Drawing selection rectangles -- rubber-banding

[Eduardo M Kalinowski <ekalin bol com br>]

Hi,

    In a project I'm working in, I will need to draw rectangles to
select a region. I've tried implementing this in the "scribble" program
of the GTK+ tutorial.
    It is almost working (using the right-button (3) for selecting). The
only problem is that sometimes the old rectangle is not erased when a
new selection is started. This happens when the button is pressed and
dragged very quickly.
    Since this rubber-banding technic is quite common in several
applications, I wonder if someone could point me how to correctly do
that.
    I'm sending attached the modified version of the "scribble" program.

--
Sometimes when you look into his eyes you get the feeling that someone
else is driving.

-- David Letterman

Eduardo M Kalinowski
ekalin bol com br -- **Novo e-mail** **New e-mail**
http://move.to/hpkb

/* example-start scribble-simple scribble-simple.c */

/* GTK - The GIMP Toolkit
 * Copyright (C) 1995-1997 Peter Mattis, Spencer Kimball and Josh MacDonald
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */

#include <gtk/gtk.h>

/* Rubber-bading */
GdkGC *select_gc;
int p1_x = 0;
int p1_y = 0;
int p2_x = 0;
int p2_y = 0;

/* Backing pixmap for drawing area */
static GdkPixmap *pixmap = NULL;

/* Create a new backing pixmap of the appropriate size */
static gint configure_event( GtkWidget         *widget,
                             GdkEventConfigure *event )
{
  if (pixmap)
    gdk_pixmap_unref(pixmap);

  pixmap = gdk_pixmap_new(widget->window,
                          widget->allocation.width,
                          widget->allocation.height,
                          -1);
  gdk_draw_rectangle (pixmap,
                      widget->style->white_gc,
                      TRUE,
                      0, 0,
                      widget->allocation.width,
                      widget->allocation.height);

  return TRUE;
}

/* Redraw the screen from the backing pixmap */
static gint expose_event( GtkWidget      *widget,
                          GdkEventExpose *event )
{
  gdk_draw_pixmap(widget->window,
                  widget->style->fg_gc[GTK_WIDGET_STATE (widget)],
                  pixmap,
                  event->area.x, event->area.y,
                  event->area.x, event->area.y,
                  event->area.width, event->area.height);

  return FALSE;
}

/* Draw a rectangle on the screen */
static void draw_brush( GtkWidget *widget,
                        gdouble    x,
                        gdouble    y)
{
  GdkRectangle update_rect;

  update_rect.x = x - 5;
  update_rect.y = y - 5;
  update_rect.width = 10;
  update_rect.height = 10;
  gdk_draw_rectangle (pixmap,
                      widget->style->black_gc,
                      TRUE,
                      update_rect.x, update_rect.y,
                      update_rect.width, update_rect.height);
  gtk_widget_draw (widget, &update_rect);
}


static void draw_select( GtkWidget *widget,
                        gint    x,
                        gint    y,
                         gboolean new)
{
  GdkRectangle update_rect;

  /* Erase */
  gdk_draw_rectangle (pixmap,
                      select_gc,
                      FALSE,
                      p1_x, p1_y,
                      p2_x - p1_x, p2_y - p1_y);

  if (!new)
    /* Draw */
    gdk_draw_rectangle (pixmap,
                        select_gc,
                        FALSE,
                        p1_x, p1_y,
                        x - p1_x, y - p1_y);

  update_rect.x = p1_x;
  update_rect.y = p1_y;
  update_rect.width = MAX(x - p1_x, p2_x - p1_x) + 1;
  update_rect.height = MAX(y - p1_y, p2_y - p1_y) + 1;
  gtk_widget_draw (widget, &update_rect);

  p2_x = x;
  p2_y = y;
}


static gint button_press_event( GtkWidget      *widget,
                                GdkEventButton *event )
{
  if (event->button == 1 && pixmap != NULL)
    draw_brush (widget, event->x, event->y);
  else if (event->button == 3 && pixmap != NULL) {
    draw_select(widget, event->x, event->y, TRUE);
    p1_x = event->x;
    p1_y = event->y;
    gdk_pointer_grab(widget->window, TRUE,
                     GDK_POINTER_MOTION_MASK,
                     widget->window,
                     NULL, GDK_CURRENT_TIME);
  }

