Next the program loops over the pixels in the smaller area. It gets the red, green, and blue components of the pixels in one image and subtracts them from the corresponding values in the other image. It divides the result by 2 and adds 128. This makes the result lie between 1 and 255, and pixels that are the same in both images become gray. When it has examined all of the pixels, the program displays the result image.

You can use this code to perform edge enhancement. Take an image, shift it one pixel to the right and down, and then use the program to compare the images. Areas with large changes in color (edges) are enhanced. Areas where the color is relatively stable are gray in the result.

Private Sub btnGo_Click(ByVal sender As System.Object, _
    ByVal e As System.EventArgs) Handles btnGo.Click
    Me.Cursor = Cursors.WaitCursor
    Application.DoEvents()

    ' Load the images.
    Dim bm1 As Bitmap = Image.FromFile(txtFile1.Text)
    Dim bm2 As Bitmap = Image.FromFile(txtFile2.Text)

    ' Make a difference image.
    Dim wid As Integer = Math.Min(bm1.Width, bm2.Width)
    Dim hgt As Integer = Math.Min(bm1.Height, bm2.Height)
    Dim bm3 As New Bitmap(wid, hgt)

    ' Create the difference image.
    Dim are_identical As Boolean = True
    Dim r1, g1, b1, r2, g2, b2, r3, g3, b3 As Integer
    Dim color1, color2, color3 As Color
    For x As Integer = 0 To wid - 1
        For y As Integer = 0 To hgt - 1
            color1 = bm1.GetPixel(x, y)
            r1 = color1.R : g1 = color1.G : b1 = color1.B

            color2 = bm2.GetPixel(x, y)
            r2 = color2.R : g2 = color2.G : b2 = color2.B

            r3 = 128 + (r1 - r2) \ 2
            g3 = 128 + (g1 - g2) \ 2
            b3 = 128 + (b1 - b2) \ 2

            color3 = Color.FromArgb(255, r3, g3, b3)
            bm3.SetPixel(x, y, color3)

            If (r1 <> r2) OrElse (g1 <> g2) OrElse (b1 <> _
                b2) Then are_identical = False
        Next y
    Next x

    ' Display the result.
    picResult.Image = bm3

    Me.Cursor = Cursors.Default
    If (bm1.Width <> bm2.Width) OrElse (bm1.Height <> _
        bm2.Height) Then are_identical = False
    If are_identical Then
        MessageBox.Show("The images are identical")
    Else
        MessageBox.Show("The images are different")
    End If

    bm1.Dispose()
    bm2.Dispose()
End Sub