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(C++) EnhanceContrast

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.

        View the code of 'EnhanceContrast' in plain text.

#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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