2002.03.07 19:01 "TIFF LAB with JPEG compression", by John Cupitt

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

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Aelbert Cuyp 13 February - 12 May 2002

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