[gdal-dev] What is the correct way to read pixel data from a GDALRasterBand in C++?

[David Strip]

the docs have moved (but apparently Google hasn't figured that out yet?) GDALRasterBand is now here. Anyway, as you've figured out, the GDALRasterBand object is the heart of the matter. You have two ways to read the data, either with the RasterIO method or the ReadBlock method. If your dataset is small enough to read into available memory, RasterIO is probably easier to use. ReadBlock can be more efficient, but your code has to handle the offsets for the origin of the block. The code example said to be in ReadBlock seems to have disappeared in the migration of the docs. Here's what it contained: The following code would efficiently compute a histogram of eight bit raster data. Note that the final block may be partial ... data beyond the edge of the underlying raster band in these edge blocks is of an undermined value. CPLErr GetHistogram( GDALRasterBand *poBand, int *panHistogram ) { int nXBlocks, nYBlocks, nXBlockSize, nYBlockSize; int iXBlock, iYBlock; GByte *pabyData; memset( panHistogram, 0, sizeof(int) * 256 ); CPLAssert( poBand->GetRasterDataType() == GDT_Byte ); poBand->GetBlockSize( &nXBlockSize, &nYBlockSize ); nXBlocks = (poBand->GetXSize() + nXBlockSize - 1) / nXBlockSize; nYBlocks = (poBand->GetYSize() + nYBlockSize - 1) / nYBlockSize; pabyData = (GByte *) CPLMalloc(nXBlockSize * nYBlockSize); for( iYBlock = 0; iYBlock < nYBlocks; iYBlock++ ) { for( iXBlock = 0; iXBlock < nXBlocks; iXBlock++ ) { int nXValid, nYValid; poBand->ReadBlock( iXBlock, iYBlock, pabyData ); // Compute the portion of the block that is valid // for partial edge blocks. if( (iXBlock+1) * nXBlockSize > poBand->GetXSize() ) nXValid = poBand->GetXSize() - iXBlock * nXBlockSize; else nXValid = nXBlockSize; if( (iYBlock+1) * nYBlockSize > poBand->GetYSize() ) nYValid = poBand->GetYSize() - iYBlock * nYBlockSize; else nYValid = nYBlockSize; // Collect the histogram counts. for( int iY = 0; iY < nYValid; iY++ ) { for( int iX = 0; iX < nXValid; iX++ ) { panHistogram[pabyData[iX + iY * nXBlockSize]] += 1; } } } } }

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