我可以将图像处理与PLINQ并行化吗？

Question

我想比较一个图像的alpha通道到~1000个其他图像。 我的比较方法如下所示：

public static unsafe double Similiarity (Bitmap a, Bitmap b)
{
    BitmapData aData = a.LockBits (
        new Rectangle (0, 0, a.Width, a.Height), 
        System.Drawing.Imaging.ImageLockMode.ReadOnly, a.PixelFormat);
    BitmapData bData = b.LockBits (
        new Rectangle (0, 0, b.Width, b.Height), 
        System.Drawing.Imaging.ImageLockMode.ReadOnly, b.PixelFormat);
    int PixelSize = 4;

    double sum = 0;
    for (int y=0; y<aData.Height; y++) {
        byte* aRow = (byte *)aData.Scan0 + (y * aData.Stride);
        byte* bRow = (byte *)bData.Scan0 + (y * bData.Stride);
        for (int x=0; x<aData.Width; x++) {
            byte aWeight = aRow [x * PixelSize + 3];
            byte bWeight = bRow [x * PixelSize + 3];
            sum += Math.Abs (aWeight - bWeight);
        }
    }
    a.UnlockBits (aData);
    b.UnlockBits (bData);

    return 1 - ((sum / 255) / (a.Width * a.Height));
}

我认为加速计算的最简单方法是使用PLINQ：

var list = from Bitmap img in imageList.AsParallel where (Similiarity (referenceImage, img) > 0.5) select img;

但在执行时，gdiplus中存在异常：

System.InvalidOperationException: The operation is invalid [GDI+ status: Win32Error]
  at System.Drawing.GDIPlus.CheckStatus (Status status) [0x00000] in <filename unknown>:0 
  at System.Drawing.Bitmap.LockBits (Rectangle rect, ImageLockMode flags, PixelFormat format, System.Drawing.Imaging.BitmapData bitmapData) [0x00000] in <filename unknown>:0 

我知道gdiplus必须在不同的进程中执行，但我认为PLINQ正在这样做。 我的假设有什么问题？

Answer 1

我建议将权重计算与相似性比较分开，如下所示：

public static unsafe byte[,] GetWeight(Bitmap a)
{
    BitmapData aData = a.LockBits(new Rectangle(0, 0, a.Width, a.Height), ImageLockMode.ReadOnly, a.PixelFormat);
    const int pixelSize = 4;

    byte[,] weight = new byte[aData.Width, aData.Height];
    for (int y = 0; y < aData.Height; y++)
    {
        byte* aRow = (byte*)aData.Scan0 + (y * aData.Stride);
        for (int x = 0; x < aData.Width; x++)
        {
            byte aWeight = aRow[x * pixelSize + 3];
            weight[x, y] = aWeight;
        }
    }
    a.UnlockBits(aData);
    return weight;
}

public static double GetSimilarity(byte[,] weightsA, byte[,] weightsB)
{
    double sum = 0;
    int height = weightsA.GetLength(1);
    int width = weightsA.GetLength(0);
    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x++)
        {
            byte aWeight = weightsA[x,y];
            byte bWeight = weightsB[x, y];
            sum += Math.Abs(aWeight - bWeight);
        }
    }

    return 1 - ((sum / 255) / (width * height));
}

调用本身将如下所示：

var referenceWeigth = GetWeight(referenceImage);    
var list =
        imageList
            .AsParallel()
            .Select(image => new {@Image = image, @Weight = GetWeight(image)})
            .Where(imageAndWeight => GetSimilarity(imageAndWeight.Weight, referenceWeigth) > 0.5)
            .Select(imageAndWeight => imageAndWeight.Image);

Answer 2

我认为问题可能是你试图同时锁定不同线程中的相同位。 一个可能的解决方案（因为您只是阅读）应首先从每个位图中提取位，并仅使用这些位并行比较它们。

Answer 3

我看到的问题是，当img取自列表时， referenceImage是在多个线程中使用的相同实例，每个线程都试图锁定它。

考虑将参考图像锁定在外面，并传递BitmapData。