Graphics code snippet to enhance the contrast of a VCL TImage.

EnhanceContrast is similar to DoHistogramEqualization, except that EnhanceContrast is milder: it tries to minimally change the average greyness (using GetAverageGreyness to first measure it).

EnhanceContrast assumes that you've already defined GetAverageGreyness.

View an example of a composition image demonstrating EnhanceContrast and DoHistogramEqualization: first, on the original (top-left), a DoFilterOperation was performed, resulting in the bottom-left image. On this result, both a DoHistogramEqualization (top-right) and EnhanceContrast (bottom-right) were performed on


#include <cassert>

#include <vcl.h>



//From http://www.richelbilderbeek.nl/CppEnhanceContrast.htm

void EnhanceContrast(const TImage * const image)

{

  //Find lowest and highest value

  assert(image!=0 && "image must not be NULL");

  assert(image->Picture->Bitmap!=0 && "image bitmap must not be NULL");

  assert(image->Picture->Bitmap->PixelFormat == pf24bit && "image bitmap must be 24 bit");



  //Get the width and height from the source

  const int width  = image->Picture->Bitmap->Width;

  const int height = image->Picture->Bitmap->Height;

  const double averageGreyness = GetAverageGreyness(image);



  double stretch = 1000.0; //Way too much for nearly all images



  for (int y=0; y!=height; ++y)

  {

    const unsigned char * const line

      = static_cast<const unsigned char *>(

        image->Picture->Bitmap->ScanLine[y]);

    for (int x=0; x!=width; ++x)

    {

      const double grey

        = static_cast<double>(line[x*3+0] + line[x*3+1] + line[x*3+2])

        / 3.0;

      const double diff = grey - averageGreyness;

      if (diff < 0.0)

      {

        //pixel was darker then average

        const double newStretch = -averageGreyness / diff;

        assert(newStretch >= 0.0);

        if (newStretch < stretch) stretch = newStretch;

      }

      else if (diff > 0.0)

      {

        //pixel was lighter then average

        const double newStretch = (255.9 - averageGreyness)  / diff;

        if (newStretch < stretch) stretch = newStretch;

      }

    }

  }



  //Enhance the contrast

  for (int y=0; y!=height; ++y)

  {

    unsigned char * const line

      = static_cast<unsigned char *>(

        image->Picture->Bitmap->ScanLine[y]);

    for (int x=0; x!=width; ++x)

    {

      const double grey

        = static_cast<double>(line[x*3+0] + line[x*3+1] + line[x*3+2])

        / 3.0;

      const double diff = grey - averageGreyness;

      const double diffGreyNew = averageGreyness + (static_cast<double>(diff) * stretch);

      const int newGrey = static_cast<int>(diffGreyNew);

      assert(newGrey >=   0);

      assert(newGrey <  256);

      line[x*3+2] = newGrey; //Red

      line[x*3+1] = newGrey; //Green

      line[x*3+0] = newGrey; //Blue

    }

  }

}

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(C++) EnhanceContrast