Sobel operator & Convolution

Feb 13, 2013 at 10:23 AM
Hey there
I'm trying to implement edge detection on an image using the sobel operator. I have successfully applied both kernels for x and y detection onto the image using .Convolute(), but now I'm stuck adding both images to one. The problem is, that all the pixels of both images seem to have the value 0 for transparency (so the A in ARGB). I can display both images on their own without a Problem, but adding them gives me just a black picture. So my questions are:
  • Why is the transparency set to 0 for every pixel after the convolution?
  • Why can I still display the Image without it being all black?
  • Why is it black when I add two Images?
For calrification, here's my code so far. I can display both wbmpY and wbmpX, but finalbmp is completely black.
    public int[,] sobelY = new int[3, 3] { { 1, 2, 1 }, { 0, 0, 0 }, { -1, -2, -1 } };
    public int[,] sobelX = new int[3, 3] { { -1, 0, 1 }, { -2, 0, 2 }, { -1, 0, 1 } };

    public void trim(WriteableBitmap wbmp)
    {
        var graybmp = wbmp.Clone();
        graybmp.ForEach(toGrayscale);

        var wbmpY = graybmp.Clone();
        var wbmpX = graybmp.Clone();

        wbmpY = wbmpY.Convolute(sobelY, 1, 0);
        wbmpX = wbmpX.Convolute(sobelX, 1, 0);

        var finalbmp = combineSobel(wbmpX, wbmpY);           
     }

    public WriteableBitmap combineSobel(WriteableBitmap img, WriteableBitmap img2)
    {
        int height = img.PixelHeight;
        int width = img.PixelWidth;
        WriteableBitmap result = img.Clone();
        for (int x = 0; x < width; x++)
        {
            for (int y = 0; y < height; y++)
            {
                Color imgColor = img.GetPixel(x, y);
                Color img2Color = img2.GetPixel(x, y);
                Color newColor = Color.FromArgb(
                    Math.Min((byte)Math.Sqrt(Math.Pow(imgColor.A, 2) + Math.Pow(img2Color.A, 2)), (byte)255),
                    Math.Min((byte)Math.Sqrt(Math.Pow(imgColor.R, 2) + Math.Pow(img2Color.R, 2)), (byte)255),
                    Math.Min((byte)Math.Sqrt(Math.Pow(imgColor.G, 2) + Math.Pow(img2Color.G, 2)), (byte)255),
                    Math.Min((byte)Math.Sqrt(Math.Pow(imgColor.B, 2) + Math.Pow(img2Color.B, 2)), (byte)255)
                );