I worked on that awhile back, and this is what I came up with for centering on waypoints ..... it may not be perfect, but it works, and also incorporates scale....which you really need to deal with.. I just came up with the algorithm by guessing. :-) .... so there may be a better way. HTH..... I'm fading out of this stuff...of course I was tempted to use my Goo Canvas and transparent images to simulate the flight of that typo dong 2 missle. Would work perfect. :-)

See 

#!/usr/bin/perl -w
use strict;
use warnings;
use Goo::Canvas;
use Gtk2 '-init';
use Glib qw(TRUE FALSE);


# slower than Gnome2::Canvas with long drags

my $draw_flag = 0;
my %lines;   # way to store multiple continuous lines
my $count = 0;

my $window = Gtk2::Window->new('toplevel');
$window->signal_connect('delete_event' => sub { Gtk2->main_quit; });
$window->set_default_size(640, 600);


my $vbox = Gtk2::VBox->new;
$vbox->set_border_width(4);
$vbox->show;

my $hbox = Gtk2::HBox->new(FALSE, 4);
$vbox->pack_start($hbox, FALSE, FALSE, 0);
$hbox->show;


$window->add($vbox);

my $swin = Gtk2::ScrolledWindow->new;
$swin->set_shadow_type('in');
$vbox->pack_start($swin, 1, 1, 0); 


my $cwidth = 4000;
my $cheight = 2000;
my $canvas = Goo::Canvas->new();
$canvas->set_size_request(900, 600); # size on screen
$canvas->set_bounds(0, 0, $cwidth, $cheight); # scrollregion
$swin->add($canvas);

my $root = $canvas->get_root_item();

#####################################################
# if you want to underlay an image
 my $im = Gtk2::Gdk::Pixbuf->new_from_file("USA.gif");
 my $w = $im->get_width;
 my $h = $im->get_height;
 my $image = Goo::Canvas::Image->new(
            $root, $im, 0, 0,
            'width' => $w,
            'height' => $h);

######################################################

$canvas->signal_connect (event => \&event_handler);


# Zoom
my $z = Gtk2::Label->new("Zoom:");
$hbox->pack_start($z, FALSE, FALSE, 0);
$z->show;

my $adj = Gtk2::Adjustment->new(1, 0.05, 100, 0.05, 0.5, 0.5);
my $sb = Gtk2::SpinButton->new($adj, 0, 2);
$adj->signal_connect("value-changed", \&zoom_changed, $canvas);
$sb->set_size_request(60, -1);
$hbox->pack_start($sb, FALSE, FALSE, 10);
$sb->show;

    
# Create PDF                                                          
+                
my $bpdf = Gtk2::Button->new_with_label('Write PDF');                 
+                      
$hbox->pack_start($bpdf, FALSE, FALSE, 0);                            
+                   
$bpdf->show;                                                          
+                   
$bpdf->signal_connect("clicked", \&write_pdf_clicked, $canvas);       
+  


$window->show_all();
Gtk2->main;


sub event_handler{
     my ( $widget, $event ) = @_;
#     print $widget ,' ',$event->type,"\n";

     my $scale = $adj->get_value;
     # print "scale->$scale\n"; 

    if ( $event->type eq "button-press" ) {
        $draw_flag = 1;
         
        #start a new line curve
        $count++;
        
        my ($x,$y) = ($event->x,$event->y);
#        print "$x  $y\n";
        
        $lines{$count}{'points'} = [$x/$scale,$y/$scale,$x/$scale,$y/$
+scale]; #need at least 2 points 
        my $points = Goo::Canvas::Points->new($lines{$count}{'points'}
+);

     $lines{$count}{'line'} = Goo::Canvas::Polyline->new(
        $root, FALSE,
         undef, # points need to be set after creation
        'stroke-color' => 'black',
        'line-width' => 3,
       );

   #setting after line creation, sets the 'points' property by name
   $lines{$count}{'line'}->set(points => $points);

   }

    if ( $event->type eq "button-release" ) {
        $draw_flag = 0;
    }


    if ( $event->type eq "focus-change" ) {
        return 0;
    }
    
    if ( $event->type eq "expose" ) {
        return 0;
    }
    
    
  if($draw_flag){
    #left with motion-notify
    if ( $event->type eq "motion-notify"){
    my ($x,$y) = ($event->x,$event->y);
#    print "$x  $y\n";
 
     push @{$lines{$count}{'points'}},$x/$scale,$y/$scale;

     my $points = Goo::Canvas::Points->new( $lines{$count}{'points'} )
+;
     $lines{$count}{'line'}->set(points => $points);
    }
  }    
  
