[C++/WinAPI] Changing bitmap contrast

It's a code of function changing kontrast of bitmap. But it don't work that I want. Maybe someone saw how loooks changing contrast in PhotoShop. I want to reach the same effect... In attaching image I show how it works in PS (right) in my function (center). On the left is source image... Any ideas how to reach effect like in PhotoShop ?

Code:

#include <windows.h>
#include "epfEngine.h"
#include <complex>

static unsigned char TransLut[256];

void build_lookup_table ( int contr )
{
	float step,step_value;

	for (int i=0; i < 256; i++)
		TransLut[i] = i;

	if (contr > 0)
	{
		unsigned int MinBin = contr;
		unsigned int MaxBin = 255 - contr;

		step = sqrt((double)contr)/contr;
		step_value = 0.0;

		for ( i = 0; i < MinBin; i++)
		{
			TransLut[i] = (unsigned char)step_value;
			step_value += step;
		}

		step = 256.0f / (float)(MaxBin-MinBin);

		for ( i = MinBin; i <= MaxBin; i++)
		{

		if (step_value > 255.0f)
		{
			step_value = 255.0f;
			step = 0.0f;
		}

		TransLut[i] = (unsigned char)step_value;
		step_value += step;
		}

		for ( i = MaxBin + 1; i < 256; i++)
			TransLut[i] = 255;

	}
	else
	{
		if (contr<0)
		{
			step = (256.0+(float)(contr*2))/256.0;
			step_value = (float)contr * -1.0;

			for (i = 0;i < 256; i++)
			{
				TransLut[i] = (unsigned char)step_value;
				step_value += step;
			}
		}
	}
}

BOOL epfChangeBitmapContrast(HBITMAP * phBitmap, int iCount)
{
	BITMAP    bm;
	BYTE    * pBits;
	RGBQUAD * pRgb;
	WORD      wByteCount;
	int       i, iPixels, gray;

	build_lookup_table(iCount);

	// Take BITMAP structure from HBITMAP

	GetObject(*phBitmap, sizeof(BITMAP), &bm);

	// Calculate bytes to read

	wByteCount = bm.bmHeight * (2 * ((bm.bmWidth * bm.bmBitsPixel + 15) / 16));

	// Alocate momory for bits od pixels and get pointers

	pBits = (BYTE *) malloc(wByteCount);
	GetBitmapBits(*phBitmap, wByteCount, pBits);

	// Convert pointer to byte to pointer to RGBQUAD

	pRgb = (RGBQUAD *) pBits;

	// Operate on pixel's colors

	iPixels = wByteCount / (bm.bmBitsPixel / 8);

	for(i = 0; i < iPixels; i++, pRgb++)
	{
		gray = (pRgb->rgbRed + pRgb->rgbGreen + pRgb->rgbBlue) / 3;
		int k = TransLut[gray]-gray;

		pRgb->rgbRed   = min(pRgb->rgbRed   + k, 255);
		pRgb->rgbGreen = min(pRgb->rgbGreen + k, 255);
		pRgb->rgbBlue  = min(pRgb->rgbBlue  + k, 255);
	}

	SetBitmapBits(*phBitmap, wByteCount, pBits);

	return TRUE;
}