2005.09.12 21:22 "[Tiff] read-by-scan-line buffers full strip, part 2", by Mark Pilon

2005.09.13 17:48 "Re: [Tiff] read-by-scan-line buffers full strip, part 3", by Mark Pilon


Thanks for this -- exactly what I need. and I'm not even playing w/ a really small-memory system; I have several megabytes of memory I can alloc for this -- but not 60+.

thanks again,


>>i.e. using libtiff part-way to get access to the compressed data, but >>perform the decompression in smaller increments than the whole strip.


> Much of my first reply stands. And you're fortunate: the LZW compression > codec isn't very complicated. Neither is your type of image, it's just a

> single plane, single strip, without any subsampling or prediction or > whatever... Still, though, this will keep you occupied for at least a

> few days I guess... ;-)
> Use TIFFGetField to get the offsets and bytecount of the single strip as
> detailed.

> The next part of your code should be thought of as a pulling pipeline > scheme. Whatever the destination of the scanlines is, this pulls the

> scanlines from the LZW decoder as they are needed. This decoder pulls > data from the strip as it needs it, small buffer at a time (say 2K).


> 'Pulling from the strip' may be implemented as calls to the seek and > read proc that are given in TIFFClientOpen, and pointed to in the

> LibTiff TIFF structure. That way, it'll work whatever IO layer is used > added to LibTiff. The LZW decoder, you'll have to build. Use tif_lzw.c

> as a source of documentation. There is also a good deal of detail on the > LZW codec in the TIFF spec (section 13, page 57 and below), but do make

> sure to also peek at tif_lzw.c as this documents also an older pre-6.0 > LZW encoding that is not detailed in the spec and that you may or may

> not need to include.
> Essentially, what you want to do is rework the decoder in tif_lzw.c,
> such that
> - it returns when it has build a single scanline

> - where it currently merely accessed the compressed data in available > memory, it makes a call to a reading helper procedure instead. This

> helper procedure will check whether data is available still in the small > buffer, and if it's not, refill it


> A particular issue to keep in mind is that neither your input buffer nor > your output buffer of your own LZW decoder respects scanline boundaries.

> For your input buffer, this is normal, of course, since you can't > predict how much input you'll need to output a scanline. Thus, simply

> keep the input buffer around, without changing its state from one > scanline-pulling call to the next.

> For your output buffer, this is because the LZW compression doesn't

> 'stop and restart' at scanline boundaries. It may very well be that a > particular LZW compressed code resolves to five uncompressed bytes,

> where your scanline has only 2 bytes left, and the next three bytes are > the first three of the next scanline. This is a nasty issue, that is

> resolved on tif_lzw with 'restart logic' with the aid of such state > variables as 'dec_restart' and related.

> Good luck!

Joris Van Damme
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Mark Pilon

Konica Minolta Printing Solutions U.S.A.

P.O. Box 37
325 Dawson County Road 227
Fallon, MT. 59326-0037
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