Sean,
Yes, GDAL has an adapter between its VSI file system abstraction and
OpenJPEG own pluggable I/O layer, so you can use any VSI virtual file
system that has read & seek capabilities (so you can potentially do
/vsizip//vsicurl/ of a JP2). But the efficiency might not be that great.
Typically if it is a huge JPEG2000 file and you just need to extract a
subset of it, even if it is tiled, locating the tile of interest might
involve browsing through all the file, and thus decompressing most of
it. For 2 reasons:
- first, support for exploiting the TLM marker, which is the equivalent
of the TIFF TileOffset index, has been added only recently in OpenJPEG
master, so not released yet. And not all tiled JPEG2000 files include
TLM markers. The minimum JPEG2000 formulation is that each tile has just
a header with its size, and so you have to go from one header to the
next one to locate all tiles.
- and even if your file has TLM markers and you use openpjeg master, if
your ZIP is a regular ZIP and not a SOZip (shameless ad for
https://github.com/sozip/sozip-spec), seeking through at an arbitrary
within a compressed file within a ZIP is a slow operation. But if
someone really needs to include a JPEG2000 file they would better
included it as a non-compressed file, because there's absolutely no file
size gain to expect from applying deflate to a JPEG2000 file, and
including it as non-compressed naturally enables fast random seeking.
Even
Le 14/09/2024 à 21:41, Sean Gillies via gdal-dev a écrit :
Hi all,
In theory, we can read anything from a zip archive without extracting
it to the filesystem, but sometimes drivers have special requirements
and limitations. Is JP2 from a zipfile supported by OpenJPEG?
--
Sean Gillies
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