[Qgis-developer] Large raster (ecw) identify very long

[Radim Blazek]

On Wed, Aug 22, 2012 at 12:29 PM, haubourg
<regis.haubourg at eau-adour-garonne.fr> wrote:
>
> Radim Blazek-2 wrote
>>
>>
>> QGIS is using GDALRasterIO() which reads a single pixel on original
>> resolution. AFAIK, ECW is using tiles internally so it should be all
>> very fast. I can imagine 2 problems:
>>
>> - the tiles in ECW file are too big - can you verify somehow how big
>> are the tiles?
>>
> Hi Radim, I'm not ecw specialist, but I understand it is a wavelet
> compression algorithm, not  a tiled one.
> There is one big image with pyramids inside.

I am not sure, but I thought that it is using wavelet, pyramids and
tiles. Without tiles it could not be fast enough on higher resolution.

> Radim Blazek-2 wrote
>>
>> - GDAL is reading bigger portion of data than necessary (cc to GDAL list)
>>
>> Is it drawing of the raster also so slow (on raster resolution zoom)?
>>
>> Radim
>>
>>
> No everything is fine when drawing data, whatever resolution I use, this is
> the big thing with ecw.
> I must say I find that it was a bit faster with 1.7.4 than 1.8 and later.
> But this could be related to other factors like internal network
> capacities.. I haven't benched on it yet.

I found in GDAL ecwdataset.cpp that it is treating  single row
requests in IRasterIO in a special way:
    if( nYSize == 1 )
    {
        return GDALRasterBand::IRasterIO(eRWFlag, nXOff, nYOff, nXSize, nYSize,
                                         pData, nBufXSize, nBufYSize,
                                         eBufType, nPixelSpace, nLineSpace );
    }
That could be the difference between draw and identify. Are you able
to test with current master if I send you a patch for QGIS GDAL
provider?

Radim

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