Why can't I create copies of these 2D arrays in C? - c

Question

Hello I am doing an assignment for Harvard CS50 please read the directions if you want to answer the question.
Here is my code for helpers.c
#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++)
{
RGBTRIPLE pixel = image[i][j];
int newgray = round((pixel.rgbtBlue + pixel.rgbtGreen + pixel.rgbtRed)/ 3.00);
image[i][j].rgbtBlue = newgray;
image[i][j].rgbtGreen = newgray;
image[i][j].rgbtRed = newgray;
}
}
return;
}
// Convert image to sepia
void sepia(int height, int width, RGBTRIPLE image[height][width])
{
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
RGBTRIPLE pixel = image[i][j];
//Reassign pixel colors based on formula
image[i][j].rgbtRed = .393 * pixel.rgbtRed + .769 * pixel.rgbtGreen + .189 * pixel.rgbtBlue;
image[i][j].rgbtGreen = .349 * pixel.rgbtRed + .686 * pixel.rgbtGreen + .168 * pixel.rgbtBlue;
image[i][j].rgbtBlue = .272 * pixel.rgbtRed + .534 * pixel.rgbtGreen + .131 * pixel.rgbtBlue;
}
}
return;
}
// Reflect image horizontally
void reflect(int height, int width, RGBTRIPLE image[height][width])
{
RGBTRIPLE temp[height][width];
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width/2; j++)
{
temp[i][j] = image[i][j];
int reflected_j = width - j;
image[i][reflected_j] = image[i][j];
image[i][j] = temp[i][j];
}
}
return;
}
// Blur image
void blur(int height, int width, RGBTRIPLE image[height][width])
{
return;
}
My first question is why is it that when I replace image[i][j].rgbtBlue = newgray; in the grayscale function with pixel.rgbtBlue = newgray; it doesn't work. Isn't the pixel variable literally a copy of image[i][j].
My second question lies in the reflection function where I make RGBTRIPLE temp[height][width]; (RGBTRIPLE is a data structure using 1 byte for each RGB color in the images picture) and assign it to copy the pixels of the original image. I made it so that it would copy the first half of the picture and reflect it to the other side from the original image then from the copy (temp) I would copy the second half and paste it in the first half of the original image. Why does the first half (left half) come up as black?
Input: image, terminal command: ./filter -r tower.bmp outfile.bmp
(tower.bmp is the input image and outfile is the output image)
Output: image

Answer 1

There are a few issues with reflect().
Only one element of temp is used at a time and never reused. Thus there is no need to create a potentially huge instance on stack.
Index reflected_j = width - j for j equal to 0 will be equal width what is out of bounds for array of length width. This invokes UB. It should be reflected_j = width - j - 1
It would be better to swap pixels while reflecting the image, not to play with half-copy.
Updated code:
void reflect(int height, int width, RGBTRIPLE image[height][width]) {
for (int y = 0; y < height; y++) {
for (int i = 0, j = width - 1; i < j; i++, j--) {
RGBTRIPLE temp = image[y][i];
image[y][i] = image[y][j];
image[y][j] = temp;
}
}
}

Answer 2

As for your first question, it is because pixel is only a value copy of image[i][j]. It has a separate memory location of it's own, so modifying it wont change the image[i][j].
As for your second question it is because this line
image[i][reflected_j] = image[i][j];
should be
image[i][j]=image[i][reflected_j]
and this line
image[i][j] = temp[i][j];
should be
image[i][reflected_j]=temp[i][j]