Hi Even, On Fri, Oct 31, 2014 at 1:01 PM, Even Rouault <even.roua...@spatialys.com> wrote:
> > * Favouring dataset.RasterIO() over band.RasterIO() to prevent up to 4x > > raster reads > > If the size of the image (or your requested window) is below the GDAL block > cache maximum size (default is 40 MB), then most drivers should cache the > data > for other bands when reading one band (particularly if decompression is > involved), if the storage on disk is pixel interleaved (i.e > R1,G1,B1,R2,G2,B2, > etc...), which is generally the case for GeoTIFF. So reading in one call > or 4 > calls will probably not make a lot of difference. > Good, that's what I suspected. > But if you know that you have to read the 4 bands, there's no reason not to > use dataset.RasterIO() in all situations. GDAL, either in its generic > implementation or in the driver specific one, will look at the declared > interleaving and will/should select the most appropriate reading strategy, > ie. > potentially operating block by block if the data is pixel interleaved, or > band > by band otherwise. > Okay, thanks. > > > * Use of GetBlockSize() to read on block boundaries, then cropping data > > later. > > I've not looked closely at the code you point to, but that depends on what > you > do. If you have allocated a big buffer to contain the whole requested > window, > then just issue a single RasterIO() and let GDAL do its job to filling it > the > best way. > If you just allocate small chunks of memory and process chunk by chunk, > then > it is indeed better to call RasterIO() on the block boundaries (or > multiple of > block boundaries). > In this case it's the former, so I guess GetBlockSize isn't relevant. > > > * Use GetMaskBand()/GetMaskFlags() to simplify nodata/alpha handling > > (probably won't impact performance but might get rid of a bunch of code) > > Yes that can be convenient to avoid having to deal both with nodata and > alpha > band in a unified way. Plus some formats can really have a mask band that > is > neither of the two. For example you can create a JPEG-compressed YCbCr TIFF > and use an internal mask band to encode transparency (although this is > admitedly quite an exotic TIFF formulation because most readers will ignore > the transparency mask) > That's what I thought (currently I use VRTs to re-alpha-ize mask bands if necessary) Thanks for your guidance, Rob :)
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