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September 2007
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2007.09.24 07:51 "Re: G3 fax: EOL needed at the beginning of each strip?", by Joris Van Damme
Martin, ComSquare Martin Osieka wrote: > > The spec clearly indicates strips have to start with EOL. > > But notin TIFF 6.0, or? > Coding Scheme > A line (row) of data is composed of a series of variable length code > words. Each code word represents a run length of all white or ... > No EOL code words are used. No fill bits are used, except for the > ignored bits at the end of the last byte of a row. RTC is not used. That sounds like G4, not G3. (Confusingly, G3 is also called T.4, and G4 is also called T.6, maybe that's what got you off track.) > > Unfortunately, I've a reasonable amount (i.e. not just a fluke) of > > testimages where garbage data preceeds the G3 block. LibTiff > > succeeds on these images by skipping to the first EOL. If we were > > to change LibTiff to start decoding right away, more unexpected > > stuff will happen on these images with preceeding garbage data that > > might just be more frequent. In AsTiff, what I do if the strip or tile > > doesn't start with an > > EOL, is actually test and see if a line of desired length can > > successfully be decoded from the preceeding data, and if that data > > then is followed by an EOL. If so, I regard that the first line. If > > not, the code starts at the first EOL instead. This works on both > > the testimages with random preceeding data, and the testimages that > > just lack the first EOL. But the LibTiff fax decoder being as it > > is, a heap of macro's with embedded macro's that nobody at this > > stage can easilly decipher, it may be hard to make it behave > > equally intelligent. > > This approach is much better than my one. > Because the algorithm implemented in libtiff works line oriented > (Macro EXPAND1D) it should not be to difficult to implement your > approach with the existing macros. EXPAND1D sounds like what might be used for 1D lines. Not all G3 is 1D lines. > May I ask you for a reference file containing garbage at the > beginning of strips? That would be hard, right now, as I'm converting some stuff and don't have a good enough search function for this particular thing available to me at the moment. I'll get you one as soon as I can, though. Probably in a couple of days or so. Best regards, Joris Van Damme info@awaresystems.be http://www.awaresystems.be/ Download your free TIFF tag viewer for windows here: http://www.awaresystems.be/imaging/tiff/astifftagviewer.html