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From: "Joris Van Damme \(AWare Systems\)" <joris.at.lebbeke@skynet.be>
To: <jcupitt@gmail.com>
References: <522c6460802150636t65f055a0v7e2665b393aee66e@mail.gmail.com><AA6DDFA4E046A24585BC828A78155B47032BFF9B@US-MAIL.ingrnet.com><522c6460802150702g3f3e3565i84f61ba6d511583@mail.gmail.com><AA6DDFA4E046A24585BC828A78155B47032BFFEC@US-MAIL.ingrnet.com><FC840EC0BF7BFA45A0F94548AF36920D020CAA33@zynlms01.ZyLAB.WAN><522c6460802180429h44f5c46atb1d0eaf0e4bbcb32@mail.gmail.com>
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Subject: Re: [Tiff] bug? JPEGQUALITY seems not to be read correctly
Date: Mon, 18 Feb 2008 15:23:08 +0100
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Status:   

John,

Joris Van Damme (AWare Systems) wrote:
> One point that makes this whole argument moat is that this LibJpeg
> quality setting we're all familiar with, ranging from 0 to 100 is
> itself a LibJpeg specific measure.

I did forget to mention one last but important detail that invalidates the 
whole concept.

JPEG, in TIFF, can use the JPEGTables tag to emit a JPEG stream with tables 
only and no image data. Doing so, it can next contain inside the strip/tiles 
JPEG streams with image data and no tables. In this case, there's one 
consistent set of tables for the whole image.

However, the JPEGTables tag can also be omitted, and the tables can be 
embedded in the strip/tile JPEG streams. In that case, there can be 
different strip/tile specific tables. You'd end up with a different quality 
estimation for each strip/tile by itself, if your estimate wants to remotely 
reflect any reality at all. Even if the JPEGTables tag is used, individual 
strips/tiles can override the image-global tables in that tag, and specify 
other tables for use on that specific strip/tile only. This can be used so 
as to have better compression on some image regions, and better quality on 
others.

Add this last fact to the ones mentioned already, and indeed you end up 
asking yourself what quality we're discussing here. The concept is moot.


Best regards,

Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be/
Download your free TIFF tag viewer for windows here:
http://www.awaresystems.be/imaging/tiff/astifftagviewer.html

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