// for each frame we take the Y plane (think of a huge "2d" array containing all the pixels)
// and the linesize (wrap - size in bytes of each picture line) if we use a byte per pixel then it's equals to width
// , width, height, and the file name
save_gray_frame(pFrame->data[0], pFrame->linesize[0], pFrame->width, pFrame->height, frame_filename);

// we must write the minimal header (providing resolution and magical string)
fprintf(f, "P5\n%d %d\n%d\n", xsize, ysize, 255);

// and finally we just need to write to the file
// since we write line by line then what we do is just to use the ysize iteration 
// (one increment for i, up to the frame height((yssize))

// FOR THAT matter, we use pointer arithmetics and
    for (i = 0; i < ysize; i++)
        // fread(void *ptr, size_t size, size_t nmemb, FILE *stream);
        // a pointer (updated/calculated each line) of size 1 but
        // writes xsize items of data on file f
        fwrite(buf + i * wrap, 1, xsize, f);