I was toying around with how to make a stationary background "appear" to stay in place, as a Tk::Canvas was scrolled. It is more of an optical illusion, but it works. In the script below, the background image appears to stay stationary with scrolling. A very fine point being the ratio used in a scrolled widget. See the $div variable. It is not obvious, because you must account for the REAL canvas size, (which includes the default border).

Anyways, this will take a photo as ARGV[0] on the commandline, or will retreive an image from the net (if no image is given).

UPDATE April 8,2006 Fixed a few pixel jitter caused on first scroll use, caused by not accounting for the canvas border.

#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::JPEG;
use Tk::PNG;

my $mw=tkinit;

my $imagin = shift;
$imagin ||= '';
my $image;

if(length $imagin){
   $image  = $mw->Photo(-file =>$imagin);
}else{
   $image = &get_photo;
}

my $c = $mw->Scrolled('Canvas',
     -width=>400,
     -height=>400,
     -bg=>'white',
     -scrollregion=>[0,0,1000,1000],
     -scrollbars=>'sw')->pack(-expand=>1, -fill=>'both');

my $real_can = $c->Subwidget('scrolled');
my $real_can_h = $real_can->reqheight; 

my $sc_h = $c->cget('-height'); 
my $pad =  ($real_can_h - $sc_h)/2;
#  pad prevents a startup jitter due to border width

my $bimage = $c->createImage( 0,$pad,   
         -anchor => 'nw',               
         -image  => $image,   
    );

for(0..500){
 if( $_ % 2 == 0){
      $c->createRectangle(0,  5 + $_ * 10, 30,  15 + $_ *10,
              -fill =>'hotpink',
            );
      
      $c->createText( 3, 5 + $_ * 10,
              -fill =>'white',
              -text => $_,
          -anchor =>'nw',
        );
  
      next;
     }
 }

my $ybar = $c->Subwidget("yscrollbar");
$ybar->configure(
           -background => "lightgreen", 
           -activebackground => "green",
       -troughcolor => "black",
           -command => \&yscrollcallback,
        );


$mw->Button(-text =>'Change Scroll Region',
            -command =>sub{
        $c->configure(-scrollregion =>[0,0,2000,2000]);
        })->pack();

MainLoop;
######################################################################
#if you specify a yscrollcallback, you will override the
#normal scroll behavior.

sub yscrollcallback{
    #restore original function
    $c->yview(@_);
 
   my($z,$z1) = $c->yview;

   my(undef,undef,undef,$sry) = $c->cget('scrollregion');

   my $real_can_h = $real_can->reqheight; 
#   print "$sry  $real_can_h\n";
   my $div = $sry/$real_can_h;  
#   print "div $div\n";

$c->coords($bimage, 0,  $div *$z * $real_can_h  );
$c->update;
   
}

######################################################################
+####3
sub get_photo{

 use LWP::Simple;
 use MIME::Base64;

 my $URL = 'http://zentara.net/2uni2.jpg';
 
 my $content = encode_base64(get($URL));
  
return $mw->Photo(-data => $content);

}

[download]

In reply to Stationary background on Scrolled Tk-Canvas by zentara

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.