CS50 blur function does not pass check50, even though image is being blurred

Question

I am working on a CS50 problem set in which I need to do a box blur for each pixel of an image. Though my code is a bit redundant, as I had created 8 if statements for special cases of pixels (like edges and corners), it blurs the image as expected, so I'm not really sure how to fix the problem. Here's the error code:

:( blur correctly filters middle pixel
    expected "127 140 149\n", not "145 160 169\n"
:( blur correctly filters pixel on edge
    expected "80 95 105\n", not "90 106 116\n"
:) blur correctly filters pixel in corner
:( blur correctly filters 3x3 image
    expected "70 85 95\n80 9...", not "70 85 95\n90 1..."
:( blur correctly filters 4x4 image
    expected "70 85 95\n80 9...", not "70 85 95\n90 1..."

There's also a more detailed error code here (only look at the "blur" errors)

Here's my code below:

// Blur image
void blur(int height, int width, RGBTRIPLE image[height][width])
{
    int blue;
    int green;
    int red;
    int counter = 0;

    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            if (i == 0 && j == 0)
            {
                // top left corner
                blue  = (image[i][j].rgbtBlue  + image[i + 1][j].rgbtBlue  + image[i][j+1].rgbtBlue  + image[i + 1][j+1].rgbtBlue);
                green = (image[i][j].rgbtGreen + image[i + 1][j].rgbtGreen + image[i][j+1].rgbtGreen + image[i + 1][j+1].rgbtGreen);
                red   = (image[i][j].rgbtRed   + image[i + 1][j].rgbtRed   + image[i][j+1].rgbtRed   + image[i + 1][j+1].rgbtRed);
                counter = 4;
            }

            else if (i == 0 && j == (width - 1))
            {
                // top right corner
                blue  = (image[i][j].rgbtBlue  + image[i + 1][j].rgbtBlue  + image[i][j-1].rgbtBlue  + image[i + 1][j-1].rgbtBlue);
                green = (image[i][j].rgbtGreen + image[i + 1][j].rgbtGreen + image[i][j-1].rgbtGreen + image[i + 1][j-1].rgbtGreen);
                red   = (image[i][j].rgbtRed   + image[i + 1][j].rgbtRed   + image[i][j-1].rgbtRed   + image[i + 1][j-1].rgbtRed);
                counter = 4;
            }

            else if (i == 0 && (j != 0 || j != (width - 1)))
            {
                // top edge
                blue  = (image[i][j - 1].rgbtBlue  + image[i][j].rgbtBlue  + image[i][j + 1].rgbtBlue  + image[i + 1][j].rgbtBlue  + image[i+1][j - 1].rgbtBlue  + image[i + 1][j + 1].rgbtBlue);
                green = (image[i][j - 1].rgbtGreen + image[i][j].rgbtGreen + image[i][j + 1].rgbtGreen + image[i + 1][j].rgbtGreen + image[i+1][j - 1].rgbtGreen + image[i + 1][j + 1].rgbtGreen);
                red   = (image[i][j - 1].rgbtRed   + image[i][j].rgbtRed   + image[i][j + 1].rgbtRed   + image[i + 1][j].rgbtRed   + image[i+1][j - 1].rgbtRed   + image[i + 1][j + 1].rgbtRed);
                counter = 6;
            }

            else if (i == (height - 1) && j == 0)
            {
                // bottom left corner
                blue  = (image[i-1][j + 1].rgbtBlue  + image[i][j + 1].rgbtBlue  + image[i - 1][j].rgbtBlue  + image[i][j].rgbtBlue);
                green = (image[i-1][j + 1].rgbtGreen + image[i][j + 1].rgbtGreen + image[i - 1][j].rgbtGreen + image[i][j].rgbtGreen);
                red   = (image[i-1][j + 1].rgbtRed   + image[i][j + 1].rgbtRed   + image[i - 1][j].rgbtRed   + image[i][j].rgbtRed);
                counter = 4;
            }

            else if (i == (height - 1) && j == (width - 1))
            {
                // bottom right corner
                blue  = (image[i][j].rgbtBlue  + image[i - 1][j].rgbtBlue  + image[i][j-1].rgbtBlue  + image[i - 1][j-1].rgbtBlue);
                green = (image[i][j].rgbtGreen + image[i - 1][j].rgbtGreen + image[i][j-1].rgbtGreen + image[i - 1][j-1].rgbtGreen);
                red   = (image[i][j].rgbtRed   + image[i - 1][j].rgbtRed   + image[i][j-1].rgbtRed   + image[i - 1][j-1].rgbtRed);
                counter = 4;
            }

            else if (i == (height - 1) && (j != 0 || j != (width - 1)))
            {
                // bottom edge
                blue  = (image[i][j].rgbtBlue  + image[i][j - 1].rgbtBlue  + image[i][j + 1].rgbtBlue  + image[i - 1][j].rgbtBlue  + image[i-1][j - 1].rgbtBlue  + image[i - 1][j + 1].rgbtBlue);
                green = (image[i][j].rgbtGreen + image[i][j - 1].rgbtGreen + image[i][j + 1].rgbtGreen + image[i - 1][j].rgbtGreen + image[i-1][j - 1].rgbtGreen + image[i - 1][j + 1].rgbtGreen);
                red   = (image[i][j].rgbtRed   + image[i][j - 1].rgbtRed   + image[i][j + 1].rgbtRed   + image[i - 1][j].rgbtRed   + image[i-1][j - 1].rgbtRed   + image[i - 1][j + 1].rgbtRed);
                counter = 6;
            }

