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;         


        

Answer 1

Despite using unsafe code, GetPixel may well be the bottleneck here. Have you looked at ways of getting all the pixels in the image in one call rather than once per pixel? For instance, Bitmap.LockBits may be your friend...

On my netbook, a very simply loop iterating 640 * 480 * 200 times only take about 100 milliseconds - so if you're finding it's all going slowly, you should take another look at the bit inside the loop.

Another optimisation you might want to look at: avoid multi-dimensional arrays. They're significantly slower than single-dimensional arrays.

In particular, you can have a single-dimensional array of size Width * Height and just keep an index:

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

Using the same simple test harness, adding a write to a 2-D rectangular array took the total time of looping over 200 * 640 * 480 up to around 850ms; using a 1-D rectangular array took it back down to around 340ms - so it's somewhat significant, and currently you've got two of those per loop iteration.