[Tagging] RFC - Feature Proposal - area of steps for pedestrians.

Christoph Hormann osm at imagico.de
Fri Mar 29 18:16:08 UTC 2019

```On Friday 29 March 2019, Martin Koppenhoefer wrote:
>
> > * you should be aware that you can't uniquely define the shape of a
> > two dimensional surface in three dimensions exclusively through the
> > shape of its outline.  You can do that in 2d (provided what you
> > have has a defined outline) but not in 3d.  That is simple
> > mathematics.  So you'd have to document what assumptions you make
> > regarding the shape of the surface, otherwise the meaning of your
> > proposal would be ill-defined.
>
> the general assumption for stairs is that all steps have the same
> height and same "depth".

That would not be a very sensible assumption since that would be
impossible for any stairs where the upper and lower end are not
equidistant across their whole length because then not all steps can
have a constant depth.

But my point was a different one.  A polygon is by definition planar.
If your modeling defines a non-planar outline (which it obviously does
when you can have arbitrarily shaped upper and lower limits) then you
need to make assumptions regarding how the shape derives from this
outline.

> While I agree with you that for 3d you need height information (the
> area proposal has a suggestion for this), [...]

You need this for any rendering of the stairs that visualizes the
individual steps in some form (because their form defines a 3d
geometry - even if you don't render it in 3d).

> in the end, all areas can be represented as polygons [...]

Well - that depends on how you define "area" obviously.  A polygon is an
attempt to describe a two dimensional planar entity through circular
linestring representations of its edges.  Even for 2d entities where
this is possible (i.e. that are planar and have a well defined inside
and outside) it is often not the most efficient way of representing
them.

--
Christoph Hormann
http://www.imagico.de/

```