public Bitmap contrast_function(Bitmap b)
{
double nContrast = 30;
double pixel = 0, contrast = (100.0 + nContrast) / 100.0;

contrast *= contrast;

int red, green, blue;

// GDI+ still lies to us – the return format is BGR, NOT RGB.
BitmapData bmData = b.LockBits(new Rectangle(0, 0, b.Width, b.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb);

int stride = bmData.Stride;
System.IntPtr Scan0 = bmData.Scan0;

unsafe
{
byte* p = (byte*)(void*)Scan0;

int nOffset = stride – b.Width * 3;

for (int y = 0; y < b.Height; ++y)
{
for (int x = 0; x < b.Width; ++x)
{
blue = p[0];
green = p[1];
red = p[2];

pixel = red / 255.0;
pixel -= 0.5;
pixel *= contrast;
pixel += 0.5;
pixel *= 255;
if (pixel < 0) pixel = 0;
if (pixel > 255) pixel = 255;
p[2] = (byte)pixel;

pixel = green / 255.0;
pixel -= 0.5;
pixel *= contrast;
pixel += 0.5;
pixel *= 255;
if (pixel < 0) pixel = 0;
if (pixel > 255) pixel = 255;
p[1] = (byte)pixel;

pixel = blue / 255.0;
pixel -= 0.5;
pixel *= contrast;
pixel += 0.5;
pixel *= 255;
if (pixel < 0) pixel = 0;
if (pixel > 255) pixel = 255;
p[0] = (byte)pixel;

p += 3;
}
p += nOffset;
}
}

b.UnlockBits(bmData);
return b;

}

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