[gdal-dev] GDALRasterBand::RasterIO returning block containing 0 for area that does not overlap the data.

[Even Rouault]

Damian,

There's no dedicated function in the GDAL API to know if a window is only no 
data without querying the pixel values. It would make only sense for drivers 
that support sparse coverages, like VRT, some formulations of TIFF with 
missing blocks, ERDAS Imagine, GeoPackage, ...

Even

> Hi,
> 
> GDALRasterBand::RasterIO returning block containing 0 for area that does
> not overlap the data.
> 
> This is slightly different from requesting data for an area that overlaps
> the data, but there is no data.
> 
> Is there away of checking for this condition before I call RasterIO?
> 
> 
> I’m using the following to read a block of data at a time:
> 
> GDALRasterBand *poBand;
> …
> poBand->RasterIO(GF_Read,startX+xoffset,startY+yoffset,blockWidth,blockHeig
> ht,buffer[band],blockWidth,blockHeight,GDT_Int32,0,0);
> 
> 
> This works fine, but I would like to be told if the block I am reading does
> not overlap the data.
> 
> There are two reasons for this;
> 
> (1) It’s an early exit, before the block is set to 0 or the no-data value.
> (2) It means I don’t have to check the block to see if it is filled with
> just 0 or no-data.
> 
> The stack trace for the calls is as follows:
> 
> tplgdal11164d.dll!VRTSimpleSource::GetSrcDstWindow(int nXOff, int nYOff,
> int nXSize, int nYSize, int nBufXSize, int nBufYSize, int * pnReqXOff, int
> * pnReqYOff, int * pnReqXSize, int * pnReqYSize, int * pnOutXOff, int *
> pnOutYOff, int * pnOutXSize, int * pnOutYSize)  Line 685 C++
>   tplgdal11164d.dll!VRTSimpleSource::RasterIO(int nXOff, int nYOff, int
> nXSize, int nYSize, void * pData, int nBufXSize, int nBufYSize,
> GDALDataType eBufType, int nPixelSpace, int nLineSpace)  Line 889 + 0xad
> bytes C++
>   tplgdal11164d.dll!VRTSourcedRasterBand::IRasterIO(GDALRWFlag eRWFlag, int
> nXOff, int nYOff, int nXSize, int nYSize, void * pData, int nBufXSize, int
> nBufYSize, GDALDataType eBufType, int nPixelSpace, int nLineSpace)  Line
> 206 + 0x9c bytes C++
>   tplgdal11164d.dll!GDALRasterBand::RasterIO(GDALRWFlag eRWFlag, int nXOff,
> int nYOff, int nXSize, int nYSize, void * pData, int nBufXSize, int
> nBufYSize, GDALDataType eBufType, int nPixelSpace, int nLineSpace)  Line
> 261 C++
> 
> In VRTSimpleSource::RasterIO the first thing that happens is a call to
> GetSrcDstWindow. If this call fails the function returns with a CE_None.
> This is after the block has been set to 0.
> 
> The simplest change would be to extend the return enum to have a CE_NoData.
> But that does not really fit with the name of the enum and does to deal
> with (1).
> 
> The other option is to provide access to GetSrcDstWindow. This would allow
> the application to check to see if there is data in the block requested.
> 
> 
> I may of course be missing a more sensible way of doing this without any
> code changes to GDAL. If I am missing something could someone let me know
> what I should be doing.
> 
> Thanks
> Damian
> 
> 
> Using: GDAL version 11.2

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