Thread: Help With Reading The Image Size Of A PPM File?

I've used this code before to test it and it did work. Unfortunately it now reads two random numbers for a pgm file and the code ends before it even gets to read the image dimensions when I try to read a ppm file in. I know I'm missing something but I'm not quite sure where yet. Any help is much appreciated. Thanks. (the code is also unfinished)

Code:

#include<stdio.h>
#include <stdlib.h>
#include <string.h>

#define RGB_COMPONENT_COLOUR 255
#define height 1080
#define width 1920

typedef struct
{
    unsigned char red, green, blue;
} PPMPixel;

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

void clear_to_end(FILE *fp)
{
    int c;
    while ((c = fgetc(fp)) != -1 && c != '\n');
}

PPMImage* load_file()
{
    FILE *fp;
    PPMImage *img;
    int d;
    char buff[16];
    char fname[100];
    printf("Enter file name: ");
    scanf("%s", fname);
    fseek(stdin,0,SEEK_END);
    fp = fopen(fname, "rb");
    if (fp == NULL || !fp || ferror(fp))
    {   
        printf("\tError while opening the file\n");
    }                        
    else    
    {
        printf("\tReading in %s\n", fname);
    }

    //Checking for comments
    //Skip whitespace and comments
    d = getc(fp);
    while (d=='#')
    {
        while(getc(fp)!='\n')
        d=getc(fp);
    }
    ungetc(d,fp);

    //Checking header
    if (!fgets(buff,sizeof(buff),fp))
    {
        perror(fname);
        exit(1);
    }

    if (buff[0] =='P' || buff[1] == 2 || buff[1] == 3)
    {
        printf("\tReading PGM/PPM image in...\n");
    }
    else
    {
        printf("\tIncorrect image format\n");
    }

    //Read image size information
    if (fscanf(fp, "%u%u", &img->y, &img->x))
    {
        printf("\tInvalid image size\n");
        printf("\tImage size is: %u x %u\n", img->y, img->x);
    }
    else
    {
        printf("\tImage size is valid\n");
        printf("\tImage size is: %u x %u\n", img->y, img->x);
    }


}
void save_file(PPMImage *img)
{
    
    
}


int main()
{
    char key = 0;
    PPMImage* img = NULL;
    do {
        puts("\tPress r to read in an image in ppm format");
        puts("\tPress s to save image in ppm format");
        puts("\tPress q to quit");
        
        scanf(" %c", &key);
        clear_to_end(stdin);
        switch (key) {
            case 'r':
                // TODO: if img != NULL, free it
                img = load_file();
                break;
            case 's':
                save_file(img);
                break;
            case 'q':
                puts("\tTerminating program...");
                break;
            default:
                puts("\tInvalid Input");
                break;
        }
    } while (key != 'q');

return0;

Originally Posted by Salem

>
> if (!fgets(buff,sizeof(buff),fp))
How long is the first line?

Netpbm - Wikipedia
You're assuming a very specific layout of spaces and newlines, which isn't at all guaranteed.

> if (buff[0] =='P' || buff[1] == 2 || buff[1] == 3)
The file contains characters, not numbers.
So you should be comparing '2' or '3'.

Also, it should be say
if (buff[0] =='P' && ( buff[1] == '2' || buff[1] == '3') )
First char is 'P'
Second char is either '2' or '3'.

Thanks for the tips! I don't know how long the first line is because any pgm or ppm file could be read in but I'm not sure how to exactly work around that yet. Also I tried using if (buff[0] =='P' && ( buff[1] == '2' || buff[1] == '3') ) before but all it seems to do is print my error message "incorrect image format".

Originally Posted by Salem

Maybe you could attach your PBM file (or post it at say Pastebin.com - #1 paste tool since 2002! ) so we can see how your code differs from the file you're trying to parse.

Well this is one of the ppm files I was having trouble with: PPM File - Pastebin.com

Thanks again.

Originally Posted by john.c

If you actually made the changes you said you made then you wouldn't get the error you say you're getting.
At any rate, you should open the file in text mode, not binary mode.
And you need to malloc some memory for the image.
Also, you're not handling comments properly at all. Comments can go anywhere but at the very beginning (which seems to be where you think they can go) or in the image data. So this is valid:

Code:

P2   # Magic number
5    # Width
2    # Height
255  # Max color value
# Image data
10 20 30 40 50
5 15 25 35 45

I forgot to add the ' ' around 2 and 3 that's why it wasn't working in the end.

Originally Posted by Salem

A few tweaks.

Code:

#include<stdio.h>
#include <stdlib.h>
#include <string.h>

#define RGB_COMPONENT_COLOUR 255
#define height 1080
#define width 1920

typedef struct
{
    unsigned char red, green, blue;
} PPMPixel;

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

void clear_to_end(FILE *fp)
{
    int c;
    while ((c = fgetc(fp)) != -1 && c != '\n');
}

PPMImage* load_file(const char *fname)
{
    FILE *fp;
    PPMImage *img = malloc(sizeof(*img));    //!! allocate space!
    int d;
    // https://en.wikipedia.org/wiki/Netpbm#PPM_example
    // according to this, max length of any line is 70 chars
    char buff[80];
    fp = fopen(fname, "r");  //!! not rb for text files!!
    if (fp == NULL)
    {   
        printf("\tError while opening the file\n");
        return NULL;
    }                        
    else    
    {
        printf("\tReading in %s\n", fname);
    }

    //Checking for comments
    //Skip whitespace and comments
    d = getc(fp);
    while (d=='#')
    {
        while(getc(fp)!='\n')
        d=getc(fp);
    }
    ungetc(d,fp);

    //Checking header
    if (!fgets(buff,sizeof(buff),fp))
    {
        perror(fname);
        exit(1);
    }

    if (buff[0] =='P' && (buff[1] == '2' || buff[1] == '3'))
    {
        printf("\tReading PGM/PPM image in...\n");
    }
    else
    {
        printf("\tIncorrect image format\n");
        return NULL;
    }

    //Read image size information
    if (fscanf(fp, "%u%u", &img->y, &img->x))
    {
        printf("\tInvalid image size\n");
        printf("\tImage size is: %u x %u\n", img->y, img->x);
    }
    else
    {
        printf("\tImage size is valid\n");
        printf("\tImage size is: %u x %u\n", img->y, img->x);
    }

    return img;
}

int main()
{
    PPMImage* img = load_file("foo.pbm");
    free(img);
}

Results

Code:

$ gcc foo.c
$ head foo.pbm 
P3
107 48
255
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 
255 255 255 255 255 255 255 255 255 255 255 255 255 255 255 
$ ./a.out 
    Reading in foo.pbm
    Reading PGM/PPM image in...
    Invalid image size
    Image size is: 107 x 48

Thanks a lot that's really helped to smooth out some problems in my code. Yet I'm still getting ridiculously high numbers for pgm image dimensions and my switch case doesn't loop after r (though the latter is kind of an unrelated issue).