Re: drawing graphs with dia

[Grégoire Dooms <dooms info ucl ac be>]

I merged both of your  answers here.
I reply in the text

Lars Clausen wrote:

> On Sat, 2005-03-12 at 20:11 +0100, Grégoire Dooms wrote:
>  
>
> > Lars Clausen wrote:
> >
> >    
> >
> > > First option is exactly what I had in mind for after 0.95.  We have code
> > > that does line-gap well for almost all objects, but only with lines.  The
> > > various straight lines would work well with it, but getting arcs and
> > > beziers to do the right thing would be tricky.
> > >
> > >
> > >      
> > For arc and bezier, I'm not sure what the right thing to do is.
> > I can suggest two things:
> > - Connect the endpoint to the central connection point,
> >  trim the curve at the edge of the connected object
> >  and draw the arrow like if the endpoint was there
> > - Preserve the slope the user set when the endpoint was connected to the
> >  central CP,
> >  connect the curve at the edge such that it aims at the central CP 
> > (with the
> >  same slope).
> >    
>
> Let's not worry about the arc yet.
>
>  
Does that meen go on with bezier ?
If so, could you comment on what is the right way to do it  ?
(one of the two aforementionned propositions ? ).


> > For the line, the gap is perfect for my intended use, why did you 
> > comment out the GUI ?
> >    
>
> It was a rather clunky half-solution to a number of problems, and only
> implemented for line.  I wanted something more complete.  I know,
> perfectionist syndrome.  
>
>  
>
> > In a general setting with an auto-gap, what do you think about this:
> > instead of absolute-gap and fractional_gap we could use gap_offset and 
> > gap_proportion (liner factor) where this latter could apply to either 
> > the line/curve length or the autogap length.
> >
> > To sum up:
> > offset / prop / target
> > .10 / 0 / self : constant gap of .10
> > 0 / .1 / self : gap is 10% of length
> > .10 / .1 / self : gap is 10% of length + .10
> > 0 / 0 /auto : arrow touches edge
> > .10 / 0 / auto : arrow is 10 away from edge
> > 0 / -.5 / auto : arrow is halfway inside the object
> >
> > positive values always increase the gap, negative decrease the gap.
> >
> >    
> Forgot to comment on this part.  One of the problems I had with the
> existing code was how many options it added.  It's an interesting idea
> to have the fractional gap action depend on being connected to the
> object, not sure yet if that would be intuitive.  I was pondering if the
> gap could be a single number, where you can either give a length or a
> percentage, and it behaves in one or the other of the above mentioned
> ways.  No way to make the combined gap, but I don't think it'd be used
> that much anyway.  I think the constant gap really is the most important
> feature, and we should go for that first -- I don't have a good use case
> for the fractional gaps yet.
Ok I'll go for gap = offset + (0/1)*auto

> > > The main thing that's been holding me off is that I wanted to let all
> > > objects have the virtual central CP.  Some objects have a dynamic number
> > > of CPs, which makes it trickier.  We have some notes on this problem on
> > > the TWiki.
> > >
> > >
> > >
> > >      
> > Why does it need to be virtual ?  ie. Why not a real central CP and 
> > multiple
> > lines connected to it like with the ellipse object ?
> >    
>
> The idea is to not have it be a normal CP, but a 'default' CP.  When a
> line end is dragged onto the object, but not onto one of the line ends,
> it connects to the virtual CP in the center, and the line ends at the
> edge of the object.  This would make it possible to make many diagrams
> without having to worry about CPs or having to rearrange connections.
>
> Maybe I've been overengineering it in my plans, and it really should
> just be that CP 0 for elements is special, and all the element objects
> just need updating to place it in the middle.
>
>  
From an UI point of view, could be that if you drop the arrow inside 
the object
it connects to the center with auto-gap by default. A modifier key could 
allow
for manual placement of the arrow inside an other object.

> > > I would love it if you started looking at this.  I'd want to keep it
> > > entirely #ifdef'ed until 0.95 is out (unless you code well and fast), but
> > > I'm very interested in seeing some action on that front.  I do feel,
> > > though, that it would be unfair not to mention the Graphviz package, which
> > > is made for making graphs.  Doesn't have the editing abilities of Dia, but
> > > is really good at autolayout.
> > >
> > >
> > >      
> > Yeah I use it a lot too. The layout engines allow to output the graph in 
> > xdot
> > format (dot format with positions and parmaters for the curves). But 
> > there is
> > one awkward thing: if you modify the xdot file there is no way/option  
> > to use
> > the graphviz renderer to display the modified graph as it always 
> > re-autolayout
> > it (AFAIK).
> >    
>
> Have you noticed that recent versions support Dia format?  Haven't tried
> it out yet.
>
>  
Just tried it out. It renders the nodes as an ellipse grouped with a text
hand-placed at the center. The arc is a bezier patch, most arcs are a 
single
bezier, some are mulltiple segment beziers the end-arrow is a black 
triangle.

Funny part:
The arcs are connected to several connection points on the edge of the 
ellipse
but they are hand placed like connected with the center CP and using a gap.
The arrow endpoint is placed about at twice the "radius" of the ellipse 
with
the slope pointing towards the center.

If one moves an ellipse the arcs are re-rendered touching those edges, 
which
is not undoable.

Looks like we should really implement this gap code for beziers.
I'll give it a try.
--
Grégoire Dooms