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ImageAttributes.SetThreshold Method (Single)

Sets the threshold (transparency range) for the default category.

Namespace:  System.Drawing.Imaging
Assembly:  System.Drawing (in System.Drawing.dll)

public void SetThreshold(
	float threshold


Type: System.Single

A real number that specifies the threshold value.

The threshold is a value from 0 through 1 that specifies a cutoff point for each color component. For example, suppose the threshold is set to 0.7, and suppose you are rendering a color whose red, green, and blue components are 230, 50, and 220, respectively. The red component (230) is greater than 0.7x255, so the red component will be changed to 255 (full intensity). The green component (50) is less than 0.7x255, so the green component will be changed to 0. The blue component (220) is greater than 0.7x255, so the blue component will be changed to 255.

An ImageAttributes object maintains color and grayscale settings for five adjustment categories: default, bitmap, brush, pen, and text. For example, you can specify a threshold for the default category, a threshold for the bitmap category, and still a different threshold for the pen category.

The default color-adjustment and grayscale-adjustment settings apply to all categories that do not have adjustment settings of their own. For example, if you never specify any adjustment settings for the pen category, the default settings apply to the pen category.

The following code example is designed for use with Windows Forms, and it requires PaintEventArgs e, which is a parameter of the Paint event handler. The code performs the following actions:

  1. Opens an Image and draws it to the screen.

  2. Creates an ImageAttributes object and sets its threshold by using the SetThreshold method.

  3. Draws the image to the screen by using the threshold of the ImageAttributes object.

private void SetThresholdExample(PaintEventArgs e)

    // Open an Image file, and draw it to the screen.
    Image myImage = Image.FromFile("Camera.jpg");
    e.Graphics.DrawImage(myImage, 20, 20);

    // Create an ImageAttributes object, and set its color threshold.
    ImageAttributes imageAttr = new ImageAttributes();

    // Draw the image with the colors bifurcated.
    Rectangle rect = new Rectangle(300, 20, 200, 200);
    e.Graphics.DrawImage(myImage, rect, 0, 0, 200, 200, 
        GraphicsUnit.Pixel, imageAttr);    

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0