Re: draw on an image

[Havoc Pennington <hp redhat com>]

On Wed, May 21, 2003 at 02:09:56AM +0200, Joel Kraehemann wrote:
> I want to draw directly on an image. I tried something like above but I didn't succeeded. 
> 
> 
> <---->
> 
>   image = gtk_image_new_from_file ((char *) str);
> 
> 
>   gdk_draw_line (GTK_IMAGE (image)->data.pixmap.pixmap,
>                  GDK_GC_BACKGROUND,
>                  8, 1,
>                  8, 3);
> 

Drawing on a window or widget should always be done in an expose_event
handler, because the X server discards the window contents anytime the
window is covered up by another window - then sends an expose event
asking them to be redrawn if the window becomes visible again.

You can draw on a pixmap without using expose_event since pixmap
contents are persistent. However, this GtkImage doesn't contain a
pixmap, you loaded it from a file (it thus contains a GdkPixbuf).

Your options are:

 - in a GtkImage expose_event handler that runs after the default 
   handler, draw to GTK_WIDGET (image)->window
 - load the pixbuf (gdk_pixbuf_new_from_file()), 
   draw it to a pixmap (gdk_draw_pixbuf() or
   gdk_pixbuf_render_pixmap_and_mask() (?)), set the 
   pixmap in the GtkImage
 - use a drawing area instead of GtkImage

Havoc