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From: Martí Maria <marti@littlecms.com>
To: <John.Cupitt@ng-london.org.uk>, <jpeg-l@ijg.org>
References: <3C87B8F8.50205@ng-london.org.uk>
Subject: Re: TIFF LAB with JPEG compression
Date: Fri, 8 Mar 2002 09:48:16 -0320
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Hi,

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

I did experiece similar problems on Lab and JPEG. Seems JPEG is not
adecuate on such high gamma spaces. If you are using Lab because its
color accurancy, you should reconsider the compression scheme. JPEG
yelds  great compression ratios, but is lossy. And since Lab is hardly
perceptually uniform, the noise comes easely evident. A better choice
would be to use RGB and embed icc profile. If the profile is accurate
enough, you will better results on this way than using Lab.

> Here's an odder problem:
>  ...
>    The rightmost two tiles are a bit mangled.

TIFF 6 states that tiles shouldn't hold any alignment. Your image seems
somehow dword aligned.

Hope this helps,
Martí Maria
The little cms project
http://www.littlecms.com
marti@littlecms.com





----- Original Message -----
From: <John.Cupitt@ng-london.org.uk>
To: <jpeg-l@ijg.org>
Sent: Thursday, March 07, 2002 3:41 PM
Subject: TIFF LAB with JPEG compression


> 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/
>
> --
> Sent via IJG mailing list, jpeg-l@ijg.org
> Administrivia: majordomo@ijg.org
>
>


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