The top face can now be shifted.

- solidify/overlap.c (scroll_event): use center of canvas as center, not
  center of the top face
- solidify/overlap.c (do_shift, shift, scroll_event): added translation
  (shifting)

solidify/overlap.c

@@ -180,25 +180,51 @@ static void rotate(struct matrix *m, double r)
 }
 
 
+static void do_shift(struct matrix *m, int dx, int dy)
+{
+	m->b[0] -= dx;
+	m->b[1] += dy;
+}
+
+
+static void shift(struct matrix *m, int dx, int dy, int dir)
+{
+	if (dx > 0 && dy < dx && dy > -dx)
+		do_shift(m, dir, 0);
+	if (dx < 0 && dy < -dx && dy > dx)
+		do_shift(m, -dir, 0);
+	if (dy > 0 && dx < dy && dx > -dy)
+		do_shift(m, 0, dir);
+	if (dy < 0 && dx < -dy && dx > dy)
+		do_shift(m, 0, -dir);
+}
+
+
 static gboolean scroll_event(GtkWidget *widget, GdkEventScroll *event,
     gpointer data)
 {
 	GtkWidget *da = gtk_bin_get_child(GTK_BIN(widget));
 	struct solid *s = data;
-	struct face *f = s->a;
+	int dx = event->x-sx(s)/2;
-	int dx = event->x-f->sx/2;
+	int dy = event->y-sy(s)/2;
-	int dy = event->y-f->sy/2;
 	double r = hypot(dx, dy);
+	int center = r/hypot(sx(s), sy(s)) < 0.25;
 
 	if (r < 1)
 		return TRUE;
 	switch (event->direction) {
 	case GDK_SCROLL_UP:
-		rotate(&f->m, r);
+		if (center)
+			shift(&s->a->m, dx, dy, 1);
+		else
+			rotate(&s->a->m, r);
 		draw_image(da, s);
 		break;
 	case GDK_SCROLL_DOWN:
-		rotate(&f->m, -r);
+		if (center)
+			shift(&s->a->m, dx, dy, -1);
+		else
+			rotate(&s->a->m, -r);
 		draw_image(da, s);
 		break;
 	default: