How to Darken an Image in C#

In order to darken an image in C# you need to look at each pixel one at a time and decrease it's red, green, and blue values.

There are a few ways to do this.

• The easiest way is just just decrement the reg, green, and blue by a fixed percentage, and limit their minimum value to 0.
• The hard way is to deal with the hue shift that can occur when one of the values "clips" at the 0 mark.

Here's the easy way.

// this code relies on the LockedBitmap class
// darkenAmount should be a value between 0 and 1
private
static void LightenImage(Bitmap bmp, double lightenAmount)
{
    var lockedBitmap = new LockBitmap(bmp);
    lockedBitmap.LockBits();

    for (int y = 0; y < lockedBitmap.Height; y++)
    {
        for (int x = 0; x < lockedBitmap.Width; x++)
        {
            var oldColor = lockedBitmap.GetPixel(x, y);
            var newColor = DarkenColor(oldColor, lightenAmount);
            lockedBitmap.SetPixel(x, y, newColor);
        }
    }
    lockedBitmap.UnlockBits();
}

public
static Color DarkenColor(Color inColor, double lightenAmount)
{
    return Color.FromArgb(
        inColor.A,
        (int)Math.Max(0, inColor.R - 255 * lightenAmount),
        (int)Math.Max(0, inColor.G - 255 * lightenAmount),
        (int)Math.Max(0, inColor.B - 255 * lightenAmount));
}

Here is a test image.

And here are the results of setting darkenAmount to 0.5.

This block of code iterates over the images width and height looking at each pixel. For each pixel it calls the function DarkenColor. DarkenColor calculates the amount of "darkenning" to apply for each of the 3 RGB elements and returns a new Color object.

You need to have the LockedBits class in your project in order to use this code. You can learn about it in our How to Load an Image in C# guide.

You might be interested in some of our other C# Image Processing Guides.

• How to Lighten an Image in C#
• How to Load an Image in C#
• C# Image Processing Guides