[gdal-dev] Question on 0th IFD offset when producing a JPEG-Compressed GeoTIFF (GDAL 1.11 vs 2.x)

[Daniele Romagnoli]

Hi List,
I'm investigating on some GeoTIFF data being produced with GDAL 1.11.3.
Basically, when setting COMPRESS=JPEG, the generated GTIFF has the 0th IFD
offset set to some KB.

these are the first 16 bytes of a GeoTIFF being generated like that:
49 49 2A 00 88 44 00 00 *10* 00 00 01 03 00 01

so:
49 49 is the endianness
00 2A is the magic number
00 00 44 88 is the offset of the 0th IFD = (17544 bytes)
Then there are several TAGs (baseline, extended and so on).
However, I think that any TIFF reader will jump to the 00 00 44 88 offset.
Moreover, the offset 00 00 00 08 (referring to the red bolded byte in my
previous sample) seems not referred by any other referencing IFD.

So it seems like that GeoTIFF has some useless bytes right after the first
8s.

I have then tried to use GDAL 2.2.1 and GDAL 2.1.0 and with that version I
see that the GeoTIFF being produced using same exact command is smaller and
the 0th IFD offset is 8 with these recent versions.
(This information is confirmed by AsTIFFTagViewer and TiffInfo too).

When activating CPL_DEBUG on all versions I see this output on GDAL
1.11.3/1.11.4 whilst I don't see it on GDAL 2.x

*GTiff: directory moved during flush in FlushDirectory()*

I have quickly checked some reference in the issue tracker but I was unable
to find anything, especially due to the fact that when searching for "IFD"
it returns hundreds of results related to "IFDefined" :-)

Do you have any feedback or comment on this?
Is it related to any bug in old code or some version update? (is a
different libtiff version somehow involved?)

Please, let me know.
Best Regards,
Daniele




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