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.
One of our vermeers, converted to sRGB and compressed with libjpeg.
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.
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:
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.
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,
Aelbert Cuyp 13 February - 12 May 2002
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