Comparing two images and get difference image

The below steps describe how to compare tow images and get difference image

Step 1: Get images to compare, both images shall be of same size

 Dim sourceBitmap As Bitmap = PictureBox2.Image
 Dim destBitmap As Bitmap = PictureBox1.Image
 
Step 2: Get byte array of each bitmap

2.1 Get first image byte array

Dim sourceBitmapData As Imaging.BitmapData = sourceBitmap.LockBits(New Rectangle(0, 0, sourceBitmap.Width, sourceBitmap.Height), Imaging.ImageLockMode.ReadOnly, Imaging.PixelFormat.Format32bppArgb)
     Dim sourceBytes As Byte() = New Byte(sourceBitmapData.Stride * sourceBitmapData.Height - 1) {}
        
Runtime.InteropServices.Marshal.Copy(sourceBitmapData.Scan0, SourceBytes, 0, SourceBytes.Length)
        
sourceBitmap.UnlockBits(sourceBitmapData) 
 2.1 Get second image byte array
  Dim destBitmapData As Imaging.BitmapData = destBitmap.LockBits(New Rectangle(0, 0, destBitmap.Width, destBitmap.Height), Imaging.ImageLockMode.ReadOnly, Imaging.PixelFormat.Format32bppArgb)
        
Dim destBytes As Byte() = New Byte(destBitmapData.Stride * destBitmapData.Height - 1) {}
        
Runtime.InteropServices.Marshal.Copy(destBitmapData.Scan0, destBytes, 0, destBytes.Length)

        
destBitmap.UnlockBits(destBitmapData)

Step 3: Create a byte array to hold differences of pixel color between the two images

Dim diffBytes As Byte() = New Byte(sourceBytes.Length - 1) {}

Step 4: Loop and compare pixel colors of both bitmap, if colors are identical fill diffByte array in step 3 with any color of your choice and if pixel colors are not identical then fill diffByte array with one of bitmap pixel colors
 
Dim i As Integer = 0
   While i < sourceBytes.Length - 1      ' RGBA: RED Green Blue Alpha
            If (sourceBytes(i) = destBytes(i)) AndAlso (sourceBytes(i + 1) = destBytes(i + 1)) AndAlso (sourceBytes(i + 2) = destBytes(i + 2)) AndAlso (sourceBytes(i + 3) = destBytes(i + 3)) Then
               
                Dim clr As Color = Color.Magenta

                diffBytes(i) = CByte(clr.R)
                diffBytes(i + 1) = CByte(clr.G)
                diffBytes(i + 2) = CByte(clr.B)
                diffBytes(i + 3) = CByte(clr.A)

            Else
               
                diffBytes(i) = sourceBytes(i)
                diffBytes(i + 1) = sourceBytes(i + 1)
                diffBytes(i + 2) = sourceBytes(i + 2)

                diffBytes(i + 3) = sourceBytes(i + 3)

            End If

            i += 4
        End While


 Step 5: Convert the byte array which holds different pixel colors to a bitmap

Dim diffBitmp As New Bitmap(sourceBitmap.Width, sourceBitmap.Height)
        
Dim data As Imaging.BitmapData = diffBitmp.LockBits(New Rectangle(0, 0, diffBitmp.Width, diffBitmp.Height), Imaging.ImageLockMode.WriteOnly, Imaging.PixelFormat.Format32bppArgb)
     
        
Runtime.InteropServices.Marshal.Copy(diffBytes, 0, data.Scan0, diffBytes.Length)
        
diffBitmp.UnlockBits(data)


Step 6: Show diff bitmap in a picture box
 Me.PictureBox3.Image = diffBitmp

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