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

[Damian Dixon]

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,blockHeight,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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