I've been taking cs50 for about under a month now and I've finally finished with "filter" in pset4. The code passes all the green checks on check50. Our task was to implement functions in helpers.c so that a user can apply grayscale, reflection, blur, or edge detection filters to their images. We were asked not to modify any of the function signatures or any files other than helpers.c.

My question is, how I could refactor the code in the edge detection block so that it is shorter with less repeated code? If there is a solution involving pointers, I'd much prefer that if possible! Any tips on other blocks are welcome as well. Thank you!

Here are links to the outline of cs50/pset4/filter and information on the sobel operator:
cs50/pset4/filter
Sobel

The files that we were asked not to modify: bmp.h

// BMP-related data types based on Microsoft's own

#include <stdint.h>

/**
 * Common Data Types
 *
 * The data types in this section are essentially aliases for C/C++
 * primitive data types.
 *
 * Adapted from http://msdn.microsoft.com/en-us/library/cc230309.aspx.
 * See http://en.wikipedia.org/wiki/Stdint.h for more on stdint.h.
 */
typedef uint8_t  BYTE;
typedef uint32_t DWORD;
typedef int32_t  LONG;
typedef uint16_t WORD;

/**
 * BITMAPFILEHEADER
 *
 * The BITMAPFILEHEADER structure contains information about the type, size,
 * and layout of a file that contains a DIB [device-independent bitmap].
 *
 * Adapted from http://msdn.microsoft.com/en-us/library/dd183374(VS.85).aspx.
 */
typedef struct
{
    WORD   bfType;
    DWORD  bfSize;
    WORD   bfReserved1;
    WORD   bfReserved2;
    DWORD  bfOffBits;
} __attribute__((__packed__))
BITMAPFILEHEADER;

/**
 * BITMAPINFOHEADER
 *
 * The BITMAPINFOHEADER structure contains information about the
 * dimensions and color format of a DIB [device-independent bitmap].
 *
 * Adapted from http://msdn.microsoft.com/en-us/library/dd183376(VS.85).aspx.
 */
typedef struct
{
    DWORD  biSize;
    LONG   biWidth;
    LONG   biHeight;
    WORD   biPlanes;
    WORD   biBitCount;
    DWORD  biCompression;
    DWORD  biSizeImage;
    LONG   biXPelsPerMeter;
    LONG   biYPelsPerMeter;
    DWORD  biClrUsed;
    DWORD  biClrImportant;
} __attribute__((__packed__))
BITMAPINFOHEADER;

/**
 * RGBTRIPLE
 *
 * This structure describes a color consisting of relative intensities of
 * red, green, and blue.
 *
 * Adapted from http://msdn.microsoft.com/en-us/library/aa922590.aspx.
 */
typedef struct
{
    BYTE  rgbtBlue;
    BYTE  rgbtGreen;
    BYTE  rgbtRed;
} __attribute__((__packed__))
RGBTRIPLE;

filter.c

#include <getopt.h>
#include <stdio.h>
#include <stdlib.h>

#include "helpers.h"

int main(int argc, char *argv[])
{

    // Define allowable filters
    char *filters = "begr";

    // Get filter flag and check validity
    char filter = getopt(argc, argv, filters);
    if (filter == '?')
    {
        fprintf(stderr, "Invalid filter.\n");
        return 1;
    }

    // Ensure only one filter
    if (getopt(argc, argv, filters) != -1)
    {
        fprintf(stderr, "Only one filter allowed.\n");
        return 2;
    }

    // Ensure proper usage
    if (argc != optind + 2)
    {
        fprintf(stderr, "Usage: filter [flag] infile outfile\n");
        return 3;
    }

    // Remember filenames
    char *infile = argv[optind];
    char *outfile = argv[optind + 1];

    // Open input file
    FILE *inptr = fopen(infile, "r");
    if (inptr == NULL)
    {
        fprintf(stderr, "Could not open %s.\n", infile);
        return 4;
    }

    // Open output file
    FILE *outptr = fopen(outfile, "w");
    if (outptr == NULL)
    {
        fclose(inptr);
        fprintf(stderr, "Could not create %s.\n", outfile);
        return 5;
    }

    // Read infile's BITMAPFILEHEADER
    BITMAPFILEHEADER bf;
    fread(&bf, sizeof(BITMAPFILEHEADER), 1, inptr);

