[gdal-dev] (no subject)

Even Rouault even.rouault at mines-paris.org
Thu Nov 12 19:19:45 EST 2009

Hi Tamas,

2 issues directly related to the big raster dimensions :

* Currently GDAL allocates an array of pointer to raster blocks of size
nBlocksPerRow x nBlocksPerColumn where nBlocksPerRow =
(nRasterXSize+nBlockXSize-1) / nBlockXSize and nBlocksPerColumn =
(nRasterYSize+nBlockYSize-1) / nBlockYSize.
When TileLevel = 20, nBlocksPerRow = nBlocksPerColumn = 1 048 576. This is too
big. Currently, we then try to use a second level of block array
("sub-blocking") as soon as nBlocksPerRow > SUBBLOCK_SIZE/2 (SUBBLOCK_SIZE=64).
This leads to allocating nSubBlocksPerRow x nSubBlocksPerColumn where
nSubBlocksPerRow = (nBlocksPerRow + SUBBLOCK_SIZE + 1)/SUBBLOCK_SIZE and 
nSubBlocksPerColumn = (nBlocksPerColumn + SUBBLOCK_SIZE + 1)/SUBBLOCK_SIZE.
In that case, nSubBlocksPerRow = nSubBlocksPerColumn = 16384, and
nSubBlocksPerRow x nSubBlocksPerColumn = 268 435 456 pointers, so 1 GB on a
32bit machine or 2 GB on a 64bit machine.

--> So we would need a three-level block cache. This would be feasable
(implementation "detail" of gdalrasterband.cpp).

Or I'm thinking that we could completely remove the array and use instead a hash
map that would map a block number to its raster block. Could cause a (small?)
slowdown of course but would be simpler to implement and maintain than a 3 level

* At TileLevel = 23, nRasterXSize = 2 147 483 648, which is ahem just one byte
more than the largest signed int, so = -2147483648
--> The proper solution would be to promote nRasterXSize/nRasterYSize to be of
type GIntBig (64bit). We would also need to update the IReadBlock(),
IWriteBlock(), IRasterIO() interfaces (and probably many others) to use GIntBig.
So this is both a large API and ABI breakage, that IMHO looks more like a GDAL
2.0 project... An alternative would be to have both the existing 'int'
variables/interfaces and introduce new GIntBIG variables/interfaces, and migrate
GDAL internals (gcore/* alg/*) and the relevant drivers such as WMS to use the
GIntBig version. Could easily turn to be very messy in the end...

I cannot think of an easy workaround. An idea would be to split the WMS extent
into 4 pieces to go under the 2 147 486 647 pixel limit, but this is not really
doable without a small hack in the TMS minidriver as you would need to add an
offset to the x and y tile number requested to the server.

Best regards,


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