Reading PPM format images

Question

P6

650 652

255

P6 indicates that it is a PPM image. The next two fields are the width and height of the image. The last field gives the maximum pixel value. At the end of the header is a \n and then the binary pixel data. The image is in color so there are three bytes (red, green, blue). The goal of my readPPM function is to return the pixel data in a one-dimensional array of unsigned chars, plus the width and height of the image. The goal of my writePPM function (I haven't done anything for that function yet) is to write the PPM format image to an empty file from the given information returned from my readPPM function. I'm still not sure how to make this program work so I don't store the 650 in width and 652 in height. I'll worry about that once I can actually read and write the text files.

Update: I've successfully stored the header (P6, 650, 652, and 255)) in their right variables (I know because I tried printing their values and they came out right). My output is shown below. I was wondering what those nonsense characters below the maximum were. Are those the binary pixel data? If so, then I just need to write my writePPM function.

Output: PSix: P6

Width: 650

Height: 652

maximum: 255

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main.cc

int main() {

char fileName[50] = "binary_pixels.txt";
int width = 0;  // width of the image
int height = 0; // heigt of the image
int size = 128; // size of the array

// read the PPM file and store its contents inside an array and return 
// the pointer to that array to pixelArray
unsigned char* pixelArray[size] = readPPM(fileName, &width, &height);

// print the contents of the array
for (int i = 0; i < 128; i++) {
    printf("%s\n", pixelArray[i]);
} // end of for loop

} // end of main

readPPM.cc

unsigned char readPPM(const char* fileName, char* pSix, int* width,         
int* height, int* maximum) { 

// open the file to read just the header reading
FILE* fr = fopen(fileName, "r");

// formatted read of header
fscanf(fr, "%s", pSix);

// check to see if it's a PPM image file
if (strncmp(pSix, "p6" , 10) != 0) {
    printf("They are not the same\n");
} else {
    printf("They are the same\n");
}

// read the rest of header
fscanf(fr, "%d\n %d\n", width, height);

fscanf(fr, "%d\n", maximum);

// check to see if they were stored properly
printf("PSix: %s\n", pSix);
printf("Width: %d\n", *width);
printf("Height: %d\n", *height);
printf("maximum: %d\n", *maximum);

// allocate array for pixels
unsigned char* pixels = new unsigned char[width * height];

// unformatted read of binary pixel data
while (fread(pixels, sizeof(pixel), 128, fr)) {
    printf("%s", pixels);
} // end of for loop

// close file
fclose(fr);

// return the array
return pixels;

} // end of readPPM

If your program shall be clean c code you may not use new. In C it is done with malloc() (include stdlib.h). — Scheff's Cat
The type of pixelArray in main is wrong. It must be unsigned char* pixelArray (without [size]). — Scheff's Cat
I've successfully stored the header (P6, 650, 652, and 255)) in their right variables (I know because I tried printing their values and they came out right). My output is shown below. I was wondering what those nonsense characters below the maximum were. Are those the binary pixel data? If so, then I just need to write my writePPM function. — asilvester635
It should be new unsigned char[width * height * 3]. Also, it should be fread(pixels, 1, width * height * 3, fr) — sameerkn

sameerkn sameerkn · Accepted Answer · 2017-02-17T07:15:09

//struct to hold a pixel.
struct rgb
{
    char r;
    char g;
    char b;
};

//struct to hold the image and its info.
struct image
{
    char p;
    int format;
    int width;
    int height;
    int intensity;
    //struct rgb **rgb;
    unsigned char *pixels;
};

int main(void) 
{
    struct image m;
    FILE *fp = //fopen(...);

    fscanf(fp, "%c%d\n", &m.p, &m.format);

    fscanf(fp, "%d %d\n", &m.width, &m.height);

    fscanf(fp, "%d\n", &m.intensity);

    printf("%c%d\n", m.p, m.format);
    printf("%d %d\n", m.width, m.height);
    printf("%d\n", m.intensity);

    //allocate array to hold the pixels
    m.pixel = (unsigned char*)malloc(m.width*m.heigth*3*sizeof(unsigned char));

    //read pixels into m.pixel
    return 0;
}

Reading PPM format images

2 Answers