[gdal-dev] Outdated external overviews
julien.michel at cnes.fr
Fri Oct 7 06:34:57 PDT 2016
I recently came accross an issue in our software which uses overviews to
speed-up navigation in image : the image can change after an external
overview has been generated (for instance it has been re-generated by an
upstream processing chain with different parameters), this can lead to
display bugs or even crash in client code (if for instance the number of
bands has changed). Of course this is not a problem for the user that
knows what she is doing : simply removing and generating the overview
again fixes the problem. But for software that offers overview
generation to the end user, this might become an issue, as the software
has no clue wether the ovr file is outdated or not.
Possible mecanism to prevent this would be :
- Check file last modification time between external overview and image
file. If image file is newer than overview files, it is probably outdated.
- Encode image checksum in ovr file, and compare it upon loading (might
be a bit intensive).
With those checks, gdal could detect that the ovr file is outdated and
simply ignore it. Then client code could be aware that there are
actually no overviews for this image and take actions to generate new ones.
Any thought ?
PS: This of course does not apply if overviews are internal
CNES - DCT/SI/AP
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