Crazypennie has the correct answer as to why you were not getting the expected results. I would just like to add an alternate routine which gives better performance when actually comparing the two images for visual equality.
The idea is the same; compare the pixels to see if they are all the same color at the same position. But GetPixel() and SetPixel() are slow. All we really need to do are get the two arrays of bytes that represent the color information in the
image and then see if there are any differences between the two arrays. We can use a System.Drawing.Imaging.BitmapData object and the System.Runtime.Interop.Marshal.Copy() method to make this fairly simple.
Note that the following routine makes some minor assumptions about the image. Look up the "Stride" or "Scan Width" of an image for more details. Although there could be some exceptions, this routine should work well with most images:
Public Function IsVisuallyEqual(ByVal sourceImage As Image, ByVal sourceBounds As Rectangle, _
ByVal targetImage As Image, ByVal targetBounds As Rectangle) As Boolean
Dim sourceBmp = New Bitmap(sourceImage)
Dim targetBmp = New Bitmap(targetImage)
Dim sourceData As System.Drawing.Imaging.BitmapData = sourceBmp.LockBits( _
sourceBounds, Imaging.ImageLockMode.ReadWrite, sourceImage.PixelFormat)
Dim targetData As System.Drawing.Imaging.BitmapData = targetBmp.LockBits( _
targetBounds, Imaging.ImageLockMode.ReadWrite, targetImage.PixelFormat)
Dim sourceLength As Integer = (sourceData.Height * sourceData.Stride) - 1
Dim sourceBytes(sourceLength) As Byte
System.Runtime.InteropServices.Marshal.Copy(sourceData.Scan0, sourceBytes, 0, sourceBytes.Length)
sourceBmp.UnlockBits(sourceData)
sourceBmp.Dispose()
Dim targetLength As Integer = (targetData.Height * targetData.Stride) - 1
Dim targetBytes(targetLength) As Byte
System.Runtime.InteropServices.Marshal.Copy(targetData.Scan0, targetBytes, 0, targetBytes.Length)
targetBmp.UnlockBits(targetData)
targetBmp.Dispose()
For i As Integer = 0 To sourceBytes.Length - 1
If Not targetBytes(i) = sourceBytes(i) Then
Return False
End If
Next
Return True
End Function
Also note that since the routine expects the bounds of each image (normally a Rectangle(0, 0, Image.Width, Image.Height) in size) you could in theory pass a rectangle within an image to see if one image is cropped out of another. You would need to
know the starting point of the cropped image, or you could write an algorithm to search for it, but this routine would allow you to match the partial image given its known starting point.
Anyway, like I said, pennie already did a good job answering the thread. Since this topic will likely get lots of search hits over time, I thought this extra bit of code might be a good addition.
Reed Kimble - "When you do things right, people won't be sure you've done anything at all"
