Thread: Reading Picture file into 2dimensional data array?

Greetings, I am new to the boards and am trying to read a pgm picture file into a 2 dimensional array, rows and columns, of the color of each pixel. the files look like... for a 4x20 pixel picture

"
P2
#imagelr.pgm
20 20
255
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100
"

My question is what is a good way to read past the 4th line (to the data) and save each number as an int in an array.

I've tried scanning the whole file using fread, then going through the giant string for the values, but i run into the problem of 100 being the chars '1' '0' '0', and no guarantee of how many digits will be in each pixel, makes saving them as ints hard.

it seems like fscanf or fread pixel by pixel would be smarter, but how do i do so, and only do so after the 4th line when data starts?

Thank you much!

The file pointer you get from fopen() maintains it's position regardless of what you do. So you can open the file, pass the pointer to a function "process_header" which reads the first four lines, and then pass the file pointer to a different function which fscanf's the pixel data, since the file pointer will now be at the fourth line, if you code everything properly.

Originally Posted by DiscoStu9

excellent idea, thank you.

Is there a simple way to read a line and have it stop after a return key? '10' i believe? so i can read lines until 4th return, then begin to fscanf

Because you are using fread(), you must find the newline yourself to do line oriented processing. Yep, ASCII 10 is the newline ('\n'). So these are equivalent:

buffer[x] == 10;
buffer[x] == '\n';

I have never worked with pgm files, but if those first four lines are of a consistent form you may want to use fgets() (which reads up to a newline automatically) in your "process_header" function, then you can parse each line the way you want, maybe with a switch/case. In other words, the function has a char buffer big enough to hold a single line, you fgets() into that, and then using a counter you can use switch() to decide how to deal with the information in each of the four lines.

Originally Posted by DiscoStu9

i see, well i am not married to fread, and yes they are always of the same header style....
so I will most likely do fgets 4 times, then move to freads for the data

Just a suggestion. There is nothing wrong with fread(). You probably want fscanf() for the data portion tho.

As I hope I implied, do this with loops in separate functions, eg:

Code:

void process_header (FILE *fp) {
    int i;
    char buffer[64];
    for (i=0; i<4; i++) {
          fgets(buffer,64,fp);
          switch (i) {
                case (0): extract your data from the line in buffer for the first line
                case (1): the second line
                [etc]
          }
     }
}

If you are processing multiple files you could have this function return a struct with the data, so the prototype might be:

Code:

struct pgm_info *process_header (FILE *fp);

Originally Posted by DiscoStu9

just curious, why did you chose to use the switch statements? why not a set of 4 instructions since we know that going in...with i<4

The switch statement would be 4 instructions. Eg: guessing that line 3 is the dimensions and that line 4 is the palette size (I'm probably wrong), you could do something like:

Code:

       struct pgm_info thisimage = malloc(sizeof(struct pgm_info));
       [...all that other stuff...]
               case (2): sscanf(buffer,"%d %d",thisimage.width, thisimage.height);
                              break;
               case (3): sscanf(buffer,"%d",thisimage.palette);

You could just use a sequence of if(i==2) statements to accomplish the same thing.

Originally Posted by DiscoStu9

Or should I use fgets and grab the whole line, then try to use atoi step by step to get the data?

If you use fscanf like this:

Code:

int val;
fscanf(fp, "%d%*c",&val);

you could just continually loop thru, pulling one number at a time, and not worry about the lines, etc. "%*c" means ignore one character after the digit, which could be either a space or a '\n'.

So how you do this depends much on what you are doing with the data. There may be an EOF issue here *but* I presume part of the header is the actual image size/length, which you could use that to set the loop length, so you grab exactly as much data as is in the file.