Lossless rotation of jpegs

  • Thread starter Thread starter Brian Smith
  • Start date Start date
B

Brian Smith

As has been documented on several sites, the CLR Imaging library supports
(theoretically) using a lossless rotation algorithm for jpeg files. This is
also used as a trick to re-Save an image without causing re-compression.

This does not seem to work in 1.1 - on most images I try, the file size is
reduced, and the resultant contents are subtly changed. Has this been broken
in the 1.1 framework, or is there some other method?

brian

---------------------------------------
sample code:
public void Rotate90(string fileName)
{
string FileNameTemp;
using (Image Pic = Image.FromFile(fileName))
{
Encoder Enc = Encoder.Transformation;
EncoderParameters EncParms = new EncoderParameters(1);
EncoderParameter EncParm;
ImageCodecInfo CodecInfo = GetEncoderInfo("image/jpeg");

// we cannot store in the same image, so use a temporary image instead
FileNameTemp = Path.GetDirectoryName(fileName) + "\\rotated " +
Path.GetFileName(fileName);
if (File.Exists(FileNameTemp))
File.Delete(FileNameTemp) ;

// for rewriting without recompression we must rotate the image 90
degrees
EncParm = new
EncoderParameter(Enc,(long)EncoderValue.TransformRotate90);
EncParms.Param[0] = EncParm;

// now save the rotated image
Pic.Save(FileNameTemp,CodecInfo,EncParms);
}
// delete the original file
File.Delete(fileName);
// rename to original
File.Move(FileNameTemp, fileName);
}
 
I've just tested this code under 1.0 and the same problem exists. So is
lossless rotation a myth?
On average, I find the jpeg is reduced by around 8%.

brian

P.S. Missing code from sample:
private ImageCodecInfo GetEncoderInfo(string mimeType)
{
ImageCodecInfo[] encoders ;
encoders = ImageCodecInfo.GetImageEncoders() ;
for (int j = 0; j < encoders.Length; j++)
{
if (encoders[j].MimeType == mimeType)
return encoders[j] ;
}
return null ;
}
 
The lossless rotation is for display only. If you're going to save the
result it gets compressed again - that's why you're seeing the size
difference.

Jerry

Brian Smith said:
I've just tested this code under 1.0 and the same problem exists. So is
lossless rotation a myth?
On average, I find the jpeg is reduced by around 8%.

brian

P.S. Missing code from sample:
private ImageCodecInfo GetEncoderInfo(string mimeType)
{
ImageCodecInfo[] encoders ;
encoders = ImageCodecInfo.GetImageEncoders() ;
for (int j = 0; j < encoders.Length; j++)
{
if (encoders[j].MimeType == mimeType)
return encoders[j] ;
}
return null ;
}


Brian Smith said:
As has been documented on several sites, the CLR Imaging library supports
(theoretically) using a lossless rotation algorithm for jpeg files. This is
also used as a trick to re-Save an image without causing re-compression.

This does not seem to work in 1.1 - on most images I try, the file size is
reduced, and the resultant contents are subtly changed. Has this been broken
in the 1.1 framework, or is there some other method?

brian

---------------------------------------
sample code:
public void Rotate90(string fileName)
{
string FileNameTemp;
using (Image Pic = Image.FromFile(fileName))
{
Encoder Enc = Encoder.Transformation;
EncoderParameters EncParms = new EncoderParameters(1);
EncoderParameter EncParm;
ImageCodecInfo CodecInfo = GetEncoderInfo("image/jpeg");

// we cannot store in the same image, so use a temporary image instead
FileNameTemp = Path.GetDirectoryName(fileName) + "\\rotated " +
Path.GetFileName(fileName);
if (File.Exists(FileNameTemp))
File.Delete(FileNameTemp) ;

// for rewriting without recompression we must rotate the image 90
degrees
EncParm = new
EncoderParameter(Enc,(long)EncoderValue.TransformRotate90);
EncParms.Param[0] = EncParm;

// now save the rotated image
Pic.Save(FileNameTemp,CodecInfo,EncParms);
}
// delete the original file
File.Delete(fileName);
// rename to original
File.Move(FileNameTemp, fileName);
}
Click to expand...
Click to expand...
 
That's true, but a bit of a red-herring. The real problem is not the
rotation but the ability to re-save an image that has not been altered
without recompressing it. Several sources have suggested that by performing
a (needless) rotation the CLR allows a re-save without re-compression. But
it doesn't actually work.
Since other tools do manage this task correctly (with or without rotation),
I'm looking for a .net solution..

brian

Jerry III said:
The lossless rotation is for display only. If you're going to save the
result it gets compressed again - that's why you're seeing the size
difference.

Jerry

Brian Smith said:
I've just tested this code under 1.0 and the same problem exists. So is
lossless rotation a myth?
On average, I find the jpeg is reduced by around 8%.

brian

P.S. Missing code from sample:
private ImageCodecInfo GetEncoderInfo(string mimeType)
{
ImageCodecInfo[] encoders ;
encoders = ImageCodecInfo.GetImageEncoders() ;
for (int j = 0; j < encoders.Length; j++)
{
if (encoders[j].MimeType == mimeType)
return encoders[j] ;
}
return null ;
}


Brian Smith said:
As has been documented on several sites, the CLR Imaging library supports
(theoretically) using a lossless rotation algorithm for jpeg files.
Click to expand...
This
is
also used as a trick to re-Save an image without causing re-compression.

This does not seem to work in 1.1 - on most images I try, the file
Click to expand...
Click to expand...
size
is
reduced, and the resultant contents are subtly changed. Has this been broken
in the 1.1 framework, or is there some other method?

brian

---------------------------------------
sample code:
public void Rotate90(string fileName)
{
string FileNameTemp;
using (Image Pic = Image.FromFile(fileName))
{
Encoder Enc = Encoder.Transformation;
EncoderParameters EncParms = new EncoderParameters(1);
EncoderParameter EncParm;
ImageCodecInfo CodecInfo = GetEncoderInfo("image/jpeg");

// we cannot store in the same image, so use a temporary image instead
FileNameTemp = Path.GetDirectoryName(fileName) + "\\rotated " +
Path.GetFileName(fileName);
if (File.Exists(FileNameTemp))
File.Delete(FileNameTemp) ;

// for rewriting without recompression we must rotate the image 90
degrees
EncParm = new
EncoderParameter(Enc,(long)EncoderValue.TransformRotate90);
EncParms.Param[0] = EncParm;

// now save the rotated image
Pic.Save(FileNameTemp,CodecInfo,EncParms);
}
// delete the original file
File.Delete(fileName);
// rename to original
File.Move(FileNameTemp, fileName);
}
Click to expand...
Click to expand...
Click to expand...
 
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