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From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us>
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To: "Adell, Albert" <aadell@icc.es>
cc: "'tiff@remotesensing.org'" <tiff@remotesensing.org>
Subject: Re: [Tiff] Tiff JPEG compression quality factor
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Date: Fri, 28 May 2004 09:35:22 -0500 (CDT)
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On Fri, 28 May 2004, Adell, Albert wrote:

>> I'm using the libtiff v3.6.1 and I can save a tiff file with Jpeg
>> compression and quality factor 80, but when i
>> try to open it, the jpeg quality factor pseudotag has a value of 75. I
>> guess I am doing something wrong,
> because I assume that I can read the Jpeg quality factor written in advance.
> Is that correct?

The notion of JPEG "quality factor" is specific to the IJG JPEG 
library.  There is no quality factor saved to the image.  However, as 
it turns out, the parameters used when saving the JPEG may be 
estimated when the image is read.

GraphicsMagick 1.1 includes some code which accurately estimates the 
IJG JPEG quality factor and sampling factors when a JPEG is read and 
allows you to save using the original parameters.  However, I believe 
that complete support is implemented for JPEG files only and not for 
JPEG in TIFF (but I could be wrong).  I have not tested JPEG in TIFF 
to see if the parameters are recorded and saved.

Bob
======================================
Bob Friesenhahn
bfriesen@simple.dallas.tx.us
http://www.simplesystems.org/users/bfriesen
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