filter.c

// when use round and do dividing, attention: int/int return int, 
//as for int/int, 
// float x = 55; // stored as 54.999999...
// x = x + 0.5 (- 0.5) (x<0); // x is now 55.499999...
// int y = (int)x; // truncated to 55
//float a;
//int b = (int)(a + 0.5);
//+0.5是四舍五入的关键
#include "helpers.h"
#include <math.h>

// Convert image to grayscale
void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            // dividing an integer by an integer. This is integer division, and will produce an integer again, ignoring the fractional part.
            int blue = image[i][j].rgbtBlue;
            int green = image[i][j].rgbtGreen;
            int red = image[i][j].rgbtRed;
            int ave = round((float)(blue + green + red) / (float)(3));
            
            image[i][j].rgbtBlue = ave;
            image[i][j].rgbtGreen = ave;
            image[i][j].rgbtRed = ave;
        }
    }
    return;
}

// Convert image to sepia
int limit(int n)
{
    if (n > 255)
    {
        return n = 255;
    }
    else
    {
        return n;
    }
}

void sepia(int height, int width, RGBTRIPLE image[height][width])
{
    int sepiaRed;
    int sepiaGreen;
    int sepiaBlue;
    
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            sepiaRed = round(.393 * image[i][j].rgbtRed + .769 * image[i][j].rgbtGreen + .189 * image[i][j].rgbtBlue);
            sepiaGreen = round(.349 * image[i][j].rgbtRed + .686 * image[i][j].rgbtGreen + .168 * image[i][j].rgbtBlue);
            sepiaBlue = round(.272 * image[i][j].rgbtRed + .534 * image[i][j].rgbtGreen + .131 * image[i][j].rgbtBlue);
            image[i][j].rgbtBlue = limit(sepiaBlue);
            image[i][j].rgbtGreen = limit(sepiaGreen);
            image[i][j].rgbtRed = limit(sepiaRed);
        }
    }
    return;
}

// Reflect image horizontally
void reflect(int height, int width, RGBTRIPLE image[height][width])
{
    for (int i = 0; i < height; i++)
    {
        int tmp[3];
        // don't need to use pointer here, because you do not copy a value or someting, you can assign it directly.
        for (int j = 0; j < width / 2; j++)
        {
            tmp[0] = image[i][j].rgbtRed;
            tmp[1] = image[i][j].rgbtGreen;
            tmp[2] = image[i][j].rgbtBlue;
            image[i][j].rgbtRed = image[i][width - 1 - j].rgbtRed;
            image[i][j].rgbtGreen = image[i][width- 1 - j].rgbtGreen;
            image[i][j].rgbtBlue = image[i][width - 1 - j].rgbtBlue;
            image[i][width - 1 - j].rgbtRed = tmp[0];
            image[i][width - 1 - j].rgbtGreen = tmp[1];
            image[i][width - 1 - j].rgbtBlue = tmp[2];
        }
    }
    return;
}

// Blur image
void blur(int height, int width, RGBTRIPLE image[height][width])
{
    RGBTRIPLE tmp[height][width];
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            int red = 0, green = 0, blue = 0, counter = 0;
            for (int k = -1; k < 2; k++)
            {
                if (k + j < 0 || k + j > width - 1)
                {
                    continue;
                }
                for (int h = -1; h < 2; h++)
                {
                    if (h + i < 0 || h + i > height - 1)
                    {
                        // the way of using continue statement: https://beginnersbook.com/2014/01/c-continue-statement/
                        continue;
                    }
                    red += image[i + h][j + k].rgbtRed;
                    green += image[i + h][j + k].rgbtGreen;
                    blue += image[i + h][j + k].rgbtBlue;
                    counter++;
                }
            }
            tmp[i][j].rgbtBlue = round((float)blue / (float)counter);
            tmp[i][j].rgbtGreen = round((float)green / (float)counter);
            tmp[i][j].rgbtRed = round((float)red / (float)counter);
        }
    }
    
    for (int i = 0; i < height; i++)
    {
        for (int j = 0; j < width; j++)
        {
            image[i][j].rgbtBlue = tmp[i][j].rgbtBlue;
            image[i][j].rgbtGreen = tmp[i][j].rgbtGreen;
            image[i][j].rgbtRed = tmp[i][j].rgbtRed;
        }
    }
    return;
}