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2010.02.08 21:39 "FFT on two TIFF images", by Richard Nolde
Deborah,
Either you are making lots of assumptions about TIFF images or your
needs are very restricted. Here are a few things to consider if you want
to use LIBTIFF.
1) G3/G4 Faxes are encoded with 8 samples per byte before compression,
not 8 eight bits per sample so you have to extract a single sample from
each byte with bit masks and shifts and then promote that bit to a
double. LIBTIFF will handle the encode/decode of the compressed data
stream into your buffer, but then you have to extract the samples from
that packed buffer into a much larger buffer. Tiffcrop contains code
that extracts single bits from the G3/G4 decoded buffer if you want to
look at it. Promoting the values to double would just be a matter of
casting the returned samples as you copy them into your complex buffer
from the default buffer returned to your application by LibTIFF.
2) Fourier Transforms only work if the width of the sample is a power of
two, which is pretty unlikely to be the case with faxes. You will need
to pad each line out to a power of two with zeros before processing your
data with an FFT.
3) If both images are of different sizes, you may need to compare only
the parts that overlap, which could require some knowledge of the
individual images to determine which parts to extract.
4) Faxes from physical fax machines (they do still exist) are often
skewed and this may effect the accuracy of your results.
5) It has been a long time since I wrote an FFT, so I'm not too sure
about this next comment. You may want to check if there is any value in
doing an FFT on bilevel images. I think it is more common to do FFTs on
grayscale images where the rate of transition from light to dark can
vary over a wide area instead of being a simple on/off transition.
A fellow developer in Germany has recommended Libfreeimage as a good way
to get access to TIFF files using LibTiff. On Fedora, the RPMs are
called freeimage and freeimage-devel. I don't know which TIFF formats it
supports and what level of access it provides, but it might be worth a
check.
Richard Nolde
Tiffcrop author.