Gtk2->main_iteration while Gtk2->events_pending;      
return 1;
}


sub write_pdf_clicked {
    my ($but, $canvas) = @_;
    print "Write PDF...\n";

    my $scale = $adj->get_value;
    print "scale->$scale\n"; 
    
    my $surface = Cairo::PdfSurface->create("$0-$scale.pdf", 
                    $scale*$cwidth, $scale*$cheight);
    
    my $cr = Cairo::Context->create($surface);

    # needed to save scaled version
    $cr->scale($scale, $scale);

    $canvas->render($cr, undef, 1);
    $cr->show_page;
    print "done\n";
    return TRUE;
}


sub zoom_changed {
    my ($adj, $canvas) = @_;
    $canvas->set_scale($adj->get_value);
}

[download]

Also of interest to centering would be: 

#!/usr/bin/perl -w
use strict;
use warnings;
use Goo::Canvas;
use Gtk2 '-init';
use Glib qw(TRUE FALSE);
use Gtk2::Gdk::Keysyms;

# Z and z zoom in and out

my $scale = 1;

my $window = Gtk2::Window->new('toplevel');
$window->signal_connect('delete_event' => sub { Gtk2->main_quit; });
#$window->set_size_request(600, 400);

my $swin = Gtk2::ScrolledWindow->new;
$swin->set_shadow_type('in');
$window->add($swin);

my ($cwidth,$cheight)= (2400,2400);
my ($cx,$cy) = ($cwidth/2,$cheight/2);
my $canvas = Goo::Canvas->new();

my ($cw,$ch) = (600,400);
$canvas->set_size_request($cw, $ch);

$canvas->set_bounds(0,0, $cwidth, $cheight);
#$canvas->set_bounds( 0, 0,2*$cwidth,2* $cheight);
my $black = Gtk2::Gdk::Color->new (0x0000,0x0000,0x0000);
my $white = Gtk2::Gdk::Color->new (0xFFFF,0xFFFF,0xFFFF);
$canvas->modify_base('normal',$white );
$swin->add($canvas);

my $root = $canvas->get_root_item();

#my ($margin_x,$margin_y) = (100,100); 

#my $g = Goo::Canvas::Group->new($root); 
#$g->scale(1,-1); #reverse direction of y axis, so graphing will be no
+rmal cartesian
#$g->translate( $margin_x, $margin_y); #notice translations are revers
+ed


my $ha1 = $swin->get_hadjustment;
my $va1 = $swin->get_vadjustment;
my $vupper = $va1->upper;
my $hupper = $ha1->upper;

my $hadjc = $ha1->value;
my $vadjc = $va1->value;

print "uppers $vupper $hupper\n";

$va1->signal_connect( value_changed => sub { my $adj = shift;
                                             #print 'vchanged ',$adj->
+get_value,"\n";
                                             #$manual = 1;
                                             return 0;
                                             } );
$ha1->signal_connect( value_changed => sub { my $adj = shift;
                                             #print 'hchanged ',$adj->
+get_value,"\n";
                                             #$manual = 1;
                                             return 0;
                                             } );

my $hbr = $swin->get_hscrollbar;
my $vbr = $swin->get_vscrollbar;
#print "$hbr $vbr\n";
$hbr->signal_connect(move_slider => sub{  
                                         #print "moving slider @_\n"
                                       }); 
$vbr->signal_connect(move_slider => sub{ 
                                          #print "moving slider @_\n"
                                       }); 

$hbr->signal_connect(event_after => sub{  #print "after slider @_\n";
                                    my($bar,$event) = @_;
                                    #print $bar,' ',$event->type,"\n";
                                   }); 
$vbr->signal_connect(event_after => sub{ # print "after slider @_\n";
                                    my($bar,$event) = @_;
                                    #print $bar,' ',$event->type,"\n";
                                  }); 

$canvas->scroll_to( $cx - $cw/2   ,$cy - $ch/2 );

print $canvas->get_bounds,"\n";

my @points1 = (0,$cy,$cwidth,$cy);
my $poly1 = Goo::Canvas::Polyline->new(
        $root, FALSE,
         \@points1, 
        'stroke-color' => 'black',
        'line-width' => 1,
       );

my @points2 = ($cx,0,$cx,$cheight);
my $poly2 = Goo::Canvas::Polyline->new(
        $root, FALSE,
         \@points2, 
        'stroke-color' => 'black',
        'line-width' => 1,
       );



$canvas->signal_connect('button-press-event',
                      \&on_can_button_press);


$canvas->signal_connect_after('key_press_event', \&on_key_press);
$canvas->can_focus(TRUE);
$canvas->grab_focus($root);

#&set_axis();
#&plot();

$window->signal_connect(event_after => \&event_after); 

$window->show_all();

