米ハーバード大学 CS50：課題「インスタグラムのような写真加工フィルタを作る」の回答例（Harvard CS50 Week 4: Problem Set 4 − Filter） 

#include "helpers.h"
#include "math.h"
// Convert image to grayscale
void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
    // Average RGB and round to nearest integer
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            // get into the 2D array, obtain value of each colour
            int red = image[i][j].rgbtRed;
            int blue = image[i][j].rgbtBlue;
            int green = image[i][j].rgbtGreen;
            // calculate the average value of each pixel, rounded
            int avg = round(((float)red + (float)blue + (float)green)/3);
            // set the calculated average to be the new value of each pixel
            image[i][j].rgbtRed = image[i][j].rgbtBlue = image[i][j].rgbtGreen = avg;
        }
    }
    return;
}
// Convert image to sepia
void sepia(int height, int width, RGBTRIPLE image[height][width])
{
    // For loop for row and column
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            // get into the 2D array, obtain value of each colour
            int red = image[i][j].rgbtRed;
            int blue = image[i][j].rgbtBlue;
            int green = image[i][j].rgbtGreen;
            // apply sepia algorithm, rounded
            // Sepia Red
            int sepiaRed = round(0.393 * red + 0.769 * green + 0.189 * blue);
            if (sepiaRed > 255)
            {
                image[i][j].rgbtRed = 255;
            }
            else
            {
                image[i][j].rgbtRed = sepiaRed;
            }
            // Sepia Blue
            int sepiaGreen = round(0.349 * red + 0.686 * green + 0.168 * blue);
            if (sepiaGreen > 255)
            {
                image[i][j].rgbtGreen = 255;
            }
            else
            {
                image[i][j].rgbtGreen = sepiaGreen;
            }
            // Sepia Green
            int sepiaBlue = round(0.272 * red + 0.534 * green + 0.131 * blue);
            if (sepiaBlue > 255)
            {
                image[i][j].rgbtBlue = 255;
            }
            else
            {
                image[i][j].rgbtBlue = sepiaBlue;
            }
        }
    }
    return;
}
// Reflect image horizontally
void reflect(int height, int width, RGBTRIPLE image[height][width])
{
    // For loop for row and column (until mid point)
    for (int i = 0; i < height; i++)
    {
        // until mid-point because pixels are swapped already
        for (int j = 0; j < width/2; j++)
        {
            RGBTRIPLE temp = image[i][j];
            //just like how [0][9]...[4][5], [i] will swap with [total-1-i]
            image[i][j] = image[i][width - (j + 1)];
            image[i][width - (j + 1)] = temp;
        }
    }
    return;
}
// Blur image
void blur(int height, int width, RGBTRIPLE image[height][width])
{
    //create a temporary image to be blurred
    RGBTRIPLE temp[height][width];
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            temp[i][j] = image[i][j];
        }
    }
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            int sum_blue;
            int sum_green;
            int sum_red;
            float counter;
            sum_blue = sum_green = sum_red = counter = 0;
// If pixel is a corner pixel -
            //corner pixel on bottom right
            if (i >= 0 && j >= 0)
            {
                sum_red += temp[i][j].rgbtRed;
                sum_green += temp[i][j].rgbtGreen;
                sum_blue += temp[i][j].rgbtBlue;
                counter++;
            }
            //corner pixel on bottom left
            if (i >= 0 && j - 1 >= 0)
            {
                sum_red += temp[i][j-1].rgbtRed;
                sum_green += temp[i][j-1].rgbtGreen;
                sum_blue += temp[i][j-1].rgbtBlue;
                counter++;
            }
            //corner pixel on top left
            if (i - 1 >= 0 && j >= 0)
            {
                sum_red += temp[i-1][j].rgbtRed;
                sum_green += temp[i-1][j].rgbtGreen;
                sum_blue += temp[i-1][j].rgbtBlue;
                counter++;
            }
            //corner pixel on top right
            if (i - 1 >= 0 && j - 1 >= 0)
            {
                sum_red += temp[i-1][j-1].rgbtRed;
                sum_green += temp[i-1][j-1].rgbtGreen;
                sum_blue += temp[i-1][j-1].rgbtBlue;
                counter++;
            }
// If pixel is located on the edges -
            //pixels on bottom edge
            if ((i >= 0 && j + 1 >= 0) && (i >= 0 && j + 1 < width))
            {
                sum_red += temp[i][j+1].rgbtRed;
                sum_green += temp[i][j+1].rgbtGreen;
                sum_blue += temp[i][j+1].rgbtBlue;
                counter++;
            }
            //pixels on top edge
            if ((i - 1 >= 0 && j + 1 >= 0) && (i - 1 >= 0 && j + 1 < width))
            {
                sum_red += temp[i-1][j+1].rgbtRed;
                sum_green += temp[i-1][j+1].rgbtGreen;
                sum_blue += temp[i-1][j+1].rgbtBlue;
                counter++;
            }
            //pixels on left edge
            if ((i + 1 >= 0 && j >= 0) && (i + 1 < height && j >= 0))
            {
                sum_red += temp[i+1][j].rgbtRed;
                sum_green += temp[i+1][j].rgbtGreen;
                sum_blue += temp[i+1][j].rgbtBlue;
                counter++;
            }
            //pixels on right edge
            if ((i + 1 >= 0 && j - 1 >= 0) && (i + 1 < height && j - 1 >= 0))
            {
                sum_red += temp[i+1][j-1].rgbtRed;
                sum_green += temp[i+1][j-1].rgbtGreen;
                sum_blue += temp[i+1][j-1].rgbtBlue;
                counter++;
            }
// If pixel is in the middle -
            //middle pixels
            if ((i + 1 >= 0 && j + 1 >= 0) && (i + 1 < height && j + 1 < width))
            {
                sum_red += temp[i+1][j+1].rgbtRed;
                sum_green += temp[i+1][j+1].rgbtGreen;
                sum_blue += temp[i+1][j+1].rgbtBlue;
                counter++;
            }
            //find average colour value
            image[i][j].rgbtRed = round(sum_red / counter);
            image[i][j].rgbtGreen = round(sum_green / counter);
            image[i][j].rgbtBlue = round(sum_blue / counter);
        }
    }
    return;
}