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From: erik@kroete2.freiburg.sub.org (Erik Corry)
Subject: Vertical differencing
To: tiff@sgi.sgi.com
Date: 	Wed, 26 Jan 1994 13:33:47 +0100
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Hello listreaders!

In Section 17 of the draft TIFF 6.0 document, the differencing predictor
for LZW compression is described. I have been using this, and it almost
always leads to smaller files.

I would like to add the option of vertical differencing to my software.
This is mentioned in Section 17, but is not thought useful, as it only
sometimes leads to smaller files. Therefore no tag value is defined.

The obvious choice is:

1: none
2: horizontal
3: both

and perhaps

4: vertical

Does anyone use these predictors? Have tag values been defined? Or private
tag values been reserved? Is there a de facto value?

One idea would have been to use the values for predictors defined for JPEG
in Section 25 (page 104 in the 1992-02-14 draft I have), but unfortunately,
the values that have already been defined for LZW are incompatible with 
these values. When I say 'both' above, I mean the JPEG scheme 4. This means
that for the pixels:


   |   |   |
 --+---+---+--
   | C | B |
 --+---+---+--
   | A | X |
 --+---+---+--
   |   |   |

the predicted value of X is A+B-C.

--
Erik Corry, Skagerrakstr. 2, D-79100 Freiburg, +49 761 406637
erik@kroete2.freiburg.sub.org 


