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January 2008
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2008.01.10 23:04 "Re: LibTiff..Reading partial strips", by Nikhil Shirahatti
Also, I may only want to read the first N pixels ..lets say the interesting data was in the first N pixels ..then why read the remaining? If the ReadStrip is done several times(lets say 10000 rows) then each time we end up reading M pixels (M >> N) rather than N..and this does cause a performance hit.. Nikhil On Jan 10, 2008 2:48 PM, Nikhil Shirahatti <shirahatti@gmail.com> wrote: > ReadScanline doesn't really help me..OK let me rephrase the question...the > reason I want to read partial strips is: > lets say a strip is of length 1000. I want to read be able to read the > strip from 0 to 100, 101 to 200...and so on so I have 10 chunks of data. > The reason why I want to do this is to convert it to our internal > structure for further processing. One may argue why not use the tiles in > libtiff...because we are not guaranteed that the tiff is tiled (they may > be > from anywhere)...Unfortunately the scan line also reads the whole scan > line...even though much less memory intensive doesn't serve our goal... > > > On Jan 10, 2008 2:08 PM, Frank Warmerdam <warmerdam@pobox.com> wrote: > > > Nikhil Shirahatti wrote: > > > I havent tried it..but the cautionary note that " It is not always > > > possible to do so due to decompression constraints" scares me. I also > > > read on the archive that "that *strip* * chopping* only works for > > > uncompressed IFDs.". How common(or uncommon) is this? When would one > > > use compressed IFD's? > > > The tiff's we get can be compressed or uncompressed formats and we > > have > > > to support them all. :) > > > > Nikhil, > > > > I think Bob means compressed images. There is no such thing as > > compressed > > IFDs. Compressed TIFF images are quite a common case, and fax > > compressed > > images are often one strip. > > > > In addition to "strip chopping", libtiff also has the TIFFReadScanline() > > > > function for reading strips one scanline at a time. This operates > > without > > needing to read and decompress the whole strip at once. > > > > Best regards, > > -- > > ---------------------------------------+-------------------------------------- > > > > I set the clouds in motion - turn up | Frank Warmerdam, > > warmerdam@pobox.com > > light and sound - activate the windows | > > http://pobox.com/~warmerdam<http://pobox.com/%7Ewarmerdam> > > and watch the world go round - Rush | President OSGeo, > > http://osgeo.org > > > > > > > -- > ----------------------------------------------------------------- > Nikhil V Shirahatti > http://vision.cs.arizona.edu/nvs > -- ----------------------------------------------------------------- Nikhil V Shirahatti http://vision.cs.arizona.edu/nvs