[gdal-dev] RasterIO in paralel

[Even Rouault]

Le mardi 08 février 2011 23:07:36, Stefano Moratto a écrit :
> Thanks for the suggestion,
> 
> I have to read png from openstreetmap. I'm trying to render an A4 paper
> size sheet so I have a lot of data to move.
> It may be better to divide the area to be read in blocks and to give to
> each thread a block. Foreach  block a thread should read all the bands.
> Probabilly I should allign the block to the tile's boundary

Unfortunately the PNG format is such that it is not possible to seek to an 
arbitrary line without decoding the data of all previous scanlines, so this 
approach won't work. The same would be true for JPEG or GIF. If you can fetch 
the data as TIFF, you could however try that approach.

> 
> Stefano
> 
> > Le mardi 08 février 2011 22:41:50, Stefano Moratto a écrit :
> > > Hello,
> > > 
> > >           given an opened GDAL dataset, is it possibile to perform
> > > 
> > > gdalrasterio calls for each band in a dedicated thread per band in a
> > > thead-safe way?
> > > E.g. I have a raster image with 3 band (r,g,b). Since I have 4 core I
> > 
> > would
> > 
> > > like to I run a thread for a band r, one for band g and one for band g.
> > 
> > This has been discussed recently. See
> > http://lists.osgeo.org/pipermail/gdal-
> > dev/2011-January/027567.html and the following messages.
> > 
> > (I'd note that if you process a pixel interleaved image, there are
> > optimizations in the GTiff driver to make the fetching of other bands
> > almost
> > 'free' if you read block by block, so there's probably little to gain,
> > but the
> > results of your experiments are welcome)
> > 
> > > Regards,
> > > Stefano