need help/guidance in finishing diamond pattern in array

#define ROWS 9
#define COLS 9
int img[ROWS][COLS];
      
void setup() {
Serial.begin(9600);
for (int r=0; r<ROWS; r++) {
  for (int c=0; c<COLS; c++) {
    img[r][c] = 0;
  }
}

void loop() {
}

void diamond() {
  for (int c=0; c<COLS; c++) {
    img[c][c] = 1;
  }
}

void print_img() {
  for (int r=0; r<ROWS; r++) {
    for (int c=0; c<COLS; c++) {
      Serial.print(img[r][c]);
     }
     Serial.println();
    }
}

The pattern you have, will look like a single diagonal line. It won't look like a diamond.

100000000 010000000 001000000 000100000 000010000 000001000 000000100 000000010 000000001