Re: Contour plots

[Hans-Bernhard B. <br...@ph...>]

Daniel J Sebald wrote:

[...]
> I would too.  However, might it be possible to use some simple 
> algorithm that does an OK job in most cases?

I rather much doubt that such a thing exists.   In contouring, I suspect
most cases are too tricky for any simple algorithm.

> The worst case scenario is when the contours are rather small oval
> shapes, or rather close together.  But if it is unacceptable to the
> user, then he or she could turn off annotation.

We really need better control than just on/off on this.

> Rather than an optimization problem where we, say, pick a trajectory
> with the smallest gradient (i.e., largest spacing between contours),
> could we just pick some line for which to intersect as the spot to
> pick the label text?  (Intersecting with lines is a not so difficult
> thing.)

A line (without ends other than where it leaves the graph box) would not
be a good idea.  It has to be a line segment, and probably an
optionally curved one at that.  Which raises a serious problem about the
command-line user interface we could provide to specify such a thing.


> The contour plot algorithm apparently already knows how to create the
> contours and whether they are lines or closed curves.  So, if it is
> a closed curve, pick the starting point or some other rule.

If it's closed, it doesn't really have a starting point, does it?

> If it is an open curve, take the line that bisects the endpoints and 
> find where that intersects the curve.

No way.  The number of intersections can easily be zero, or could be
a dozen.

Some other random points:

*) I'm strictly against any GUI-only feature.  Once we have a suitable
command line method of specifying the locus of those contour labels, we 
can think of a mouse interaction method of doing the same.  We may even 
have to keep in mind that a GUI should be possible while setting up the 
command line syntax.  But command line is still *mandatory*.

*) We're going to paint labels in the middle of the data area.  I.e. we 
risk covering up critical aspects of the plotted data, which I consider 
a strict no-no.  That's why a simple on/off control would be 
insufficient.  We can't seriously expect users to accept that if the 
default placement puts on of  a dozen labels in some undesirable place, 
their only option is to give up on in-graph contour labels altogether 
(or roll their own by plotting an extra file 'with labels')

*) In-graph contour labelling must take into account whether a terminal 
can render rotated text or not.  There will be a considerably large 
number of cases where horizontal-only labels are hard or impossible to 
place, while text oriented parallel to the contour line would still work.