What is the best way to adjust Contrast level on an Image as found in Photoshop

Question

Do you think the following one is best for adjusting Contrast level? In fact, I would like to get the same/close perfromance for adjusting contrast as found in Photoshop.

public static Bitmap AdjustContrast(Bitmap Image, float Value)
{
    Value = (100.0f + Value) / 100.0f;
    Value *= Value;
    System.Drawing.Bitmap NewBitmap = Image;

    for (int x = 0; x < NewBitmap.Width; ++x)
    {
        for (int y = 0; y < NewBitmap.Height; ++y)
        {
            Color Pixel = NewBitmap.GetPixel(x, y);
            float Red = Pixel.R / 255.0f;
            float Green = Pixel.G / 255.0f;
            float Blue = Pixel.B / 255.0f;
            Red = (((Red - 0.5f) * Value) + 0.5f) * 255.0f;
            Green = (((Green - 0.5f) * Value) + 0.5f) * 255.0f;
            Blue = (((Blue - 0.5f) * Value) + 0.5f) * 255.0f;
            NewBitmap.SetPixel(x, y, Color.FromArgb(Clamp((int)Red, 255, 0), Clamp((int)Green, 255, 0), Clamp((int)Blue, 255, 0)));
        }
    }

    return NewBitmap;
}

public static T Clamp<T>(T Value, T Max, T Min)
     where T : System.IComparable<T>
{
    if (Value.CompareTo(Max) > 0)
        return Max;
    if (Value.CompareTo(Min) < 0)
        return Min;
    return Value;
}

The above code is not mine and I forgot the source of the code.

Answer 1

Basically, no. Get and Set pixel are really slow. Try something along this lines of this. It locks the Bitmap's pixels in memory and then directly manipulates that area of memory with a contrast algorithm. It does require unsafe code but if you want any sort of performance out of your solution, that's the way you have to go.

Alternatively, you can use built in matrix manipulations to do basically the same thing. I don't particularly like using them as sometimes it's hard to get the perfect effect. But they might be the easiest thing in your case. Link.