[Gdal-dev] Several questions

Romano J M Silva rmagacho at predialnet.com.br
Tue Apr 13 18:15:10 EDT 2004

> There is no such documentation.  Some memory issues are driver specific,
> the general implementation of RasterIO() (which makes cachable block
> to the low level driver) will read one block request at a time, copy the
> data required out into the working buffer (passed in by the user) and then
> release that block into the cache.  The caching code will attempt to flush
> the least recently used block if adding a new block puts things over the
> GDAL caching limit.

What is this caching limit?

> > 4) What are tiled images and how can I create one?
> A tiled images is one where the "block size" or natural unit of access
> is less wide than a scanline and less high than the image.  The most
> (non-tiled) image organization is to treat each scanline as a block.
> accessing a small window of data is inefficient on a large image with this
> organization because so much uneeded scanline data is fetched and ignored.

I am commonly loading small window of data in a scanline mode. Is it
possible to  "create" a tiled GeoTiff?

> A tiled image splits the image into smallish squares (often 128x128 or
> 256x256) so that local read operations can be done efficiently even on
> large images.
> > 6) When getRasterXSize and getRasterBandXSize will generate different
> > results?
> The overviews of a band also have GDALRasterBand objects associated with
> them.  If you fetch the overview band objects they will have a different
> xsize and ysize than the base band or the dataset.

How do I retrieve an overview to get its bands?


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