[gdal-dev] Overriding IRasterIO in class derived from GDALRasterBand

[Jorge Arévalo]

Hello,

2009/8/12 Tamas Szekeres <szekerest at gmail.com>:
>
>
> 2009/8/12 Jorge Arévalo <jorge.arevalo at gmail.com>
>>
>> >> I'm afraid I have not quite followed the discussion of the IRasterIO
>> >> implementation, but I find the above statement quite alarming. It is
>> >> imperative that we *not* end up reading back all blocks if only a few
>> >> are requested.  Imagine a 100GB image split up into blocks in a
>> >> table.  If an application using GDAL is run and only needs a small
>> >> area of the file it is important that the blocks not requested never
>> >> be pulled from the DB.
>> >
>> > I can only second Frank's comment.
>> > IOW, general idea is to query minimum (sub)set of tiles requested,
>> > where request is based on spatial extent (i.e. geographic coordiantes)
>> > or extent within pixel space of a coverage (specified as range of rows
>> > and columns of a raster).
>> >
>>
>> OK. Then, and correct me if I'm wrong, the point is to query only the
>> extent asked by the program using GDAL driver. Then, I should query
>> only the blocks requested in, for example, the IReadBlock method, just
>> as I did. But Tamas' suggestion was to minimize the number of server's
>> rounds implementing a kind of cache system at driver's level.
>
>
> I may have been misunderstood somewhere, but I've never talked about the
> requirement to fetch all the data (the entire extent) of the raster data
> table.
> We have been talking about fetching all the tiles requested by a single
> RasterIO call which tiles would definitely be requested by the subsequent
> IReadBlock calls if we were rely on the default RasterIO implementation.
> Just like what the WCS driver currently implements in DirectRasterIO to
> prevent from accessing the server for each tiles.
>

Yes, of course. The idea of fetching all the data covered by the
extent of the raster was my (wrong) own suggestion.

In WCS, RasterBand::IRasterIO calls Dataset::DirectRasterIO, that
constructs exactly one GetCoverage petition for the given window,
instead of constructing one petition for each small block.

My generalization was to create a single query to get all the extent
of the raster after opening the dataset, instead of all the extent
needed in a IRasterIO call.
That was the error. I'm sorry.

Then, I should follow my first option: override the IRasterIO method
in RasterBand class (and in Dataset too, I suppose).

Best regards,
Jorge


> Best regards,
>
> Tamas
>
>
>