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/ _______________________________________________ Tagging mailing list Tagging@openstreetmap.org https://lists.openstreetmap.org/listinfo/tagging
