[gdal-dev] Overviews using GDAL

[Dmitry Baryshnikov]

Hi Hari,

26.12.2012 9:16, HariPrasad ?????:
> Hi Dmitry
>
> I was inspecting various GDAL sources
> (http://trac.osgeo.org/gdal/browser/trunk/gdal/frmts). I was bit confused
> how to implement RasterIO method. Which format to look for better idea.
By my opinion the best format is geotiff
> Should i call IRasterIO() method in IReadBlock?
>
> /CPLErr RBDRasterBand::IReadBlock( int nBlockXOff, int nBlockYOff, void *
> pImage )
> {
>    return IRasterIO( GF_Read, nBlockXOff, nBlockYOff, nBlockXSize,
> nBlockYSize,
> 	                      pImage, nBlockXSize, nBlockYSize, eDataType,
> 	                      GDALGetDataTypeSize(eDataType)/8, 0 );
> }/
>
> /CPLErr RBDRasterBand::IRasterIO( GDALRWFlag eRWFlag,
> 	                                 int nBlockXOff, int nBlockYOff, int
> nXSize, int nYSize,
> 	                                 void * pData, int nBufXSize, int
> nBufYSize,
> 	                                 GDALDataType eBufType,
> 	                                 int nPixelSpace, int nLineSpace )
> {
>      //my logic
> }/
>
>
> Earlier we were using ERDAS method to get the handle
> /long LayerRasterRead( void *lHandle, unsigned long bRow, unsigned long
> bCol,
> 			            unsigned char **pixels )
> {
> //logic
> }/
>
> Is there any method similar to get the handle in GDAL?
The best starting point is http://gdal.org/gdal_tutorial.html but in 
example shown reading of one line of the image.
Reading the whole image will looks like this:

     int nPixelSpace(0);
     int nLineSpace(0);
     int nBandSpace(0);
     if(nBandCount > 1)
     {
         int nDataSize = GDALGetDataTypeSize(eDT) / 8;
         nPixelSpace = nDataSize * nBandCount;
         nLineSpace = nBufXSize * nPixelSpace;
         nBandSpace = nDataSize;
     }
     void*    data = CPLMalloc (nWidth * nHeight * nDataSize * 
nBandCount); //allocate memory for pixels
     pGDALRaster->RasterIO( GF_Read, nXOff, nYOff, nXSize, nYSize, data, 
nWidth, nHeight, eDT, nBandCount, panBandList, nPixelSpace, nLineSpace, 
nBandSpace);
Details: 
http://gdal.org/classGDALRasterBand.html#a5497e8d29e743ee9177202cb3f61c3c7

If nWidth != nXSize and nYSize != nHeight the closest overview will be 
used for fast access to pixel data.

If eDT == /GDT_Byte/you can write: unsigned char* pixels = (unsigned 
char*)   data;
> --
> View this message in context:http://osgeo-org.1560.n6.nabble.com/Overviews-using-GDAL-tp5022226p5024588.html
> Sent from the GDAL - Dev mailing list archive at Nabble.com.
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>
Best regards,
     Dmitry
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