Is it necessary to do all arithmetic operations for filtering images in one line for C? for CS50 fil-CodePudding

I've been doing the "filter-less" problem for CS50 and noticed that, this code:

void grayscale(int height, int width, RGBTRIPLE image[height][width])
{
    for (int i = 0; i < height; i  )
    {
        for (int j = 0; j < width; j  )
        {
            image[i][j].rgbtBlue = (image[i][j].rgbtBlue   image[i][j].rgbtRed   image[i][j].rgbtGreen) / 3;

            image[i][j].rgbtRed = image[i][j].rgbtBlue;
            image[i][j].rgbtGreen = image[i][j].rgbtBlue;
        }
    }
    return;
}

and this one,

// 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  )
        {
            image[i][j].rgbtBlue = (image[i][j].rgbtBlue   image[i][j].rgbtRed   image[i][j].rgbtGreen);

            // Notice this line
            image[i][j].rgbtBlue /= 3;

            image[i][j].rgbtRed = image[i][j].rgbtBlue;
            image[i][j].rgbtGreen = image[i][j].rgbtBlue;
        }
    }
    return;
}

Have entirely different outputs, and the former gives better results. Can someone explain the reason behind that? Why is the difference when the arithmetic operation is done in separate lines? Do correct me if I am doing something wrong here.

The program is supposed to take an input image in BMP format, apply filters (In this case Greyscale) and create an output, the function takes the image's height height and width width and the RGB values of the said image in the array image.

This is from CS50's Introduction to computer science course Week 4 by the way.

[EDIT]

typedef uint8_t  BYTE;

typedef struct
{
    BYTE  rgbtBlue;
    BYTE  rgbtGreen;
    BYTE  rgbtRed;
} __attribute__((__packed__))
RGBTRIPLE;

More information regarding the question is here:

https://cs50.harvard.edu/x/2022/psets/4/filter/less/

CodePudding user response：

image[i][j].rgbtBlue image[i][j].rgbtRed image[i][j].rgbtGreen is larger than uint8_t can cope with. And

image[i][j].rgbtBlue = (image[i][j].rgbtBlue   image[i][j].rgbtRed   image[i][j].rgbtGreen) / 3;

assigns, for example, 417/3 (or 139, operations here are done with int) to the member

image[i][j].rgbtBlue = (image[i][j].rgbtBlue   image[i][j].rgbtRed   image[i][j].rgbtGreen);
image[i][j].rgbtBlue /= 3;

first assigns 417 % 256 (or 161) to the member, then divides that by 3