an jpg or exif file without recompressing the bitmap?
The trick is to rotate the picture by 90 degrees. In this case the framework supplies a lossless rewriting of a jpeg file:
private static ImageCodecInfo GetEncoderInfo(String mimeType)
{
int j;
ImageCodecInfo[] encoders;
encoders = ImageCodecInfo.GetImageEncoders();
for(j = 0; j < encoders.Length; ++j)
{
if(encoders[j].MimeType == mimeType)
return encoders[j];
} return null;
}
private void WriteNewDescriptionInImage(string Filename,string NewDescription)
{
Image Pic;
PropertyItem[] PropertyItems;
byte[] bDescription=new Byte[NewDescription.Length];
int i;
string FilenameTemp;
Encoder Enc=Encoder.Transformation;
EncoderParameters EncParms=new EncoderParameters(1);
EncoderParameter EncParm;
ImageCodecInfo CodecInfo=GetEncoderInfo("image/jpeg");
// copy description into byte array
for (i=0;i<NewDescription.Length;i++) bDescription[i]=(byte)NewDescription[i];
// load the image to change
Pic=Image.FromFile(Filename);
// put the new description into the right property item
PropertyItems=Pic.PropertyItems;
PropertyItems[0].Id=0x010e; // 0x010e as specified in EXIF standard
PropertyItems[0].Type=2;
PropertyItems[0].Len=NewDescription.Length;
PropertyItems[0].Value=bDescription;
Pic.SetPropertyItem(PropertyItems[0]);
// we cannot store in the same image, so use a temporary image instead
FilenameTemp=Filename+".temp";
// for lossless rewriting must rotate the image by 90 degrees!
EncParm=new EncoderParameter(Enc,(long)EncoderValue.TransformRotate90);
EncParms.Param[0]=EncParm;
// now write the rotated image with new description
Pic.Save(FilenameTemp,CodecInfo,EncParms);
// for computers with low memory and large pictures: release memory now
Pic.Dispose();
Pic=null;
GC.Collect();
// delete the original file, will be replaced later
System.IO.File.Delete(Filename);
// now must rotate back the written picture
Pic=Image.FromFile(FilenameTemp);
EncParm=new EncoderParameter(Enc,(long)EncoderValue.TransformRotate270);
EncParms.Param[0]=EncParm;
Pic.Save(Filename,CodecInfo,EncParms);
// release memory now
Pic.Dispose();
Pic=null;
GC.Collect();
// delete the temporary picture
System.IO.File.Delete(FilenameTemp);
}
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When saving to JPEG images, you can also control the compression ratio of the algorithm. You must use a different overload of the Save method:
public void Save(
string filename,
ImageCodecInfo encoder,
EncoderParameters encoderParams
);
The first step is to get the ImageCodecInfo structure for the JPEG image. The GDI+ interface provides no direct method to get this object. You must resort to a little trick—enumerate all the image encoders and check their MIME type properties against the JPEG MIME type string (image/jpeg). The ImageCodecInfo structure contains information inherent in the encoding and decoding of the image.
The EncoderParameters argument represents an array of encoding parameters. Each element of the array is an EncoderParameter type. Possible parameters are listed as members of the static class Encoder. For example, the parameter Compression lets you choose a compression engine for TIFF images. The Quality parameter lets you choose the desired quality of the JPEG compression. This code compresses a JPEG with a 40:1 ratio:
// Set the quality to 40 (must be a long)
Encoder qualityEncoder = Encoder.Quality;
EncoderParameter ratio = new EncoderParameter(qualityEncoder, 40L);
// Add the quality parameter to the list
codecParams = new EncoderParameters(1);
codecParams.Param[0] = ratio;
// Save to JPG
bmp.Save(fileName, jpegCodecInfo, codecParams);
Additionally, this MSDN page details more information, specifically that JPEG images must have a width and height in multiples of 16 in order to have completely "lossless" rotation:
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/gdicpp/GDIPlus/usingGDIPlus/usingimageencodersanddecoders/transformingajpegimagewithoutlossofinformation.asp
Hope this is useful to others!
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