VB返回小數字溢出

Question

花了一個小時試圖解決這個問題，我決定作爲一個問題。

我在圖像相關的項目中使用VB.Net 2010。在這一步中，我必須比較兩幅圖像像素的顏色，以確定它們的相似程度。要做到這一點，我設計了下面的代碼：

  ''get both images as bitmaps 

      Dim act As New Bitmap("cam.bmp") 
      Dim map As New Bitmap(bmap) 

      ''declare the variable that counts the different pixels 

      Dim count As Integer = 0 

      ''declare the variables that represent the colors 

      Dim val As Long = 0 
      Dim valr As Long = 0 
      Dim valg As Long = 0 
      Dim valb As Long = 0 

      ''analize both images pixel by pixel 

      For x = 1 To act.Width - 1 Step 1 

       For y = 1 To act.Height - 1 Step 1 

        valr = 0 
        valg = 0 
        valb = 0 

        ''determine how different the pixels are 

        valr = Math.Abs(act.GetPixel(x, y).R - map.GetPixel(x, y).R) 
        valg = Math.Abs(act.GetPixel(x, y).G - map.GetPixel(x, y).G) 
        valb = Math.Abs(act.GetPixel(x, y).B - map.GetPixel(x, y).B) 

        val = Long.Parse(Int((valr + valg + valb)/3).ToString) 

        ''if enough different, add 1 to count 

        If val > 50 Then 

         count += 1 

        End If 

       Next 

      Next 

      ''show how many pixels are different in proportion 

      MsgBox((count/(act.Width * act.Height)).ToString) 

運行此代碼時，溢出的部分valr = Math.Abs(act.GetPixel(x, y).R - map.GetPixel(x, y).R)發生。沒有其他問題發生。

Answer 1

問題是，GetPixel返回Color和Color.R返回Byte。當你從另一個字節中減去一個字節時，結果是一個字節，但字節是無符號的，不能包含負值。如果第二個值大於第一個值，那麼在調用方法之前，您會收到OverflowException。

爲了避免這種情況，您可以使用CInt將字節轉換爲整數，然後再減去它們。

valr = Math.Abs(CInt(act.GetPixel(x, y).R) - CInt(map.GetPixel(x, y).R)) 
valg = Math.Abs(CInt(act.GetPixel(x, y).G) - CInt(map.GetPixel(x, y).G)) 
valb = Math.Abs(CInt(act.GetPixel(x, y).B) - CInt(map.GetPixel(x, y).B))