Speed up Matrix Addition in C#

Question

I'd like to optimize this piece of code :

public void PopulatePixelValueMatrices(GenericImage image,int Width, int Height)
{            
        for (int x = 0; x < Width; x++)
        {
            for (int y = 0; y < Height; y++)
            {
                Byte  pixelValue = image.GetPixel(x, y).B;
                this.sumOfPixelValues[x, y] += pixelValue;
                this.sumOfPixelValuesSquared[x, y] += pixelValue * pixelValue;
            }
        }
}

This is to be used for image processing, and we're currently running this for about 200 images. We've optimized the GetPixel value to use unsafe code, and we're not using image.Width, or image.Height, as those properties were adding to our runtime costs.

However, we're still stuck at a low speed. The problem is that our images are 640x480, so the middle of the loop is being called about 640x480x200 times. I'd like to ask if there's a way to speed it up somehow, or convince me that it's fast enough as it is. Perhaps a way is through some fast Matrix Addition, or is Matrix Addition inherently an n^2 operation with no way to speed it up?

Perhaps doing array accesses via unsafe code would speed it up, but I'm not sure how to go about doing it, and whether it would be worth the time. Probably not. Thanks.

EDIT : Thank you for all your answers.

This is the GetPixel method we're using:

 public Color GetPixel(int x, int y)
    {
        int offsetFromOrigin = (y * this.stride) + (x * 3);
        unsafe
        {
            return Color.FromArgb(this.imagePtr[offsetFromOrigin + 2], this.imagePtr[offsetFromOrigin + 1], this.imagePtr[offsetFromOrigin]);
        }
    }

Answer 1

Where are images stored? If each is on disk, then a bit of your processing time issue may be in fetching them from the disk. You might examine this to see if it is an issue, and if so, then rewrite to pre-fetch the image data so that the array procesing code does not have to wait for the data...

If the overall application logic will allow it (Is each matrix addition independant, or dependant on output of a previous matrix addition?) If they are independant, I'd examine executing them all on separate threads, or in parallel..

Answer 2

Although it's a micro-optimization and thus may not add much you might want to study what the likelihood is of getting a zero when you do

Byte  pixelValue = image.GetPixel(x, y).B;

Clearly, if pixelValue = 0 then there's no reason to do the summations so your routine might become

public void PopulatePixelValueMatrices(GenericImage image,int Width, int Height)
  {
  for (int x = 0; x < Width; x++)
    {
    for (int y = 0; y < Height; y++)
      {
       Byte  pixelValue = image.GetPixel(x, y).B;

       if(pixelValue != 0)
         {
         this.sumOfPixelValues[x, y] += pixelValue;
         this.sumOfPixelValuesSquared[x, y] += pixelValue * pixelValue;
         }}}}

However, the question is how often you're going to see pixelValue=0, and whether the saving on the compute-and-store will offset the cost of the test.

Answer 3

If you only do matrix addition, you'd like to consider using multiple threads to speed up by taking advantage of multi-core processors. Also use one dimensional index instead of two.

If you want to do more complicated operations, you need to use a highly optimized math library, like NMath.Net, which uses native code rather than .net.