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How to: Rotate Colors

Rotation in a four-dimensional color space is difficult to visualize. We can make it easier to visualize rotation by agreeing to keep one of the color components fixed. Suppose we agree to keep the alpha component fixed at 1 (fully opaque). Then we can visualize a three-dimensional color space with red, green, and blue axes as shown in the following illustration.

Recoloring

A color can be thought of as a point in 3-D space. For example, the point (1, 0, 0) in space represents the color red, and the point (0, 1, 0) in space represents the color green.

The following illustration shows what it means to rotate the color (1, 0, 0) through an angle of 60 degrees in the Red-Green plane. Rotation in a plane parallel to the Red-Green plane can be thought of as rotation about the blue axis.

Recoloring

The following illustration shows how to initialize a color matrix to perform rotations about each of the three coordinate axes (red, green, blue).

Recoloring

The following example takes an image that is all one color (1, 0, 0.6) and applies a 60-degree rotation about the blue axis. The angle of the rotation is swept out in a plane that is parallel to the red-green plane.

The following illustration shows the original image on the left and the color-rotated image on the right.

Rotate Colors

The following illustration shows a visualization of the color rotation performed in the following code.

Recoloring
Private Sub RotateColors(ByVal e As PaintEventArgs)
    Dim image As Bitmap = New Bitmap("RotationInput.bmp")
    Dim imageAttributes As New ImageAttributes()
    Dim width As Integer = image.Width
    Dim height As Integer = image.Height
    Dim degrees As Single = 60.0F
    Dim r As Double = degrees * System.Math.PI / 180 ' degrees to radians
    Dim colorMatrixElements As Single()() = { _
       New Single() {CSng(System.Math.Cos(r)), _
                     CSng(System.Math.Sin(r)), 0, 0, 0}, _
       New Single() {CSng(-System.Math.Sin(r)), _
                     CSng(-System.Math.Cos(r)), 0, 0, 0}, _
       New Single() {0, 0, 2, 0, 0}, _
       New Single() {0, 0, 0, 1, 0}, _
       New Single() {0, 0, 0, 0, 1}}

    Dim colorMatrix As New ColorMatrix(colorMatrixElements)

    imageAttributes.SetColorMatrix( _
       colorMatrix, _
       ColorMatrixFlag.Default, _
       ColorAdjustType.Bitmap)

    e.Graphics.DrawImage(image, 10, 10, width, height)

    ' Pass in the destination rectangle (2nd argument), the upper-left corner  
    ' (3rd and 4th arguments), width (5th argument),  and height (6th  
    ' argument) of the source rectangle.
    e.Graphics.DrawImage( _
       image, _
       New Rectangle(150, 10, width, height), _
       0, 0, _
       width, _
       height, _
       GraphicsUnit.Pixel, _
       imageAttributes)
End Sub

The preceding example is designed for use with Windows Forms, and it requires PaintEventArgs e, which is a parameter of the Paint event handler. Replace RotationInput.bmp with an image file name and path valid on your system.

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