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February 2007
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2007.02.06 05:03 "Re: The fake input in fax2tiff", by Joris Van Damme
John,
John Whitington wrote:
> I'm modifying the fax2tiff source to decode CCITT streams from PDF
> files into raw data. Thus, I don't need to produce an output TIFF. It
> works, but now I'm paring it down by removing the output code.
>
> The fake input fax2tiff produces to make the fax input work is
> initialised in the original source like this:
>
> faxTIFF = TIFFClientOpen("(FakeInput)", "w",
> /* TIFFClientOpen() fails if we don't set existing value here */
> TIFFClientdata(out),
> TIFFGetReadProc(out), TIFFGetWriteProc(out),
> TIFFGetSeekProc(out), TIFFGetCloseProc(out),
> TIFFGetSizeProc(out), TIFFGetMapFileProc(out),
> TIFFGetUnmapFileProc(out));
>
> (Where 'out' is the TIFF pointer being used for output).
>
> How can I synthesize the values (TIFFClientdata etc.) without having
> to open an actual output TIFF?
Perhaps I'm misunderstanding, but it seems to me there is a much easier
approach. Work out a fake file header and fake IFD that for convinience you
place right after the file header. Make the fake IFD have all appropriate
settings for this kind of file, like appropriate Photometric and Compression
etc. Make it specify one strip, located right after the fake IFD. This fake
data, file header and IFD, make it a resource or even a constant array in
your code, it shouldn't be too long. I'll call that length FakeLength from
this point on.
Now make fake read and seek procs. For instance, when you receive a read for
data that lies before FakeLength, return the appropriate part of your fake
file header and IFD data. When you receive a read for data that lies beyond
FakeLength, substract FakeLength from the offset, and provide the
appropriate portion of the CCITT compressed data.
This should enable you to use a completely normal LibTiff decoding session,
except that you call TIFFClientOpen and pass pointers to your modified seek
and read procs.
Or am I missing something?
Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be/
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