Gtk2->main;

sub on_can_button_press {
     #print "@_\n";
     my ($widget, $event ) = @_;
     #print $widget ,' ',$event->type, '  ','button',' ',$event->butto
+n,"\n";
     my ($x,$y) = ($event->x,$event->y);
      print "$x  $y\n";
    return 0;
}


sub on_key_press {
    # print "@_\n";
     my ( $canvas, $event ) = @_;
    # print $event->type,"\n";

    if ( $event->keyval == $Gtk2::Gdk::Keysyms{Z} ) {
        $scale += .1;
        $canvas->set_scale($scale);
        print "$scale\n";
    #    $canvas->scroll_to($cx - $cw/2,$cy- $ch/2);
    
    #    $canvas->scroll_to( $cx + $scale*$cx - $cw/2   ,$cy + $scale*
+$cy - $ch/2 );
        #$canvas->scroll_to( 0 - $cw/2   ,0 - $ch/2 );
        
    #    $ha1->upper($hupper * $scale);
    #    $hupper = $ha1->upper;
    #    print "new hupper $hupper\n";
    #    $ha1->clamp_page(0,$hupper);
    
    $hadjc = $ha1->value;
    print "$hadjc\n";
        
    #    $va1->upper($vupper * $scale);
    #    $vupper = $va1->upper;
    #    print "new vupper $vupper\n";
    #    $va1->clamp_page(0,$vupper);
    
    $vadjc = $va1->value;
    print "$vadjc\n";
    
     #$canvas->scroll_to( $cx + $scale*$cx - $cw/2   ,$cy + $scale*$cy
+ - $ch/2 );
        
        #$canvas->scroll_to( $hupper/2   , $vupper/2 );
            
    }

    if ( $event->keyval == $Gtk2::Gdk::Keysyms{z} ) {
        $scale -= .1;
        $canvas->set_scale($scale);
        #canvas->scroll_to(0,$cheight);
        #$canvas->scroll_to( 0 - $cw/2   ,0 - $ch/2 );
        #$canvas->scroll_to( $cx + $scale*$cx - $cw/2   ,$cy + $scale*
+$cy - $ch/2 );
    }

    if ( $event->keyval == $Gtk2::Gdk::Keysyms{s} ) {
        write_pdf($canvas);
    }

    if ( $event->keyval == $Gtk2::Gdk::Keysyms{c} ) {
        #$canvas->scroll_to( 0 - $cw/2, 0 - $ch/2);
        
        # $canvas->scroll_to( $cx - $cw/2   ,$cy - $ch/2 );
        # print $cx * $scale,' ',$cy * $scale,"\n";
         
#         $canvas->scroll_to( $cx - $cw/2  , $cy - $ch/2 );

#        $canvas->scroll_to( $cx , $cy );

        $canvas->scroll_to( $cx - $cw/(2*$scale)  , $cy - $ch/(2*$scal
+e) );

#    my $current   = $ha1->get_value;
#    my $increment = $ha1->step_increment;
#    my $new       = $current ;#+ $increment ;

#    $ha1->set_value( $new );


}    



#    print "key was ", chr( $event->keyval ), "\n";

    return 0;
}


sub write_pdf {
    #print "@_\n";
    my $canvas = shift;
    print "Write PDF...\n";

    my $scale = $canvas->get_scale;
    print "scale->$scale\n"; 
    
    my $surface = Cairo::PdfSurface->create("$0-$scale.pdf", 
                    $scale*$cwidth, $scale*$cheight);
    
    my $cr = Cairo::Context->create($surface);

    # needed to save scaled version
    $cr->scale($scale, $scale);

    $canvas->render($cr, undef, 1);
    $cr->show_page;
    print "done\n";
    return TRUE;
}

sub event_after {
  my ($mw, $event) = @_;

#  print "\t",'canvas ',$event->type,"\n";

# if( ($event->type eq 'configure') or ($event->type eq 'expose' ) ){
#     $resize_flag = 1
#   }else{ $resize_flag = 0  }

#  return FALSE if $resize_flag; 
  
#  if ($manual){ $manual = 0; return FALSE} 

 my $rect = $canvas->allocation;
 $cw = $rect->width;
 $ch = $rect->height;
# print "$cw $ch\n"; 
 
#  my ($x, $y) = $mw->get_size;
#  print "$x $y\n";;

#  $va1->set_value( ($vupper - $y)/2 + 5);
#  $ha1->set_value( ($hupper - $x)/2 + 5);

#$canvas->scroll_to( 0 - $cw/2   ,0 - $ch/2 );

  return FALSE;
}
__END__

[download]
I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku

In reply to Re: Goo::Canvas zoom correction by zentara
in thread Goo::Canvas zoom correction by deadpickle

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