Ionut Sandric Ionut Sandric - 3 months ago 55
C++ Question

GDAL C++ RasterIO by blocks

I am new to gdal and c++ and I am trying to create a function that reads a raster by blocks (using RasterIO and not ReadBlock) do something with the values and writes the result to new raster file

I am getting an error:


Unhandled exception at 0x00007FFDFB517BC4 (gdal201.dll) in Test.exe:
0xC0000005: Access violation writing location 0x00000230E2593000 at
line rasterBand->RasterIO(GDALRWFlag::GF_Read, j * readCols, i *
readRows, readCols, readRows, rasterBlock, readCols, readRows,
GDALDataType::GDT_CFloat32, 0, 0)


I have a very similar function that works ok, but it reads the pixel values row by row

Any help is appreciated

The function is bellow

Thank you

#include <iostream>
#include "gdal_priv.h"
using namespace std;

void SlopeBlock()
{
int readRows = 2048;
int readCols = 2048;
int nBlockXSize;
int nBlockYSize;
int rowOff = 0;
int colOff = 0;
int nRows = 0;
int nCols = 0;
double noData = -9999;

GDALAllRegister();

GDALDataset* dem = (GDALDataset*)GDALOpen("path/EUD_CP-DEMS_4500025000-AA.tif", GA_ReadOnly);

GDALDriver *gTIF = GetGDALDriverManager()->GetDriverByName(dem->GetDriverName());

double geoTransform[6];
dem->GetGeoTransform(geoTransform);

GDALRasterBand* rasterBand = dem->GetRasterBand(1);

nCols = rasterBand->GetXSize();
nRows = rasterBand->GetYSize();
noData = rasterBand->GetNoDataValue();

//rasterBand->GetBlockSize(&nBlockXSize, &nBlockYSize);

int nXBlocks = (rasterBand->GetXSize() + readCols - 1) / readCols;
int nYBlocks = (rasterBand->GetYSize() + readRows - 1) / readRows;

cout << nYBlocks << "___" << nXBlocks << "\n";


//Slope
GDALDataset *slope = gTIF->Create("path/slopeBlock.tif", nCols, nRows, 1, GDT_Float32, NULL);
slope->SetGeoTransform(geoTransform);

for (int i = 0; i < nYBlocks; i++)
{
for (int j = 0; j < nXBlocks; j++)
{
float* rasterBlock = (float*)CPLMalloc(readCols*readRows*sizeof(float));
float* rasterBlockOut = (float*)CPLMalloc(readCols*readRows*sizeof(float));

if (rasterBand != nullptr)
rasterBand->RasterIO(GDALRWFlag::GF_Read, j * readCols, i * readRows, readCols, readRows, rasterBlock, readCols, readRows, GDALDataType::GDT_CFloat32, 0, 0);

//Not sure if is working
for (int jb = 0; jb < readCols; jb++)
{
if (rasterBlock[jb] == noData)
{
rasterBlockOut[jb] = noData;
}
else
{
rasterBlockOut[jb] = rasterBlock[jb];
}
}

slope->GetRasterBand(1)->RasterIO(GDALRWFlag::GF_Write, j * readCols, i * readRows, readCols, readRows, rasterBlockOut, readCols, readRows, GDALDataType::GDT_CFloat32, 0, 0);

CPLFree(rasterBlock);
CPLFree(rasterBlockOut);
}
}

GDALClose(dem);
GDALClose(slope);


}

Answer

This may not be the cause of the crash, but you're asking for a 2048*2048 block even if your image is smaller. This code should only work for images with multiples of 2048 as size.

You should do something like:

nXSize = min(readCols, rasterBand->GetXSize() - j * readCols);
nYSize = min(readRows, rasterBand->GetYSize() - i * readRows);

rasterBand->RasterIO(GDALRWFlag::GF_Read, j * readCols, i * readRows, 
    nXSize, nYSize, rasterBlock, nXSize, nYSize, GDALDataType::GDT_CFloat32, 0, 0);
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