    // Read infile's BITMAPINFOHEADER
    BITMAPINFOHEADER bi;
    fread(&bi, sizeof(BITMAPINFOHEADER), 1, inptr);

    // Ensure infile is (likely) a 24-bit uncompressed BMP 4.0
    if (bf.bfType != 0x4d42 || bf.bfOffBits != 54 || bi.biSize != 40 ||
        bi.biBitCount != 24 || bi.biCompression != 0)
    {
        fclose(outptr);
        fclose(inptr);
        fprintf(stderr, "Unsupported file format.\n");
        return 6;
    }

    int height = abs(bi.biHeight);
    int width = bi.biWidth;

    // Allocate memory for image
    RGBTRIPLE(*image)[width] = calloc(height, width * sizeof(RGBTRIPLE));
    if (image == NULL)
    {
        fprintf(stderr, "Not enough memory to store image.\n");
        fclose(outptr);
        fclose(inptr);
        return 7;
    }

    // Determine padding for scanlines
    int padding = (4 - (width * sizeof(RGBTRIPLE)) % 4) % 4;

    // Iterate over infile's scanlines
    for (int i = 0; i < height; i++)
    {
        // Read row into pixel array
        fread(image[i], sizeof(RGBTRIPLE), width, inptr);

        // Skip over padding
        fseek(inptr, padding, SEEK_CUR);
    }

    // Filter image
    switch (filter)
    {
        // Blur
        case 'b':
            blur(height, width, image);
            break;

        // Edges
        case 'e':
            edges(height, width, image);
            break;

        // Grayscale
        case 'g':
            grayscale(height, width, image);
            break;

        // Reflect
        case 'r':
            reflect(height, width, image);
            break;
    }

    // Write outfile's BITMAPFILEHEADER
    fwrite(&bf, sizeof(BITMAPFILEHEADER), 1, outptr);

    // Write outfile's BITMAPINFOHEADER
    fwrite(&bi, sizeof(BITMAPINFOHEADER), 1, outptr);

    // Write new pixels to outfile
    for (int i = 0; i < height; i++)
    {
        // Write row to outfile
        fwrite(image[i], sizeof(RGBTRIPLE), width, outptr);

        // Write padding at end of row
        for (int k = 0; k < padding; k++)
        {
            fputc(0x00, outptr);
        }
    }

    // Free memory for image
    free(image);

    // Close infile
    fclose(inptr);

    // Close outfile
    fclose(outptr);

    return 0;
}

My helpers.c

#include "helpers.h"
#include <stdio.h>
#include <ctype.h>
#include <math.h>
#include <string.h>

// Prototypes
void swap(RGBTRIPLE *a, RGBTRIPLE *b);

// Convert image to grayscale
void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
    // Iterate through the height or also known as each row
    for (int i = 0; i < height; i++)
    {
        // Iterate through the width or also known as each pixel/column
        for (int j = 0; j < width; j++)
        {
            // Calculate the average of the R, G, and B values and round to nearest integer
            int average = round(((double) image[i][j].rgbtBlue + (double) image[i][j].rgbtGreen + (double) image[i][j].rgbtRed) / 3);
            // Set the values of R, G, and B to the average, making them the same, to produce the correct shade of gray
            image[i][j].rgbtBlue = average;
            image[i][j].rgbtGreen = average;
            image[i][j].rgbtRed = average;
        }
    }
    return;
}

// Reflect image horizontally
void reflect(int height, int width, RGBTRIPLE image[height][width])
{
    // Iterate through the height or also known as each row
    for (int i = 0; i < height; i++)
    {
        // Iterate through the width or also known as each pixel/column
        for (int j = 0; j < width; j++)
        {
            // Perform swap up until the middle
            if (j < width / 2)
            {
                swap(&image[i][j], &image[i][width - (j + 1)]);
            }
        }
    }
    return;
}

// Function to swap two elements
void swap(RGBTRIPLE *a, RGBTRIPLE *b)
{
    RGBTRIPLE temp;
    temp = *a;
    *a = *b;
    *b = temp;
}

// Blur image
void blur(int height, int width, RGBTRIPLE image[height][width])
{
    // Initialize copy of image
    RGBTRIPLE temp[height][width];

