Improve Image Comparison Function - vb.net

Question

I'm working on Visual Inspection System. One of my key function is to compare an image captured with an image from a data base. The comparison would reveal missing parts or damaged part. I have tried using pixel comparison, but this method is not reliable as it needs exactly similar image captured every time. Is there a way to improve this function to be more versatile. In a way it has to detect the difference in image even if the image captured is slightly offset or rotated. Please guide me using VB.Net. Below is my current code.
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("C:\Users\pnasguna\Desktop\A56.jpg")
Dim bm2 As Bitmap = Image.FromFile("C:\Users\pnasguna\Desktop\A54.jpg")
' 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 eq_color As Color = Color.White
Dim ne_color As Color = Color.Red
For x As Integer = 0 To wid - 1
For y As Integer = 0 To hgt - 1
If bm1.GetPixel(x, y).Equals(bm2.GetPixel(x,y)) Then
bm3.SetPixel(x, y, eq_color)
Else
bm3.SetPixel(x, y, ne_color)
are_identical = False
End If
Next y
Next x
' Display the result.
PictureBox1.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

Answer 1

You could take the XnaFan ImageComparison library to inspectionate the sourcecode as an example for your needs
It reveals the difference between pixels and can compare images with a Similarity coefficient, I've write a basic example of both:
Imports XnaFan.ImageComparison
' ===================================================
' Get percentage difference value between two images:
' ===================================================
Dim img1 As Image = Image.FromFile("C:\Image1.jpg")
Dim img2 As Image = Image.FromFile("C:\Image2.jpg")
Dim PercentageDifference As Single =
ImageTool.PercentageDifference(img1:=img1, img2:=img2, threshold:=3)
MessageBox.Show(String.Format("Percentage Difference: {0}%",
CSng(PercentageDifference * 100I).ToString("n1")))
' ========================================
' Get difference image between two images:
' ========================================
Dim img1 As Image = Image.FromFile("C:\Image1.jpg")
Dim img2 As Image = Image.FromFile("C:\Image2.jpg")
Dim DifferenceBitmap As Bitmap =
ImageTool.GetDifferenceImage(img1:=img1,
img2:=img2,
adjustColorSchemeToMaxDifferenceFound:=True,
absoluteText:=False)
PictureBox1.Image = DifferenceBitmap
If you want something more complex you could use AForge (Imaging) library to do the similarity comparison