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Hi all, first ... thanks for all the great software.

I've noticed some small problems with TIFF JPEG compression.

http://cima.ng-london.org.uk/~john/vermeer_rgb_jpeg.jpg

   One of our vermeers, converted to sRGB and compressed with
   libjpeg.

http://cima.ng-london.org.uk/~john/vermeer_cielab_jpeg.jpg

   The same image, but converted to TIFF LAB and saved with JPEG
   compression. Uncompressed again and resaved as sRGB JPEG.

   Hopefully the red/green mottling in the neutrals is visible.

http://cima.ng-london.org.uk/~john/vermeer_cielab_tiff.tif

   For reference, the TIFF LAB with JPEG compression.

This is all with quality 75, and (I think) fairly recent library versions.

libtiff saves non-rgb images with Y compression for all channels. The 
jpeg Y compressor does not know that it needs to be very careful around 
value 128 (a/b == 0), so compression noise is moving things between red 
and green.

So ... am I doing anything stupid, and is there a fix? I guess the best 
solution would be to tweak the cb/cr compressors for the a/b channels, 
but I imagine this would be a library change.

Here's an odder problem:

http://cima.ng-london.org.uk/~john/vermeer_tiled.jpg

   This is a 250 pixel across detail of the sRGB image, saved
   as JPEG-compressed tiled TIFF with 128x128 pixel tiles,
   then uncompressed and resaved as a plain jpeg.

   The rightmost two tiles are a bit mangled.

http://cima.ng-london.org.uk/~john/vermeer_tiled.tif

   For reference, the tiled TIFF.

Here it seems to be a multiple-of-8 problem. If you adjust the image 
width so that the cropped-down tiles on the right and bottom edges are a 
multiple of 8 across, the mangling vanishes.

Thanks for any advice,

John

========================================================== 
Aelbert Cuyp 13 February - 12 May 2002 

For information and tickets: 
http://www.nationalgallery.org.uk/exhibitions/cuyp/


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	LibTiff Mailing List TIFF and LibTiff Mailing List Archive March 2002 Previous Thread Next Thread Previous by Thread Next by Thread Previous by Date Next by Date Contact The TIFF Mailing List Homepage This list is run by Frank Warmerdam Archive maintained by AWare Systems	Thread 2002.03.07 19:01 "TIFF LAB with JPEG compression", by <john.cupitt@ng-london.org.uk> 2002.03.07 20:15 "Re: TIFF LAB with JPEG compression", by Tom Lane 2002.03.08 12:40 "Re: TIFF LAB with JPEG compression", by Vipin Aravind 2002.03.08 12:48 "Re: TIFF LAB with JPEG compression", by Martí Maria 2002.03.07 19:01 "TIFF LAB with JPEG compression", by <john.cupitt@ng-london.org.uk> Hi all, first ... thanks for all the great software. I've noticed some small problems with TIFF JPEG compression. http://cima.ng-london.org.uk/~john/vermeer_rgb_jpeg.jpg One of our vermeers, converted to sRGB and compressed with libjpeg. http://cima.ng-london.org.uk/~john/vermeer_cielab_jpeg.jpg The same image, but converted to TIFF LAB and saved with JPEG compression. Uncompressed again and resaved as sRGB JPEG. Hopefully the red/green mottling in the neutrals is visible. http://cima.ng-london.org.uk/~john/vermeer_cielab_tiff.tif For reference, the TIFF LAB with JPEG compression. This is all with quality 75, and (I think) fairly recent library versions. libtiff saves non-rgb images with Y compression for all channels. The jpeg Y compressor does not know that it needs to be very careful around value 128 (a/b == 0), so compression noise is moving things between red and green. So ... am I doing anything stupid, and is there a fix? I guess the best solution would be to tweak the cb/cr compressors for the a/b channels, but I imagine this would be a library change. Here's an odder problem: http://cima.ng-london.org.uk/~john/vermeer_tiled.jpg This is a 250 pixel across detail of the sRGB image, saved as JPEG-compressed tiled TIFF with 128x128 pixel tiles, then uncompressed and resaved as a plain jpeg. The rightmost two tiles are a bit mangled. http://cima.ng-london.org.uk/~john/vermeer_tiled.tif For reference, the tiled TIFF. Here it seems to be a multiple-of-8 problem. If you adjust the image width so that the cropped-down tiles on the right and bottom edges are a multiple of 8 across, the mangling vanishes. Thanks for any advice, John ========================================================== Aelbert Cuyp 13 February - 12 May 2002 For information and tickets: http://www.nationalgallery.org.uk/exhibitions/cuyp/

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2002.03.07 19:01 "TIFF LAB with JPEG compression", by <john.cupitt@ng-london.org.uk>