cairo program alpha

//gcc example.c -o example `pkg-config --cflags --libs gtk+-3.0`
//export PKG_CONFIG_PATH="$PKG_CONFIG_PATH:/usr/local/lib/pkgconfig"

#include <cairo.h>
#include <gtk/gtk.h>
#include <time.h>
const int MAX_WIDTH = 512;
const int MAX_HEIGHT = 512;
const int MAX_SIZE = 512;
const int GRID = 16;
const double ADJ_COEF = MAX_SIZE/GRID;

struct vl_pixel{
    int hor;
    int ver;
    int r;
    int g;
    int b;
};

static void do_drawing(cairo_t *);

static gboolean on_draw_event(GtkWidget *widget, cairo_t *cr,
    gpointer user_data)
{
  do_drawing(cr);

  return FALSE;
}

void vl_pixel(cairo_t *cr,struct vl_pixel pixel){
    cairo_set_source_rgb(cr,pixel.r/255.0,pixel.g/255.0,pixel.b/255.0);
    cairo_rectangle(cr,ADJ_COEF*pixel.hor,ADJ_COEF*pixel.ver,ADJ_COEF,ADJ_COEF);
    cairo_fill(cr);

}
// struct vl_pixel *vl_pixel_read(FILE *file){
//  char ch = getchar(FILE);
//  while(ch != EOF){

//  }
// }
void vl_pixel_save(FILE *file, struct vl_pixel pixel){
    fprintf(file, "%d %d %d %d %d\n", pixel.ver,pixel.hor,pixel.r,pixel.g,pixel.b);
}
static void do_drawing(cairo_t *cr)
{
    cairo_set_line_width (cr, 0.1);
    cairo_set_source_rgb (cr, 1, 0 , 0);
    int hor=0,ver=0;
    FILE *file;
    file = fopen("pattern1.txt","w+");
    for (hor=0;hor*ADJ_COEF<MAX_SIZE;hor++){
        for (ver=0;ver*ADJ_COEF<MAX_SIZE;ver++){
            struct vl_pixel pixel = {0};
            pixel.hor=hor;
            pixel.ver = ver;
            pixel.r = rand()%255;
            pixel.g = 0;
            pixel.b = 0;
            vl_pixel_save(file,pixel);
            vl_pixel(cr,pixel);
            // vl_pixel(cr,hor,ver,r,g,b);
        }
    }

}


int main(int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *darea;
  srand(time(NULL));
  gtk_init(&argc, &argv);

  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  darea = gtk_drawing_area_new();
  gtk_container_add(GTK_CONTAINER(window), darea);

  g_signal_connect(G_OBJECT(darea), "draw",
      G_CALLBACK(on_draw_event), NULL);
  g_signal_connect(window, "destroy",
      G_CALLBACK(gtk_main_quit), NULL);

  gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
  gtk_window_set_default_size(GTK_WINDOW(window), MAX_SIZE, MAX_SIZE);
  gtk_window_set_title(GTK_WINDOW(window), "GTK window");

  gtk_widget_show_all(window);

  gtk_main();

  return 0;
}