將循環轉換爲並行循環

Question

我寫了一個計算圖像焦點值的代碼。但要花5秒以上才能完成。

public double GetFValue(Image image) 
     { 
      Bitmap source = new Bitmap(image); 
      int count = 0; 
      double total = 0; 
      double totalVariance = 0; 
      double FM = 0; 
      Bitmap bm = new Bitmap(source.Width, source.Height); 
      Rectangle rect = new Rectangle(0,0,source.Width,source.Height); 
       Bitmap targetRect = new Bitmap(rect.Width, rect.Height); 

      // converting to grayscale 
      for (int y = 0; y < source.Height; y++) 
      { 
       for (int x = 0; x < source.Width; x++) 
       { 
        count++; 
        Color c = source.GetPixel(x, y); 
        int luma = (int)(c.R * 0.3 + c.G * 0.59 + c.B * 0.11); 
        source.SetPixel(x, y, Color.FromArgb(luma, luma, luma)); // the image is now gray scaled 
        var pixelval = source.GetPixel(x, y); 
        // targetRect.Save(@"C:\Users\payam\Desktop\frame-42-rectangle.png", System.Drawing.Imaging.ImageFormat.Png); 
        int pixelValue = pixelval.G; 
        total += pixelValue; 
        double avg = total/count; 
        totalVariance += Math.Pow(pixelValue - avg, 2); 
        double stDV = Math.Sqrt(totalVariance/count); // the standard deviation, which is also the focus value 
        FM = Math.Round(stDV, 2); 
       } 
      } 
      return FM; 
     } 

我想將此代碼轉換爲並行計算。我最終得到的錯誤是我無法將他們的頭圍繞在他們身上。任何建議？

public double CalculateFvalue (Image image) 
    { 
     Bitmap myimage = new Bitmap(image); 
     int count = 0; 
     int total = 0; 
     double totalVariance = 0; 
     double FM = 0; 
     Parallel.For(0, image.Height, y => 
      { 

      for (int x = 0; x < myimage.Width; x++) 
        { 
          count++; 
          Color c = myimage.GetPixel(x, y); 
          int luma = (int)(c.R * 0.3 + c.G * 0.59 + c.B * 0.11); 
          myimage.SetPixel(x, y, Color.FromArgb(luma, luma, luma)); // the image is now gray scaled 
          var pixelval = myimage.GetPixel(x, y); 
          int pixelValue = pixelval.G; 
          total += pixelValue; 
          double avg = total/count; 
          totalVariance += Math.Pow(pixelValue - avg, 2); 
          double stDV = Math.Sqrt(totalVariance/count); // the standard deviation, which is also the focus value 
          FM = Math.Round(stDV, 2); 
        } 


    }); 

     return Math.Round(FM,2); 
    } 

Answer 1

爲了擴展我的評論，不要嘗試並行運行GetPixel，而是使用lockBits。

使用您的代碼lockbits：

public double GetFValue(Image image) 
    { 
     Bitmap source = new Bitmap(image); 
     int count = 0; 
     double total = 0; 
     double totalVariance = 0; 
     double FM = 0; 
     Bitmap bm = new Bitmap(source.Width, source.Height); 
     Rectangle rect = new Rectangle(0, 0, source.Width, source.Height); 
     //Bitmap targetRect = new Bitmap(rect.Width, rect.Height); 

     //new 
     ///* 
     BitmapData bmd = source.LockBits(rect, ImageLockMode.ReadWrite, source.PixelFormat); 
     int[] pixelData = new int[(rect.Height * rect.Width) -1]; 
     System.Runtime.InteropServices.Marshal.Copy(bmd.Scan0, pixelData, 0, pixelData.Length); 

     for (int i = 0; i < pixelData.Length; i++) 
     { 
      count++; 
      Color c = Color.FromArgb(pixelData[i]); 
      int luma = (int)(c.R * 0.3 + c.G * 0.59 + c.B * 0.11); 
      //Probably a formula for this 
      pixelData[i] = Color.FromArgb(luma, luma, luma).ToArgb(); 
      total += luma; 
      double avg = total/count; 
      totalVariance += Math.Pow(luma - avg, 2); 
      double stDV = Math.Sqrt(totalVariance/count); 
      FM = Math.Round(stDV, 2); 
     } 
     source.UnlockBits(bmd); 
     return FM; 
    } 

在使用1024×768的JPG從win7的樣本圖象的快速測試（Chrysanthemum.jpg）：

lockbits：241毫秒

getPixel： 2208毫秒

注意轉換你的代碼時，我注意到一些奇怪的東西（比如getpixel，setpixel，getpixel在同一像素上？）但我猜你k現在你想要達到什麼目的，這段代碼和你的完全相同

Answer 2

發生這種情況是因爲您聲明的變量超出了Parallel.For的範圍。由於它們的訪問（和寫入）是非確定性的，因此可能會使用錯誤的數據覆蓋值（如FM）。

我建議你讓每個迭代產生有關其結果的信息，然後使用收集的數據以線程安全的方式處理外部變量。你也可以通過使用幾個lock聲明逃脫，但我個人會避免這種情況。