            else if (j == 0 && (i != 0 || i != (height - 1)))
            {
                // left edge
                blue  = (image[i][j].rgbtBlue  + image[i - 1][j].rgbtBlue  + image[i+1][j].rgbtBlue  + image[i][j + 1].rgbtBlue  + image[i-1][j + 1].rgbtBlue  + image[i + 1][j + 1].rgbtBlue);
                green = (image[i][j].rgbtGreen + image[i - 1][j].rgbtGreen + image[i+1][j].rgbtGreen + image[i][j + 1].rgbtGreen + image[i-1][j + 1].rgbtGreen + image[i + 1][j + 1].rgbtGreen);
                red   = (image[i][j].rgbtRed   + image[i - 1][j].rgbtRed   + image[i+1][j].rgbtRed   + image[i][j + 1].rgbtRed   + image[i-1][j + 1].rgbtRed   + image[i + 1][j + 1].rgbtRed);
                counter = 6;
            }

            else if (j == (width - 1) && (i != 0 || i != (height - 1)))
            {
                // right edge
                blue  = (image[i][j].rgbtBlue  + image[i-1][j].rgbtBlue  + image[i + 1][j].rgbtBlue  + image[i][j - 1].rgbtBlue  + image[i + 1][j-1].rgbtBlue  + image[i-1][j - 1].rgbtBlue);
                green = (image[i][j].rgbtGreen + image[i-1][j].rgbtGreen + image[i + 1][j].rgbtGreen + image[i][j - 1].rgbtGreen + image[i + 1][j-1].rgbtGreen + image[i-1][j - 1].rgbtGreen);
                red   = (image[i][j].rgbtRed   + image[i-1][j].rgbtRed   + image[i + 1][j].rgbtRed   + image[i][j - 1].rgbtRed   + image[i + 1][j-1].rgbtRed   + image[i-1][j - 1].rgbtRed);
                counter = 6;
            }

            else
            {
                blue  = (image[i][j].rgbtBlue  + image[i - 1][j].rgbtBlue  + image[i + 1][j].rgbtBlue  + image[i][j - 1].rgbtBlue  + image[i][j + 1].rgbtBlue  + image[i - 1][j + 1].rgbtBlue  + image[i-1][j - 1].rgbtBlue  + image[i + 1][j - 1].rgbtBlue  + image[i + 1][j + 1].rgbtBlue);
                green = (image[i][j].rgbtGreen + image[i - 1][j].rgbtGreen + image[i + 1][j].rgbtGreen + image[i][j - 1].rgbtGreen + image[i][j + 1].rgbtGreen + image[i - 1][j + 1].rgbtGreen + image[i-1][j - 1].rgbtGreen + image[i + 1][j - 1].rgbtGreen + image[i + 1][j + 1].rgbtGreen);
                red   = (image[i][j].rgbtRed   + image[i - 1][j].rgbtRed   + image[i + 1][j].rgbtRed   + image[i][j - 1].rgbtRed   + image[i][j + 1].rgbtRed   + image[i - 1][j + 1].rgbtRed   + image[i-1][j - 1].rgbtRed   + image[i + 1][j - 1].rgbtRed   + image[i + 1][j + 1].rgbtRed);
                counter = 9;
            }

            image[i][j].rgbtBlue  = round((float) blue / counter);
            image[i][j].rgbtGreen = round((float) green / counter);
            image[i][j].rgbtRed   = round((float) red / counter);
        }
    }
    return;
}

Looking at other people's answers for this same problem, I see that some have made a second nested for loop to store the original value of the pixel. I initially tried to implement this, but it ended up causing problems, so I figured it was not necessary. Is this the problem with my code, and if so, how would I properly implement the "original value" pixel within my code? If not, does anyone know the problem? Thanks in advance.

score 3 · Accepted Answer · answered Jun 05 '20 at 07:17

You are modifying your image as you apply blur function to the pixels. This means when you modify few pixels, the adjacent pixels blur values are calculated with "blurred pixel values". This is wrong. All the calculations must be done within original image pixel values. For this, you should create a copy of image in the beginning (such as temp) and make all this calculation within that temp image which has unmodified pixel values.

Add this to the beginning of your code;

RGBTRIPLE temp[height][width]; // create a temporary array to store a duplicate of image.

// save a new copy of image as temp per color.
for (int i = 0; i < height; i++) //Loop for height of image.
{
    for (int j = 0; j < width; j++) //Loop for width of image and save color values in temp.
    {
        temp[i][j] = image[i][j];
    }
}

And replace image with temp in your calculations (except the assignment at the end).

Thanks, I knew in the back of my mind that I needed two nested loops (one for reading and one for writing), but it didn't work for some reason when I tried implementing it myself. But your solution worked perfectly and passed all checks, so thanks again! — qwert9988, Jun 05 '20 at 15:19

CS50 blur function does not pass check50, even though image is being blurred

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