Gary Lucas created SANSELAN-80:
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Summary: Incorrect code for tiled TIFF files applyPredictor method
Key: SANSELAN-80
URL: https://issues.apache.org/jira/browse/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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