From: Josef Wolf <jw@...>
Date: 2009-09-08T07:20:23+09:00
Subject: Re: Zoomable TkCanvas?

On Tue, Sep 08, 2009 at 04:26:32AM +0900, Joel VanderWerf wrote:
> Josef Wolf wrote:
>> It looks like TkCanvas has no methods for zooming in and out. For Perl,
>> I have found the Tk::Worldcanvas and Tk::Abstractcanvas modules. Anybody
>> knows about something like that for ruby?
>
> Those perl modules sound interesting--maybe it would be useful to have
> them in ruby if someone hasn't done that yet.

Yeah, they make life a whole lot easier. In addition to zooming, they:
 - maintain original coordinates at all zoom factors. Thus even after
   zoom operations, events as well as querying/moving/adding items are
   done as if the canvas is at zoom factor 1.0. So zoom is handled
   completely transparent to the user of the module..
 - turn around the y-axis (origin is at left bottom)
 - handle scroll bars + panning

> I've used Tk's Canvas from tcl and from ruby, and I've always had to 
> implement zooming myself in wrapper classes.

Strange, that such a powerful widget is missing such a basic functionality.

> One way to do it is tag all 
> canvas objects (or just the ones that zoom), and use the Canvas#scale 
> method on that tag. You have to keep track of the current zoom level (Tk 
> doesn't), and use that to calculate the arguments to #scale. You also have 
> to adjust the scroll bars (using 'configure :scrollregion => ...').  Then 
> use xview/yview to keep the current view position in sync with the zoom 
> level.

Sounds easy enough. Is it really that easy? I think this would work only
if you do not add or move any items after you have made zoom operations.
Looks like above mentioned modules do a whole lot more of work. They
override all of the item creation and modification methods to fix movement
or addition of new items to the current zoom factor.

> I think that covers it, but if you're interested, take a look at canvas.rb 
> in my tkar project. Tkar is a process, rather than a library, but it 
> abstracts out details like zooming and provides a basic user interface for 
> controlling zoom, pan, etc.
>
> http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/343516
>
> http://rubyforge.org/projects/tkar

Sounds very interesting.

Thanks, I'll check that out!