Image Processing: Contrast Enhancement 2

It shows how to manipulate the parameters for the contrast enhancement directly in your program instead of using the built-in dialog of the filter that provides the contrast enhancement functionality.

The sample application's window looks as follows:

The filter applies a histogram stretching function to the image. That means that the pixel values between a min and max value are mapped to the entire value range of the image. The pixel values that are below the min value are set to 0. The pixel values that are above the max value are set to 255. The min and max value can be manipulated with 2 scrollbars (sldLowerBound and sldUpperBound). The beginning of the program is extended by 2 function calls that initialize the controls on the main form.

InitControls
UpdateControls

The function InitControls initializes the min and max values of the scrollbars.

Private Sub InitControls()
    sldLowerBound.Min = 0
    sldLowerBound.Max = 255
    sldUpperBound.Min = 0
    sldUpperBound.Max = 255
End Sub

The function UpdateControls assigns the filter parameters "Enable", "Lower Bound" and "Upper Bound" to the checkbox cbEnable and the scrollbars sldLowerBound and sldUpperBound.

Private Sub UpdateControls()
    If filter.Parameter("Enable") Then
        cbEnable.Value = Checked
    Else
        cbEnable.Value = Unchecked
    End If
    sldLowerBound.Value = filter.Parameter("Lower Bound")
    txLowerBound.Text = sldLowerBound.Value
    sldUpperBound.Value = filter.Parameter("Upper Bound")
    txUpperBound.Text = sldUpperBound.Value
End Sub

The event handler for the scrollbar sldLowerBound ensures that its value is equal or less to the value of the scrollbar sldUpperBound . After this check, the scrollbar value is assigned to the filter parameter "Lower Bound" and the text field to the right of the scrollbar is updated. The event handler for the scrollbar sldUpperBound works accordingly.

Private Sub sldLowerBound_Scroll()
    If sldLowerBound.Value >= sldUpperBound.Value Then
        sldLowerBound.Value = sldUpperBound.Value - 1
    End If
    filter.Parameter("Lower Bound") = sldLowerBound.Value
    txLowerBound.Text = sldLowerBound.Value
End Sub