Re: Rounded corners

[Don Blaheta <blahedo blahedo org>]

Quoth Hans Breuer:
> Am 29.05.2008 05:37, Don Blaheta schrieb:
> > The biggest, I think, is that "corner radius" is being used to mean two
> > different things: the radius of rounding for a rounded rectangle, and
> > the radius of rounding for the cap of a regular rectangle.  
>
> If I understand the terminology correctly 'caps' are only at the end of 
> lines, see e.g.:  [...] ; but we need 'joins' here because we are
> talking about the radius between line segments, see: [...]

Aha, I learn something every day.  Yes, I've been using the word "cap"
but I meant "join".  Reading the rest of your email, I'm definitely
convinced that trying to merge the join style with the corner radius is
a losing proposition, but I like this:

> > "Rounded" would make a radius of
> >   exactly half the linewidth.  Polylines and zigzaglines get the
> >   checkbox as well instead of a corner radius option.
>
> The almost independent parameter which could be given to all the renderers 
> is the line join. Maybe we should allow the user to set it, instead of 
> hardcoding the behaviour for radius>=0 && radius<line_width?

Yes!  I didn't realise that JOIN_MITER and so forth were already-available
options, but having a single parameter for all objects-with-corners that
said whether to render it as pointy or round would be pretty much what I
was hoping for before I got carried away with the separate radius stuff.

> > How do Visio and other such programs handle any of this?
>
> I Looked at some other renderers Dia has - namely antialised vias cairo and 
> wmf and the implmentation looks like the radius is applied at the control 
> rectangle. For radius 0 it gives sharp edges, for 0.1 it has an outer 
> radius of almost half of the line width.

Almost half?  Surely it is slightly more than half?  This still strikes
me as a bit hacky (in that it's only a convention that has "0.0" meaning
"mitred join"---telling it to round the join at a radius of 0 in from
the control point would be perfectly sensible too).  Its meaning is also
not quite transparent, which triggers a usability tradeoff: permitting
some range of roundedness in the joins (though not a _full_ range of
roundedness), at the cost of having a single numeric parameter whose
*primary* modality is really just zero/nonzero.  Even if the underlying
renderers *can* handle the join-radius parameter as you describe, it
might be better to just have a "join type" pulldown (mitre or round).
It would be irrelevant to an actual rounded rectangle (no joins!) but
relevant to polylines and zigzaglines.  Oh, and polygons.  Oh! and
cusp-controlled bezierlines and beziergons.  (Which, as I check right
now, appear to have a *bevel* join, actually, at least in the gdk
renderer.)

-- 
-=-Don Blaheta-=-=-blahedo blahedo org-=-=-=-<http://www.blahedo.org/>-=-
"Yeah, I never realized what fun it would be to do interviews until I
remembered how neurotic, self-important and generally unable to converse
in a relaxed casual way law students are."      --Jonathan Wagner