| Re: /ALIAS and drawing in different draw_widgets [message #31700 is a reply to message #31687] |
Mon, 05 August 2002 05:32   |
lyubo
Messages: 34
Registered: March 2002
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"David Fanning" <david@dfanning.com> wrote in message
news:MPG.17b5e9dbc44fa19a989950@news.frii.com...
> lyubo (lzagorch@cs.wright.edu) writes:
>
>> I have the following situation:
>> Model M1 contains objects A,B, and C
>> Model M2 contains an alias of A
>> Model M3 contains an alias of B
>> Model M4 contains an alias of C
>> The models are added to different views and the views are drawn in
>> different draw widgets.
>>
>> The problem comes when I update the objects (A,B, and C) and try
>> to redraw the views. It looks like I can't redraw M1,M2,M3,and M4
>> correctly in that sequence. Redrawing only M1 works fine, redrawing
>> M2,M3, and M4 (without M1) also works, but if I try to redisplay M1,
>> M2,M3,and M4, M1 has only Cs.
>> Furthermore, there is a memory leakage when I redraw M1. I guess
>> that it has something to do with the aliases, but I have no idea what
>> could be causing that. What exactly happens when you add an alias
>> of an object to a model? Isn't it just a copy of the same object? If it
>> isn't which one gets updated first - the alias or the object?
>
> I don't have too many specific suggestions, except that
> in my experience about 99% of the odd things that happen
> when you are programming in object graphics are caused by
> programming mistakes rather than than the software. I would
> always look there first.
>
> Having said that, this whole notion of aliases is interesting
> to me. It occurred to me that Dave Burridge and I have
> developed something very much like it in the object widget
> system we have developed. In fact, it is so much like it that
> I wonder if we didn't independently come up with the same
> solution RSI uses.
>
> (This is not the first coincidence that has caused me to think
> this. Just watching the system perform sometimes brings the
> object graphics system to mind. At the very least, I like to
> think building this system has brought a little deeper insight
> into how an object system is suppose to work in general.)
>
> In any case, in our system we have the notion of an object
> hierarchy, based on container objects. There is a container
> at the top, it has children, they have children, etc. When
> you create an object you tell it who its "parent" is. All this
> is very much like widget programming.
>
> But, of course, you might create an object, say an image object,
> that you would like to belong to two different draw widget objects,
> to give a simple example. You wouldn't want to duplicate or copy
> the object, because then you would have to duplicate the data
> and this would be poor memory management. (Let's say, for sake
> of argument, that the image was 3.4 GBytes in size.)
>
> In our system, only one object can be the "parent" object, but
> you can add a reference to that object to another container. In
> the IDL vernacular, this would be the "alias" object. But now we
> have the problem of who controls this object, and more specifically,
> who destroys it? Clearly you don't want the image object destroyed
> until everyone else is finished using it.
>
> We have solved the problem by reference counting. Every time the
> object is "added" to a container, be it the parent object or some
> other object, its reference count is increased by one. When the
> object is "destroyed" (say by having one of the containers that
> holds it destroyed), we intercept this and decrement the object's
> reference count. An object is only really destroyed when its
> reference count goes to zero.
>
> So, if the IDL system works anything like ours, what happens when
> you add an alias object to a model is that you simply add the
> object reference to the model container. I suspect that both
> models are then working with the same object. If you change it
> one place, you change it somewhere else.
>
> Why this fails to do what you want it to do in your program
> is a mystery to me. But then a LOT of object programming is
> a mystery to me, even when I've just spent the whole day
> writing the damn things. :-)
>
> Cheers,
>
> David
>
>
>
> --
> David W. Fanning, Ph.D.
> Fanning Software Consulting, Inc.
> Phone: 970-221-0438, E-mail: david@dfanning.com
> Coyote's Guide to IDL Programming: http://www.dfanning.com/
> Toll-Free IDL Book Orders: 1-888-461-0155
David, first I'd like to thank you for your reply.
Even though I am not an expert I know what I've been doing,
and I still don't have any idea exactly what was causing the problem.
I found a workaround though, I just draw the objects and the aliases in
the same window in different viewports, and now I don't have memory
leaks or any other problems drawing them. I am sorry because it is a
big program and I can't uncouple that part so easily, otherwise I would
have posted an example here. But I definitely keep a copy, just in case
I have the chance to meet you again in the future :-)
Cheers,
Lyubo
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