BitmapExtensionsAdjustContrast Method

Adjusts the contrast of the specified bitmap.


Namespace: KGySoft.Drawing
Assembly: KGySoft.Drawing (in KGySoft.Drawing.dll) Version: 8.1.0
public static void AdjustContrast(
	this Bitmap bitmap,
	float contrast,
	IDitherer? ditherer = null,
	ColorChannels channels = ColorChannels.Rgb


bitmap  Bitmap
The Bitmap to be transformed.
contrast  Single
A float value between -1 and 1, inclusive bounds. Positive values increase the contrast, while negative values decrease the it.
ditherer  IDitherer  (Optional)
An optional IDitherer instance to dither the result of the transformation if the transformed colors are not compatible with the PixelFormat of the specified bitmap. This parameter is optional.
Default value: .
channels  ColorChannels  (Optional)
The ColorChannels, on which the adjustment has to be performed. This parameter is optional.
Default value: Rgb.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type Bitmap. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).


If bitmap has an indexed PixelFormat and ditherer is , then its palette entries will be transformed instead of the actual pixels. To transform the colors of an indexed Bitmap without changing the palette specify a non- ditherer. Transforming the palette is both faster and provides a better result.

The ditherer is ignored for PixelFormats with more than 16 bits-per-pixel and for the Format16bppGrayScale format.


The following example demonstrates how to use this method:
using Bitmap original = Icons.Shield.ExtractBitmap(new Size(256, 256));

// starting with an indexed image using an optimized 8 BPP palette
using Bitmap bmp = original.ConvertPixelFormat(PixelFormat.Format8bppIndexed,


// Decreasing the contrast. By specifying a ditherer the original palette is preserved
// (which is not so optimal for the transformed image anymore). The ditherer tries
// to approximate the desired result with the original palette as much as possible.
// Try it also without a ditherer to transform only the palette entries.
// Try different contrast values and ColorChannels, too.
bmp.AdjustContrast(-0.5f, ErrorDiffusionDitherer.FloydSteinberg, ColorChannels.Rgb);

// As ditherer was not null now the result is generated using the original palette

The example above produces the following results:

before.gifShield icon quantized to 256 colors using the Median Cut algorithm
after.gifShield icon with decreased contrast using Floyd-Steinberg dithering and a palette optimized for the untransformed image


ArgumentNullExceptionbitmap is .
ArgumentOutOfRangeExceptioncontrast is not between -1 and 1
channels is out of the defined flags.

See Also