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https://issues.apache.org/jira/browse/IMAGING-33?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13271490#comment-13271490
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Gary Lucas commented on IMAGING-33:
-----------------------------------
Thanks. Nice job on the changes, they're exactly what I had in mind... Now
the code is both more correct than it was before and also a little bit faster
as well.
> Incorrect code for tiled TIFF files applyPredictor method
> ----------------------------------------------------------
>
> Key: IMAGING-33
> URL: https://issues.apache.org/jira/browse/IMAGING-33
> Project: Apache Commons Imaging
> Issue Type: Bug
> Components: Format: TIFF
> Reporter: Gary Lucas
> Priority: Minor
> Fix For: 1.0
>
>
> 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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