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2012.11.24 00:09 "help writing thumbnails to TIFF file?", by Paul Heckbert
2012.11.24 00:46 "Re: help writing thumbnails to TIFF file?", by Joris Van Damme
2012.11.25 13:01 "Re: help writing thumbnails to TIFF file?", by Edward Lam
2012.11.25 15:08 "Re: help writing thumbnails to TIFF file?", by Joris Van Damme
2012.11.27 02:10 "Re: help writing thumbnails to TIFF file?", by Paul Heckbert
2012.11.27 03:25 "Re: help writing thumbnails to TIFF file?", by Bob Friesenhahn
2012.11.27 13:12 "Re: help writing thumbnails to TIFF file?", by Joris Van Damme
2012.11.27 14:38 "Re: help writing thumbnails to TIFF file?", by Olivier Paquet
2012.11.28 03:06 "Re: help writing thumbnails to TIFF file?", by Olivier Paquet
2012.12.08 04:35 "Re: help writing thumbnails to TIFF file?", by Paul Heckbert
2012.11.27 18:09 "Re: help writing thumbnails to TIFF file?", by Daniel Mccoy

2012.11.27 02:10 "Re: help writing thumbnails to TIFF file?", by Paul Heckbert

We have several opinions so far:
Daniel McCoy said he implemented TIFFCheckpointDirectory years ago to  
allow partially-written TIFF files to be read.
Olivier Paquet and Edward Lam said that they use this function for  
that purpose, and it works for them.  Edward thinks that most other  
people should not use TIFFCheckpointDirectory.
Joris Van Damme referred to the function as a hack that served a  
mysterious function for its original developers.  He added that it  
would be unlikely to suit me or anyone else but he didn't explain why  
it wouldn't, other than to compare TIFFCheckpointDirectory to  
Stonehenge.  He argued that people shouldn't care about the order of  
data in their TIFF files.

I side with those that are saying that TIFFCheckpointDirectory is  
useful and (at least sometimes) works properly.  Without  
TIFFCheckpointDirectory, the directory would always appear at the end  
of a TIFF file, after the pixels, and reading partial TIFF files would  
be impossible.  Some control of data order in the file IS useful,  
namely: does the directory appear before or after the pixels?

Other situations where it's useful to have the directory first: if a  
file is being streamed in, you might want to inspect the directory and  
cancel the streaming.  And for very large files (I currently work with  
TIFFs up to 4 gigabytes, and soon, larger) it's nice to maximize  
performance and minimize seek times by putting directories,  
thumbnails, and other header-like information at the beginning of a  
file.  That's also the order in which most of the rest of the world  
writes their TIFF files.

PS: If we're interested in distant history, I worked with Sam Leffler  
at Pixar in 1986 and he and I wrote the Pixar Software Standards  
document together.


Returning to my code:

I wonder if others have attempted to write two directories (IFDs)  
using TIFFCheckpointDirectory.  Anyone got pointers to code that does  
this?  Am I using TIFFSetDirectory properly?

My current hypothesis: TIFFCheckpointDirectory works fine if you're  
writing only one directory, but it's buggy when there are multiple  
directories.

I tested further and it seems that my program, when run with "thumb - 
nocheckpoint", yields a TIFF that "exiftool -hexdump" can parse  
properly even though the directories follow the pixels.  I don't know  
if other TIFF-reading software can parse it properly, however.