Problems with a subroutine to match bitmaps.

Duane

I have developed a sub to match two bitmaps pixel-by-pixel. Bm1 is a newly created bitmap saved to a temporary directory. Bm2 represents the existing bitmaps stored in a permanent directory. The problem is I am getting a Match response for two bitmaps even when I know they are not the same. Can somebody tell me what I am doing wrong? If you need more information, please respond and tell me what you need to know. Thanks.

Private Sub btnMatch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnMatch.Click

PatternArrayCreator.BmpTempSave()

Dim dir As New DirectoryInfo("C:\Documents and Settings\Owner\My Documents\Pentominos\")

Dim fileArr As FileInfo() = dir.GetFiles("*.bmp")

Dim fiInfo As FileInfo

Me.Cursor = Cursors.WaitCursor

Application.DoEvents()

Dim bm1 As Bitmap = Image.FromFile("C:\bmpt.bmp")

Dim bm2 As Bitmap = Nothing

Dim bm3 As Bitmap = New Bitmap(121, 201)

Dim are_identical As Boolean = True

For Each fiInfo In fileArr

bm2 = Image.FromFile(fiInfo.FullName)

For x As Integer = 0 To bm3.Width - 1

For y As Integer = 0 To bm3.Height - 1

If bm1.GetPixel(x, y).Equals(bm2.GetPixel(x, y)) Then

are_identical = True

Else

are_identical = False

End If

If are_identical = False Then

MsgBox("The images are different.")

Continue For

Else

MsgBox("The images match.")

Exit Sub

End If

Me.Cursor = Cursors.Default

bm1.Dispose()

bm2.Dispose()

End Sub

9 Replies

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Duane14y ago
I am temporarily leaving this area of exploration (LOL). When I get back to this project I will make new attempts to find a solution and if necessary, start a new thread at that time. Thank you all for your assistance.
Vulpes14y ago
Well, this is what the code should do:

When it finds a pixel which doesn't match, it should set are_identical to false, exit the 'y' for loop and then the 'x' for loop.

It should then pop up a messagebox saying that "The images are different" and continue the for each loop by comparing the next image. When it does so, are_identical should be reset to true.

If all the pixels are the same, then are_identical will still be true on exiting the 'y' and 'x' for loops. It should then pop up a messagebox saying that "The Images match" and exit the subroutine altogether. Incidentally, it would be better here to use Exit For so that the cursor is restored and the bitmaps are disposed of (also need to dispose of bmp3) before the Sub ends 'naturally'.

The only thing I can find in the code which could conceivably be suspect is that the 'x' and 'y' for loops are using bmp3's width and height rather than that of bmp1.

Is it possible that the other bitmaps could have a different size to this?
Duane14y ago
I think I must have been unclear in my response. Yes, it is true that when a loop finds mismatch in the bitmaps it should exit (with a message). However, it is not doing this and I can't figure out why. The program suggestions you made should have provided the result mentioned above but that is not what's happening. Any further ideas as to why this is happening?
Vulpes14y ago
It will improve processing, because on average, it's not examining as many pixels as before. As soon as it finds one which doesn't match, it will exit the loop.

I'd certainly have thought that for two images to be regarded as equal, corresponding pixels should have the same value, but you seem to regard this criterion as being too strong.

What then is your criterion for regarding the two images as being the same - that a high percentage of corresponding pixels are equal?
Duane14y ago
Still not working properly, but it did improve processing. I also figured out that since all the bitmaps individual pixels (24000!) either have to be exactly the same at exactly the same location or not, any program that checks individual pixels and provides a result based on that particular pixel will provide an incorrect response in almost all cases. I need to figure out how to get the result of the ENTIRE analysis and base my results on that. If you know a way to do this please let me know.

Likewise if I'm wrong about the above and you can explain why, I'd like to hear about it.
Duane14y ago
Thanks, I'll try that and let you know how it came out.
Vulpes14y ago
Looking at it again, it's just dawned on me what (I think) the problem is.

Even if the two bitmaps are not the same, are_identical will be true if the last pixel(s) examined happen to be the same.

To avoid this you'll need to rejig the code a bit:

Private Sub btnMatch_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnMatch.Click
PatternArrayCreator.BmpTempSave()
Dim dir As New DirectoryInfo("C:\Documents and Settings\Owner\My Documents\Pentominos\")
Dim fileArr As FileInfo() = dir.GetFiles("*.bmp")
Dim fiInfo As FileInfo
Me.Cursor = Cursors.WaitCursor
Application.DoEvents()

Dim bm1 As Bitmap = Image.FromFile("C:\bmpt.bmp")
Dim bm2 As Bitmap = Nothing

Dim bm3 As Bitmap = New Bitmap(121, 201)

For Each fiInfo In fileArr

Dim are_identical As Boolean = True 'reset to true each time through the loop

bm2 = Image.FromFile(fiInfo.FullName)
For x As Integer = 0 To bm3.Width - 1
For y As Integer = 0 To bm3.Height - 1
If bm1.GetPixel(x, y).ToArgb() <> bm2.GetPixel(x, y).ToArgb() Then
are_identical = False
Exit For ' no point iterating further
End If
Next

If are_identical = False Then
Exit For ' exit outer for loop
End If
Next

If are_identical = False Then
MsgBox("The images are different.")
Continue For
Else
MsgBox("The images match.")
Exit Sub
End If
Next

Me.Cursor = Cursors.Default

bm1.Dispose()
bm2.Dispose()

End Sub
Duane14y ago
This does not seen to be the problem. The arguments are getting passed properly, but the result is still the same. Even when I try to match a bitmap with no color information whatever the program returns that it matches an existing file! This isn't possible, therefore my code is not executing properly at some point. If someone wants the entire program, in order to attempt to solve this problem, please let me know. I am working with Visual Basic Express 2010 on Windows XP (if that makes a difference).
Vulpes14y ago
Rather than:

If bm1.GetPixel(x, y).Equals(bm2.GetPixel(x, y)) Then

I'd try:

If bm1.GetPixel(x, y).ToArgb() = bm2.GetPixel(x, y).ToArgb() Then

See the remarks in the MSDN docs for the Color.Equals() method:

http://msdn.microsoft.com/en-us/library/e03x8ct2(VS.100).aspx