Hi Even,
Le 19/05/2014 20:01, Even Rouault a écrit :
Guillaume,
I am using the OpenJPEG driver from GDAL and I noticed some differences
with the behaviour of an other OpenJPEG "driver" that I used before in
the library Orfeo ToolBox. The GDAL driver fails to pass a conformance
test : using the input file "jpeg2000_conf_p1_06.j2k"
Are you refering to
http://code.google.com/p/openjpeg/source/browse/data/input/conformance/p1_06.j2k
? I see this is a 12 x 12 pixel image ...
Yes, this is a 12x12 image.
, reading the
resolution level 4 (zero being the original image) , it should produce a
1x1 image with the pixel value [195, 36, 100].
The behaviour of GDAL drivers differs on the following points :
* The number of overviews detected for a jpeg2000 dataset is limited
(no overview if its dimensions are both lower than 256). Even if the
the overviews can be computed by GDAL, it won't use the wavelet
coefficients for the highest levels, but rather perform the default
nearest neighbor interpolation.
Yes, this is to avoid exposing overviews that are too small and do not make
pratical sense. Other JPEG2000 drivers, such as JP2KAK or JP2ECW, have similar
logic.
* The size of the overview at level 'n' is computed as : ovr_size =
raster_size / 2^n , whereas it should be ovr_size = ceil(
raster_size / float(2^n) )
Hum, any reason to particularly round to the upper value rather than the lower
value ? The JP2ECW driver does the rounding to the lower value too.
This rounding to the upper value is done in OpenJpeg (for instance :
src/lib/openjp2/j2k.c:7648 ). I think it is also used to compute the
maximum number of resolutions. I am not sure if using a lower-value
rounding in GDAL driver has an impact on the decompressed images (other
than a possible crop).
I am using GDAL 1.10.1 and OpenJpeg 2.0.
Is it relevant to report a bug on this topic ?
IMHO the test case seems to be a bit too artificial to justify any change,
unless there are real world cases where that would cause issues.
I agree that this is a corner case, and I guess you would like to keep a
consistent behaviour between the different jpeg2000 drivers. My worry is
more about the compliance with the standard.
Even
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