on DrawingArea, pixmap and pixbuf

[David Eduardo Gomez Noguera <davidgn servidor unam mx>]

Hello.
browsing the scribble source code, i see that it creates the pixmap used
in the callbacks, and that its a global.

My program creates the pixmap on the main() function, but when passing
drawing_area->window, the compiled program tells of "Gdk-CRITICAL **:
file gdkpixmap.c: line 62 (gdk_pixmap_new): assertion `(window != NULL)
|| (depth != -1)' failed.", so drawing_area->window is null.
why does it happens? is it because its outside the gtk_main() thread?
it doesnt happen if i pass mainwindow->window.

also, i pass -1 as depth.

later on, a call to gdk_pixbuf_get_from_drawable and to
gdk_pixbuf_get_bits_per_sample returns an 8.
Ive done all this to create a png from the GdkPixmap.

the code to make the png is:

/*********************/
GtkWidget *area;
    GdkPixmap *pixmap;
    GdkPixbuf *pixbuf;
    GdkDrawable *drawable;
    gint bpp;
    guchar *buffer;

    FILE *imagen;
    png_structp png_ptr;
    png_infop info_ptr;
    png_uint_32 k;
      png_bytep row_pointers[ALTO];
      
    area = gtk_object_get_data(GTK_OBJECT(user_data), "dibujo");
    pixmap = gtk_object_get_data(GTK_OBJECT(area), "mapa");
    drawable = pixmap;
    pixbuf =
	gdk_pixbuf_get_from_drawable(NULL, drawable,
				     gtk_widget_get_colormap(user_data), 0,
				     0, 0, 0, 100, 100);
      imagen=fopen("imagen.png","wb");
    bpp = gdk_pixbuf_get_bits_per_sample(pixbuf);
    printf("%d\n", bpp);
    buffer = gdk_pixbuf_get_pixels(pixbuf);

    png_ptr = png_create_write_struct
	(PNG_LIBPNG_VER_STRING, NULL, NULL, NULL);
    if (!png_ptr)
	return (ERROR);

    info_ptr = png_create_info_struct(png_ptr);
    if (!info_ptr) {
	png_destroy_write_struct(&png_ptr, (png_infopp) NULL);
	return (ERROR);
    }

    if (setjmp(png_jmpbuf(png_ptr))) {
	png_destroy_write_struct(&png_ptr, &info_ptr);
	fclose(imagen);
	return (ERROR);
    }

    png_init_io(png_ptr, imagen);

    png_set_IHDR(png_ptr, info_ptr, ANCHO, ALTO, bpp,
PNG_COLOR_TYPE_RGB,
		 PNG_INTERLACE_NONE, PNG_COMPRESSION_TYPE_DEFAULT,
		 PNG_FILTER_TYPE_DEFAULT);
     png_write_info(png_ptr, info_ptr);

      for (k = 0; k < ALTO; k++)
           row_pointers[k] = buffer + k*ANCHO*3;
       png_write_image(png_ptr, row_pointers);
       png_write_end(png_ptr, info_ptr);
       png_destroy_write_struct(&png_ptr, &info_ptr);
      fclose(imagen);
/**************************/

I am using Gtk+ 1.4

What am i doing wrong? some values i am just gessing (like k*ANCHO*3).

Any suggestions, corrections?

Note: ANCHO=width ALTO=height

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