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February 2008
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2008.02.15 14:57 "Re: bug? JPEGQUALITY seems not to be read correctly", by Ed Grissom
There is no place in the header for a "quality" setting. This is not something that is in the JPEG spec or the TIFF spec. What is in both specs is a quality table (q-table). This q-table is generated from the q-setting. It is difficult to recreate the q-setting that was used to create a q-table. -- ed grissom ed.grissom@intergraph.com -----Original Message----- From: tiff-bounces@lists.maptools.org [mailto:tiff-bounces@lists.maptools.org] On Behalf Of jcupitt@gmail.com Sent: Friday, February 15, 2008 8:36 AM To: libtiff - Liste Subject: [Tiff] bug? JPEGQUALITY seems not to be read correctly I think I might have found a bug (or misfeature) in JPEG quality handling. If I write a file with TIFFTAG_JPEGQUALITY set to 100, it seems to work: john@mm-jcupitt2:~/pics$ im_copy healthygirl.jpg test.tif:jpeg:100 john@mm-jcupitt2:~/pics$ im_copy healthygirl.jpg test2.tif:jpeg:75 john@mm-jcupitt2:~/pics$ ls -l test* -rw-r--r-- 1 john john 643587 2008-02-15 14:29 test2.tif -rw-r--r-- 1 john john 2267003 2008-02-15 14:29 test.tif So we get a much larger file, as expected. But if I look at the files with tiffinfo, the TIFFTAG_JPEGQUALITY tag is not displayed: john@mm-jcupitt2:~/pics$ tiffinfo test.tif TIFF Directory at offset 0x2291ae (2265518) Image Width: 1166 Image Length: 1944 Resolution: 118.11, 118.11 pixels/cm Bits/Sample: 8 Compression Scheme: JPEG Photometric Interpretation: RGB color YCbCr Subsampling: 2, 2 Orientation: row 0 top, col 0 lhs Samples/Pixel: 3 Rows/Strip: 16 Planar Configuration: single image plane JPEG Tables: (289 bytes) and if I copy the file, quality seems to revert to the default (75): john@mm-jcupitt2:~/pics$ tiffcp test.tif test3.tif john@mm-jcupitt2:~/pics$ ls -l test*.tif -rw-r--r-- 1 john john 643587 2008-02-15 14:29 test2.tif -rw-r--r-- 1 john john 244444 2008-02-15 14:30 test3.tif -rw-r--r-- 1 john john 2267003 2008-02-15 14:29 test.tif So it seems that TIFFTAG_JPEGQUALITY is either not being written, or that on reading it is always set to the default 75. This is with libtiff 3.8.2. John