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August 2005
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2005.08.04 20:50 "Re: TIFFWriteScanline and G4 compressed block concatenation", by Joris Van Damme
Philip, Would you mind posting in plain text message format, and providing a good subject line in the future? That is the standard way to mail to a mailing list, plus, it is also efficient for both human readers and post-processing code. > 1. When generating a TIFF file and I'm using TIFFWriteScanLine(), is > each call to TIFFWriteScanLine() generating a strip within the TIFF > file? That is determined by the value of the RowsPerStrip tag, http://www.awaresystems.be/imaging/tiff/tifftags/rowsperstrip.html. In case you're familiar with strip-based access, you may be tempted to think the scanline-based access is just that, with strips spanning a single line. Well, it's not. The scanline-base access is build so as to access the multi-scanline buffers written and read using normal strip-based routines (multi meaning 1 or more). > 2.I want to extract a CCITT Group 4 image from a TIFF file. If I obtain > the strip offsets using TIFFGetField(), can I concatenate the strips > together to create a valid CCITT Group 4 image file? A good way to find out is to try. I'm not enterily sure and would have to double-check, but I don't think you can do that. Two CCITT group 4 compressed blocks, concatenated, is not one valid block I think. For one thing, I'm seeing a bit accumulator that is cleared and reset in the decoder, when the same decoder state is reused on a subsequent strip, so, presumably and without additionally consulting specs, scanlines aren't aligned to byte offsets in a single group 4 compressed block. 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