Matt,
Le 16/03/2021 à 21:45, [email protected] a écrit :
I recently learned that *jpegtran* from the JPEG library has the
ability to crop and save a new jpeg losslessly:
[...]
Can GDAL take advantage of this for jpeg-in-geotiff images?
to some limited extent, yes. I thought this was documented in
https://gdal.org/drivers/raster/gtiff.html but I can't find it.
So the most relevant doc is from this comment of the GeoTIFF driver
// When CreateCopy'ing() from a JPEG dataset, and asking for
COMPRESS=JPEG,
// use DCT coefficients (unless other options are incompatible, like
// strip/tile dimensions, specifying JPEG_QUALITY option, incompatible
// PHOTOMETRIC with the source colorspace, etc.) to avoid the lossy
steps
// involved by decompression/recompression.
Basically, you can do JPEG to JPEG-in-TIFF lossless transcoding if doing:
gdal_translate in.jpg out.tif -co COMPRESS=JPEG [[-co TILED=YES] [-co
BLOCKXSIZE=...] [-co BLOCKYSIZE=...]]
where the block size should be a multiple of 16.
Most other gdal_translate generic option (like -projwin / -srcwin) or
GTiff driver creation option are incompatible with the implementation of
this special mode (subsetting could be in theory possible on boundaries
of JPEG DCT blocks, but isn't implemented).
If adding "--debug on", you should see a line "GTiff: Using special copy
mode from a JPEG dataset" when this lossless transcoding mode works.
Otherwise the generic JPEG decoding / recoding is used.
JPEG-in-TIFF to JPEG-in-TIFF or JPEG lossless transcodingcould also be
in theory supported, but isn't implemented . With modest effort though
with some scripting it is possible to extract a JPEG file from a tiled
JPEG-in-TIFF file by reassembling the TIFF tile content with the
huffman/quantization tables in the JPEGTABLE TIFF tag.
If someone wants to propose a pull request with a doc addition to the
GTiff driver doc with the above content, that'd be welcome.
Even
*Matt Wilkie*
Geomatics Analyst
Environment | Technology, Innovation and Mapping
T 867-667-8133 | _Yukon.ca <http://yukon.ca/>_
/Hours: 08:30-16:30, Tue-Wed: Office, Thu-Fri: Remote./
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