    // Make a copy of image to preserve original values
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            temp[i][j] = image[i][j];
        }
    }

    // Iterate through the height or also known as each row
    for (int i = 0; i < height; i++)
    {
        // Iterate through the width or also known as each pixel/column
        for (int j = 0; j < width; j++)
        {
            // Variable that counts how many numbers added to arrive at the sum
            int count = 0;

            // Sum variables for each colour
            double sum_blue = 0;
            double sum_green = 0;
            double sum_red = 0;

            // Loop to check the surrounding pixels within 1 column and 1 row
            for (int k = i - 1; k <= i + 1; k++)
            {
                for (int l = j - 1; l <= j + 1; l++)
                {
                    // Only adds pixels that are within the image boundaries
                    if (k >= 0 && l >= 0 && k < height && l < width)
                    {
                        sum_blue += temp[k][l].rgbtBlue;
                        sum_green += temp[k][l].rgbtGreen;
                        sum_red += temp[k][l].rgbtRed;
                        count++;
                    }
                }
            }
            // Use the averages from the surrounding pixels and set the new colour values for the iterated pixel
            image[i][j].rgbtBlue = round(sum_blue / count);
            image[i][j].rgbtGreen = round(sum_green / count);
            image[i][j].rgbtRed = round(sum_red / count);
        }
    }
    return;
}

// Detect edges
void edges(int height, int width, RGBTRIPLE image[height][width])
{
    RGBTRIPLE temp[height][width];

    // Make copy of image
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            temp[i][j] = image[i][j];
        }
    }

    // Sobel Operator matrices for Gx and Gy
    int kernel_Gx[3][3] = {{-1, 0, 1}, {-2, 0, 2}, {-1, 0, 1}};
    int kernel_Gy[3][3] = {{-1, -2, -1}, {0, 0, 0}, {1, 2, 1}};

    // Iterate through the height or also known as each row
    for (int i = 0; i < height; i++)
    {
        // Iterate through the width or also known as each pixel/column
        for (int j = 0; j < width; j++)
        {
            // Initialize values for weighted sums in the x direction
            double gx_blue = 0;
            double gx_green = 0;
            double gx_red = 0;
            // Initialize values for weighted sums in the y direction
            double gy_blue = 0;
            double gy_green = 0;
            double gy_red = 0;
            // Counter to detect what row of the 3x3 the loop is iterating
            int row = 0;

            // Loop to check the surrounding pixels within 1 row
            for (int k = i - 1; k <= i + 1; k++)
            {
                // Counter to detect what column of the 3x3 grid the loop is iterating
                int column = 0;
                // Loop to check the surrounding pixels within 1 column
                for (int l = j - 1; l <= j + 1; l++)
                {
                    // Only adds pixels that are within the image boundaries
                    if (k >= 0 && l >= 0 && k < height && l < width)
                    {
                        // Calculate Gx
                        gx_blue += (kernel_Gx[row][column] * temp[k][l].rgbtBlue);
                        gx_green += (kernel_Gx[row][column] * temp[k][l].rgbtGreen);
                        gx_red += (kernel_Gx[row][column] * temp[k][l].rgbtRed);
                        // Calculate Gy
                        gy_blue += (kernel_Gy[row][column] * temp[k][l].rgbtBlue);
                        gy_green += (kernel_Gy[row][column] * temp[k][l].rgbtGreen);
                        gy_red += (kernel_Gy[row][column] * temp[k][l].rgbtRed);
                    }
                    column++;
                }
                row++;
            }
            // Combine Gx and Gy
            int sobel_blue = round(sqrt(pow(gx_blue, 2) + pow(gy_blue, 2)));
            int sobel_green = round(sqrt(pow(gx_green, 2) + pow(gy_green, 2)));
            int sobel_red = round(sqrt(pow(gx_red, 2) + pow(gy_red, 2)));

            // Set the new colour values for the iterated pixel and cap at 255 if necessary
            image[i][j].rgbtBlue = (sobel_blue > 255) ? 255 : sobel_blue;
            image[i][j].rgbtGreen = (sobel_green > 255) ? 255 : sobel_green;
            image[i][j].rgbtRed = (sobel_red > 255) ? 255 : sobel_red;
        }
    }
    return;
}