Sigma Formula for image C++
I'm trying to make a certain tone curve which is known as Sigma. 
 
Unfortunately, I do not know the formula for Sigma, i somehow cant find the formula for it.
Can anyone help me? Thanks  

Based on a graph x and y axis and this website : 3rd link:   <http://www.mediachance.com/pseam/help/curves.html>

Using Windows Visual Studio 2010  

			

    for(int y=0; y<bih.biHeight; y++)
    			{						
    				for(int x=0; x<bih.biWidth; x++)
    				{	
    				SetPixel(hdc, ((double)x/(double)(x+20))*bih.biWidth, bih.biHeight-x, red);
    				}
Sigma Formula for image C++
===

I'm trying to make a certain tone curve which is known as Sigma. 
 
Unfortunately, I do not know the formula for Sigma, i somehow cant find the formula for it.
Can anyone help me? Thanks  

Based on a graph x and y axis and this website : 3rd link:   <http://www.mediachance.com/pseam/help/curves.html>

Using Windows Visual Studio 2010  

			

    for(int y=0; y<bih.biHeight; y++)
    			{						
    				for(int x=0; x<bih.biWidth; x++)
    				{	
    				SetPixel(hdc, ((double)x/(double)(x+20))*bih.biWidth, bih.biHeight-x, red);
    				}