  return TRUE;
}


static gint button_release_event( GtkWidget      *widget,
                                GdkEventButton *event )
{
  if (event->button == 3 && pixmap != NULL) {
    gdk_pointer_ungrab(GDK_CURRENT_TIME);
  }

  return TRUE;
}

static gint motion_notify_event( GtkWidget *widget,
                                 GdkEventMotion *event )
{
  int x, y;
  GdkModifierType state;

  if (event->is_hint)
    gdk_window_get_pointer (event->window, &x, &y, &state);
  else
    {
      x = event->x;
      y = event->y;
      state = event->state;
    }
    
  if (state & GDK_BUTTON1_MASK && pixmap != NULL)
    draw_brush (widget, x, y);
  else if (state & GDK_BUTTON3_MASK && pixmap != NULL)
    draw_select(widget, x, y, FALSE);
  
  return TRUE;
}

void quit ()
{
  gtk_exit (0);
}

int main( int   argc, 
          char *argv[] )
{
  GtkWidget *window;
  GtkWidget *drawing_area;
  GtkWidget *vbox;

  GtkWidget *button;

  GdkColor red = { 0, 0, 0xFFFF, 0xFFFF };

  gtk_init (&argc, &argv);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_widget_set_name (window, "Test Input");

  vbox = gtk_vbox_new (FALSE, 0);
  gtk_container_add (GTK_CONTAINER (window), vbox);
  gtk_widget_show (vbox);

  gtk_signal_connect (GTK_OBJECT (window), "destroy",
                      GTK_SIGNAL_FUNC (quit), NULL);

  /* Create the drawing area */

  drawing_area = gtk_drawing_area_new ();
  gtk_drawing_area_size (GTK_DRAWING_AREA (drawing_area), 200, 200);
  gtk_box_pack_start (GTK_BOX (vbox), drawing_area, TRUE, TRUE, 0);

  gtk_widget_show (drawing_area);

  /* Signals used to handle backing pixmap */

  gtk_signal_connect (GTK_OBJECT (drawing_area), "expose_event",
                      (GtkSignalFunc) expose_event, NULL);
  gtk_signal_connect (GTK_OBJECT(drawing_area),"configure_event",
                      (GtkSignalFunc) configure_event, NULL);

  /* Event signals */

  gtk_signal_connect (GTK_OBJECT (drawing_area), "motion_notify_event",
                      (GtkSignalFunc) motion_notify_event, NULL);
  gtk_signal_connect (GTK_OBJECT (drawing_area), "button_press_event",
                      (GtkSignalFunc) button_press_event, NULL);
  gtk_signal_connect (GTK_OBJECT (drawing_area), "button_release_event",
                      (GtkSignalFunc) button_release_event, NULL);
  
  gtk_widget_set_events (drawing_area, GDK_EXPOSURE_MASK
                         | GDK_LEAVE_NOTIFY_MASK
                         | GDK_BUTTON_PRESS_MASK
                         | GDK_BUTTON_RELEASE_MASK
                         | GDK_POINTER_MOTION_MASK
                         | GDK_POINTER_MOTION_HINT_MASK);

  /* GC for selection rectangle */
  gtk_widget_realize(drawing_area);
  select_gc = gdk_gc_new(drawing_area->window);
  gdk_colormap_alloc_color(gdk_colormap_get_system(), &red,
                           FALSE, TRUE);
  gdk_gc_set_foreground(select_gc, &red);
  gdk_gc_set_function(select_gc, GDK_XOR);

  /* .. And a quit button */
  button = gtk_button_new_with_label ("Quit");
  gtk_box_pack_start (GTK_BOX (vbox), button, FALSE, FALSE, 0);

  gtk_signal_connect_object (GTK_OBJECT (button), "clicked",
                             GTK_SIGNAL_FUNC (gtk_widget_destroy),
                             GTK_OBJECT (window));
  gtk_widget_show (button);

  gtk_widget_show (window);

  gtk_main ();

  return 0;
}
/* example-end */