# Multispectral.cxx¶

Example usage:

./Multispectral Input/qb_RoadExtract.tif Output/qb_blue.tif Output/qb_shiftscale.tif


Example source code (Multispectral.cxx):

//  First, we are going to use \doxygen{otb}{VectorImage} instead of the now

// We also include some other header which will be useful later. Note that we
// are still using the \doxygen{otb}{Image} in this example for some of the
// output.

#include "otbImageFileWriter.h"
#include "otbMultiToMonoChannelExtractROI.h"
#include "itkShiftScaleImageFilter.h"
#include "otbPerBandVectorImageFilter.h"

int main(int argc, char* argv[])
{
if (argc != 4)
{
std::cerr << "Usage: " << argv[0] << " <input_filename> <output_extract> <output_shifted_scaled>" << std::endl;
}

// We want to read a multispectral image so we declare the image type and the
// reader. As we have done in the previous example we get the filename from
// the command line.

using PixelType       = unsigned short;
using VectorImageType = otb::VectorImage<PixelType, 2>;

//  Sometime, you need to process only one spectral band of the image. To get
// only one of the spectral band we use the
// \doxygen{otb}{MultiToMonoChannelExtractROI}. The declaration is as usual:

using ExtractChannelType                   = otb::MultiToMonoChannelExtractROI<PixelType, PixelType>;
ExtractChannelType::Pointer extractChannel = ExtractChannelType::New();
//  We need to pass the parameters to the filter for the extraction. This
// filter also allow extracting only a spatial subset of the image. However,
// we will extract the whole channel in this case.
//
// To do that, we need to pass the desired region using the
// \code{SetExtractionRegion()} (method such as \code{SetStartX},
// \code{SetSizeX} are also available). We get the region from the reader with
// the \code{GetLargestPossibleRegion()} method. Before doing that we need to
// read the metadata from the file: this is done by calling the
// \code{UpdateOutputInformation()} on the reader's output. The difference with the
// \code{Update()} is that the pixel array is not allocated (yet !) and reduce
// the memory usage.

//  We chose the channel number to extract (starting from 1) and we plug the
// pipeline.

extractChannel->SetChannel(3);

//  To output this image, we need a writer. As the output of the
// \doxygen{otb}{MultiToMonoChannelExtractROI} is a \doxygen{otb}{Image}, we
// need to template the writer with this type.

using ImageType            = otb::Image<PixelType, 2>;
using WriterType           = otb::ImageFileWriter<ImageType>;
WriterType::Pointer writer = WriterType::New();

writer->SetFileName(argv[2]);
writer->SetInput(extractChannel->GetOutput());

writer->Update();

// After this, we have a one band image that we can process with most OTB
// filters.
//
// In some situation, you may want to apply the same process to all bands of
// the image. You don't have to extract each band and process them separately.
// There is several situations:
//
// \begin{itemize}
// \item the filter (or the combination of filters) you want to use are doing
// operations that are well defined for \doxygen{itk}{VariableLengthVector}
// (which is the pixel type), then you don't have to do anything special.
// \item if this is not working, you can look for the equivalent filter
// specially designed for vector images.
// \item some of the filter you need to use applies operations undefined for
// \doxygen{itk}{VariableLengthVector}, then you can use the
// \doxygen{otb}{PerBandVectorImageFilter} specially designed for this
// purpose.
// \end{itemize}
//
// Let's see how this filter is working. We chose to apply the
// \doxygen{itk}{ShiftScaleImageFilter} to each of the spectral band. We start
// by declaring the filter on a normal \doxygen{otb}{Image}. Note that we
// don't need to specify any input for this filter.

using ShiftScaleType               = itk::ShiftScaleImageFilter<ImageType, ImageType>;
ShiftScaleType::Pointer shiftScale = ShiftScaleType::New();
shiftScale->SetScale(0.5);
shiftScale->SetShift(10);

// We declare the \doxygen{otb}{PerBandVectorImageFilter} which has three
// template: the input image type, the output image type and the filter type
// to apply to each band.
//
// The filter is selected using the \code{SetFilter()} method and the input
// by the usual \code{SetInput()} method.

using VectorFilterType                 = otb::PerBandVectorImageFilter<VectorImageType, VectorImageType, ShiftScaleType>;
VectorFilterType::Pointer vectorFilter = VectorFilterType::New();
vectorFilter->SetFilter(shiftScale);

// Now, we just have to save the image using a writer templated over an
// \doxygen{otb}{VectorImage}:

using VectorWriterType                 = otb::ImageFileWriter<VectorImageType>;
VectorWriterType::Pointer writerVector = VectorWriterType::New();

writerVector->SetFileName(argv[3]);
writerVector->SetInput(vectorFilter->GetOutput());

writerVector->Update();

return EXIT_SUCCESS;
}