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From "Gary Lucas (JIRA)" <>
Subject [jira] [Created] (SANSELAN-80) Incorrect code for tiled TIFF files applyPredictor method
Date Sun, 06 May 2012 20:17:48 GMT
Gary Lucas created SANSELAN-80:

             Summary: Incorrect code for tiled TIFF files applyPredictor method 
                 Key: SANSELAN-80
             Project: Commons Sanselan
          Issue Type: Bug
          Components: Format: TIFF
            Reporter: Gary Lucas
            Priority: Minor

I believe that the DataReaderTiled class used for reading tiled TIFF files invokes the applyPredictor
method with incorrect arguments and will not be able to properly decode TIFF files that use
predictors.  The bug was found during a code inspection. Unfortunately, I do not have any
samples of data in this format (there are none in the Apache Imaging test files) and cannot
verify that this is the case.

Some Background
TIFF files are often used to store images in technical applications where data must be faithfully
preserved, so lossy compression methods like JPEG are inappropriate and non-lossy method like
LZW must be used. However, continuous tone images like satellite images or photographs often
do not compress well since there is little apparent redundancy in the data. To improve the
redundancy of the data, TIFF uses a simple predictor.  The first pixel (gray tone or RGB value)
in a tile is stored as a literal value.  All subsequent pixels are stored as differences.
 To see how this works, imagine a monochrome picture where the gray tones gradually fade from
white to black at a steady rate. Although no particular data value is ever repeated (so there
is little apparent redundancy in the source data) the delta values remain constant (so a set
of delta values will compress very well). When transformed in this matter, certain images
show substantial improvements in compression ratio.

The Probem
The DataReaderTiff class uses a method called applyPredictor that takes an argument telling
it whether the sample passed in is the first value, and should be treated as a literal, or
whether it is a subsequent value and should be treated as a delta.   Unfortunately, the parameter
it uses is the x coordinate of the pixel to be decoded.  While this approach works for TIFF
strip files (where the first pixel always has a coordinate of zero), it does not work for
tiles where the first pixel in the tile could fall anywhere in the image. 
The Fix
While we could simply fix the argument passed into the predictor, there is a better solution.
The predictor performs an if/then operation on the input parameter to find out if it is the
first sample in the tile. Once it unpacks a sample, it retains it as the "last" value so that
it may be added to the next delta value.  Why not simply get rid of the if/then operation
and just ensure that the last value gets zeroed out before beginning the processing of a strip
or tile.  This would save an if/then operation and fix the bug.

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