Some algorithms produce an indexed image as output. In such images, each pixel is given a value according to the region number it belongs to. This value starting at 0 or 1 is usually an integer value. Often, such images are produced by segmentation or classification algorithms.

If such regions are easy to manipulate – it is easier and faster to compare two integers than a RGB value – it is different when it comes to displaying the results.

Here we present a convient way to convert such indexed image to a color image. In such conversion, it is important to ensure that neighboring regions, which are likely to have consecutive number have easily dicernable colors. This is done randomly using a hash function by ScalarToRGBPixelFunctor.



The original indexed image (left) and the conversion to color image.

Example usage:

./IndexedToRGBExample Input/buildingExtractionIndexed.tif Output/buildingExtractionRGB.png Output/buildingExtractionIndexed_scaled.png

Example source code (IndexedToRGBExample.cxx):

#include "otbImage.h"
#include "otbImageFileReader.h"
#include "otbImageFileWriter.h"
#include "itkUnaryFunctorImageFilter.h"
#include "itkScalarToRGBPixelFunctor.h"
#include "itkRescaleIntensityImageFilter.h"

int main(int argc, char* argv[])
  if (argc != 4)
    std::cerr << "Usage: " << argv[0] << " <inputImageFile> ";
    std::cerr << " <outputRGBImageFile> <outputScaledImageFile>" << std::endl;
    return EXIT_FAILURE;
  const char* inputFilename        = argv[1];
  const char* outputRGBFilename    = argv[2];
  const char* outputScaledFilename = argv[3];

  using ImageType    = otb::Image<unsigned long, 2>;
  using RGBImageType = otb::Image<itk::RGBPixel<unsigned char>, 2>;

  using ReaderType           = otb::ImageFileReader<ImageType>;
  ReaderType::Pointer reader = ReaderType::New();


  // The UnaryFunctorImageFilter is the filter in charge of calling the functor
  // we specify to do the work for each pixel. Here it is the ScalarToRGBPixelFunctor
  using ColorMapFunctorType               = itk::Functor::ScalarToRGBPixelFunctor<unsigned long>;
  using ColorMapFilterType                = itk::UnaryFunctorImageFilter<ImageType, RGBImageType, ColorMapFunctorType>;
  ColorMapFilterType::Pointer colormapper = ColorMapFilterType::New();


  using WriterType           = otb::ImageFileWriter<RGBImageType>;
  WriterType::Pointer writer = WriterType::New();


  // The following is just to produce the input image for the software guide
  using OutputImageType          = otb::Image<unsigned char, 2>;
  using RescalerType             = itk::RescaleIntensityImageFilter<ImageType, OutputImageType>;
  RescalerType::Pointer rescaler = RescalerType::New();

  using UCharWriterType            = otb::ImageFileWriter<OutputImageType>;
  UCharWriterType::Pointer writer2 = UCharWriterType::New();