Blur function for cs50 PSET4, where are the errors? - c

Question

I have been stuck with this function for days now. I looked at other people questions, I watched youtube tutorials videos, but I can't get it right.
The task is part of Harvard's CS50 course (https://cs50.harvard.edu/x/2020/psets/4/filter/less/).
Any kind of help would be much appreciated! I really don't want to go on with the course without understanding what the problem is.
//check if pixels are valid
bool valid_pixel(int r, int c, int height, int width)
{
return r >= 0 && c >= 0 && r < height && c < width;
}
// Blur image
void blur(int height, int width, RGBTRIPLE image[height][width])
{
//create a copy of the original image
RGBTRIPLE temp[height][width];
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
temp[i][j] = image[i][j];
}
}
int red, green, blue, count;
red = green = blue = count = 0;
//iterate through rows
for (int i = 0; i < height; i++)
{
//iterate through columns
for (int j = 0; j < width; j++)
{
//move one pixel up to one pixel down in the rows
for (int r = i - 1; r <= i + 1; r++)
{
//move one pixel left to one pixel right in the columns
for (int c = j - 1; c <= j + 1; c++)
{
//check if they are unvalid pixels
if (valid_pixel(r, c, height, width))
{
//count every valid pixel
count ++;
//"store" every pixel color
red += image[r][c].rgbtRed;
green += image[r][c].rgbtGreen;
blue += image[r][c].rgbtBlue;
}
}
}
//calculate average values
temp[i][j].rgbtRed = round((float)red / count);
temp[i][j].rgbtGreen = round((float)green / count);
temp[i][j].rgbtBlue = round((float)blue / count);
}
}
for (int i = 0; i < height; i++)
{
for (int j = 0; j < width; j++)
{
image[i][j] = temp[i][j];
}
}
return;

Answer 1

The primary problem is that you forget to reset red, blue and green variables (that you sum up) after each outer loop iteration. You should put this line inside the main loop-
red = green = blue = count = 0;
Also, you're copying the image into another temporary image and copying that temporary image into the original image again in the end. This is very in-efficient. You should not copy the pixels from the original image into the blurred image at first. You can put the modified values directly into this temporary image. And in the end, use memmove to efficiently move entire rows to the original image at once. (Remember to #include <string.h>)
void blur(int height, int width, RGBTRIPLE image[height][width])
{
RGBTRIPLE blurred_img[height][width];
//iterate through rows
for (int i = 0; i < height; i++)
{
//iterate through columns
for (int j = 0, red, green, blue, count; j < width; j++)
{
// Reset the variables
red = blue = green = count = 0;
//move one pixel up to one pixel down in the rows
for (int r = i - 1; r <= i + 1; r++)
{
//move one pixel left to one pixel right in the columns
for (int c = j - 1; c <= j + 1; c++)
{
//check if they are unvalid pixels
if (valid_pixel(r, c, height, width))
{
//count every valid pixel
count++;
//"store" every pixel color
red += image[r][c].rgbtRed;
green += image[r][c].rgbtGreen;
blue += image[r][c].rgbtBlue;
}
}
}
//calculate average values
blurred_img[i][j].rgbtRed = round((float)red / count);
blurred_img[i][j].rgbtGreen = round((float)green / count);
blurred_img[i][j].rgbtBlue = round((float)blue / count);
}
}
for (int i = 0; i < height; i++)
{
// Copy the new image over to the original, row by row
memmove(image[i], blurred_img[i], sizeof(RGBTRIPLE) * width);
}
return;
}
This is assuming valid_pixel is correct. To determine whether the pixel position is valid you can just do-
if (k > 0 && k < height && l > -1 && l < width))
Notice however, when r (the row) is invalid, the innermost loop (column loop) is still iterated until c == width even though the entirety of this loop is useless, because r is invalid and it'll stay that way until the innermost loop completes and r increments.
For efficiency, you should break whenever r is invalid-
if (r < 0 || r > height - 1)
{
break;
}
if (c > -1 && c < width)
{
//count every valid pixel
count++;
//"store" every pixel color
red += image[r][c].rgbtRed;
green += image[r][c].rgbtGreen;
blue += image[r][c].rgbtBlue;
}