Stuck implementing boundary checks on frame windows for mean filtering

Question

I have a function that successfully reads rgb values from a ppm and a function that successfully writes to a ppm. What I am trying is a function called denoiseImage that changes rgb values from a ppm using mean filtering with a frame window size n by n where n is odd. My intent is to go through each pixel, using it as the center point for the window n by n that surrounds it. I then take the mean values for each color (r,g,b) and divide by the number of pixels in the window and assign those new values to the rgb of every pixel in the window. However, I am unable to implement a check for the cases where the frame does not fully fit into pixels (for example, the frame center point is the top right pixel, a window of 3x3 will go to non existent points.) When it does not fit fully, I intend to use the available pixels that fit and take the mean of those numbers instead. So far, my code will only work for cases where the frame fully fits. My function:

RGB *denoiseImage(int width, int height, const RGB *image, int n)
{
int firstPos, lastPos, i = 0, j = 0, k, numofPix;
int sumR=0,sumG=0,sumB=0;
numofPix = (width * height);
RGB *pixels = malloc(numofPix * sizeof(RGB));
if (n == 1)                  //Case where the window size is 1 and therefore the image does not get changed.
{
    return pixels;
}

for (j=0;j < numofPix;j++)                  
{
    firstPos = (j - width) - ((n - 1)/2);
    lastPos = (j + width) + ((n - 1)/2);

    //Need to check boundary cases to prevent segmentation fault

    for (k=firstPos;k<=lastPos;k++)      //Seg fault. Unable to shrink frame to compensate for cases where the frame does not fit.
    {
        sumR+=image[k].r;
        sumG+=image[k].g;
        sumB+=image[k].b;
        i++;
        if (i = n)                                      //Used to skip elements not in frame
        {
            j += (width-n);
            i = 0;
        }
    }

    sumR = sumR/(n*n);                                   //Calculating mean values
    sumG = sumG/(n*n);
    sumB = sumB/(n*n);

    for (k=firstPos;k<=lastPos;k++)                     //Assigning the RGB values with the new mean values.
    {
        pixels[k].r=sumR;
        pixels[k].g=sumG;
        pixels[k].b=sumB;
        printf("%d %d %d ",pixels[k].r, pixels[k].g, pixels[k].b);
    }
}
return pixels;
}

int main()
{
RGB *RGBValues;
int width, height, max;
int j = 0,testemp=3;             //test temp is a sample frame size
char *testfile = "test.ppm";
char *testfile2 = "makeme.ppm";
RGBValues = readPPM(testfile, &width, &height, &max);              //Function reads values from a ppm file correctly                     
RGBValues = denoiseImage(width,height, RGBValues, testemp,testing);
writePPM(testfile2,width,height,max,RGBValues);   //Function writes values to a ppm file correctly                             
}

How would I implement a way to check if the frame fits or not?

Answer 1

This is a great question and luckily known in the image processing community. Edges are always treated differently when it comes to 2D filtering.

One way to look at it is to extend the space in 2D and to fill the edges with extrapolated values from the middle.

For example, you may look into the http://www.librow.com/articles/article-1 and search for a media filter.

I am sure that you will find solution soon, since you are going into right direction.