C :: Find Darkest Point In PPM File Using RGB

I have write a program to read ppm file and write a new ppm file. However, I'm not sure how to find the darkest point in this ppm file.

Note:
1. there are 610200 RGB point, but my new file only created 176196 RGB point.
2. RGB range: 0-255. (0,0,0)= black; (255,255,255)= white
3. When reporting the location of your dark pixels, assume the upper left corner of the image is location 1, 1

Below is my code:

#include<stdio.h>#include<stdlib.h>
typedef struct {
unsigned char red,green,blue;
} pixel_t;

typedef struct {
int x, y;
pixel_t *data;
} PPMImage;

[Code] .....

Here is ppm file: [URL] .....