[COG] Tiff and JPEG2000 - COW

[Even Rouault]

Norman,

> While this work is still experimental, we see a number of clear benefits to
> COW over HTTP:
> 
> * Progressive in spatial position, resolution and quality

As far as I understand JPEG2000, the issue is that you can't have at the same time fast spatial 
sub-window access and fast overview access in the same file layout. I believe the former is 
obtained with PCRL (Position-Component-Resolution-Layer) progression order (but even 
with that, I believe you'd need several non-contiguous ranges as the precincts corresponding 
to the small overview level are common to several spatial regions), and the later with RPCL 
(Resolution-Position-Component-Layer)

> * Region of Interest encoding

Is there a real use case for this ? If so, that could also be done with JPEG-in-TIFF. You can use 
different JPEG quality tables for each TIFF tile.

> 
> The biggest stumbling block with JPEG2000 has always been understanding the
> core spec which is very complex.

Indeed. Having spent several weeks trying to improve OpenJPEG performance last summer, 
this involves huge headaches. Currently one big limitation of openjpeg regarding to what you 
mention above is that the lib needs to ingest the whole codestream of a JPEG2000 tile (so 
the whole file for a single-tiled JPEG2000 file). This could perhaps be enhanced, but this is a 
non trivial work. But this would have to be done since COW without an open source 
implementation wouldn't get much traction.

To check quickly if COW is something that has a potential, I'd suggest you try with the GDAL 
JP2KAK driver and /vsicurl/ and look at the range requests done.

Another issue is that even with the best implementation, the CPU time needed to decode 
JPEG2000 is way higher than with JPEG.

Even

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com
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