RGB48 compression anomaly at Quality 4 and up
libPGF is an implementation of the Progressive Graphics File (PGF)
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Hello
On the Digikam website I saw the announcement, from Gilles Caulier, that PGF file format was going to be included.
My main interest is to find a "near lossless" compression system for 16bpc (RGB48) photo images, that will give better compression than lossless systems without degrading the dynamic range from the original 12bit linear camera raw format.
I think that is equivalent to a PSNR of about 75dB.
Fortunately you have provided the Xeraina Windows GUI Image Viewer application and a console application, so it should have been easy. But things did not quite work out like that!
The image viewer won't load RGB48 from 16 bit TIFF or PNG, but I found something that would create a PPM from my raw image and this worked, partly.
Unfortunately there seems to be an anomaly in the compression/PSNR results for compression levels of 4 and above.
Image Viewer Version Nos:3.8.09.09, 5.08.49, 2.08.49
Format Compression File size PSNR
PPM 45.5MB
PGF 0 30.1MB
PGF 1 29.4MB 100.7dB
PGF 2 27.1MB 95.0dB
PGF 3 24.1MB 90.2dB
PGF 4 13.5MB 44.3dB
PGF 5 12.0MB 44.3dB
PGF 6 10.5MB 44.3dB
PGF 7 8.9MB 44.3dB
PGF 8 7.2MB 44.3dB
From level 4 the PSNR plummets to 44.3db and then stays constant, although the file size continues decreasing. It is as though the algorithm has introduced some fixed pixel errors or lost some bits. About 6MB judging from the change in file size!
I also tried encoding with the console application, but that loaded the PPM files with the R & B channels swapped so the PSNR was always 27dB. However the file size and the PSNR for the green channel were similar to the Image Viewer results.
The console application loads 8bit TIFF and JPEG correctly.
I hope you can fix the encoder and that it is not an inherent limitation of the algorithm. The results looked very good up to the point it went wrong.
It would also be useful if the Image Viewer could load RGB48 from a 16bit TIFF, since this is a common interchange format in Windows, understood by most 16bit software.
Thanks!
Hello
>> The image viewer won't load RGB48 from 16 bit TIFF or PNG, but I found something that would create a PPM from my raw image and this worked, partly.
Thats almost true. RGB48 PNG and TIFF images are loaded as 8 bit only. We should change this in future.
>> Unfortunately there seems to be an anomaly in the compression/PSNR results for compression levels of 4 and above.
Try higher quality values, for example 15. 16 bit images usually use higher compression values (= quality).
>> I also tried encoding with the console application, but that loaded the PPM files with the R & B channels swapped so the PSNR was always 27dB. However the file size and the PSNR for the green channel were similar to the Image Viewer results.
We fixed this problem and also fixed wrong PSNR numbers for RGB48 images.
Please download the new PGF console application, version 6.09.33, and the new ImageViewer 3.8.09.33: http://www.xeraina.ch/pgf/pgfdownload.html