I have written a proposal (3D tagging for building base shapes) to introduce a new set of tags for representing the shape of building undersides, for example archways and spherical building parts, which cannot currently be accurately represented in regards to 3D rendering. I am seeking constructive criticism and comments about the tags I have proposed, particularly concerning the suffix :inverted, where I am open to changing the recommended tagging for differentiating between a downwards arch shape and an upwards arch shape, please see the Examples section for how this tag is intended to be used.
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Strongly support this concept, it is very much required. I’d had something very similar in mind, including the choice of base: prefix and eg. base:direction sense all as described.
It’s been a shame to be able to nicely define the upper half of a fancy building and then for the bottom half to ‘go all Minecraft’ due to the current assumption that (modern) buildings are all biggest at the bottom, with only flat-topped holes.
I’d not considered directly copying all the roof:shape names but rather adding what is needed with appropriate names. ‘round’ and ‘dome’ and ‘pyramidal’(or ’cone’) - and default ‘flat’ of course - make sense as natural mirrors of the roof ones. I was imagining ‘slope’ rather than the very roof-related term ‘skillion’ for a non-flat planar base, and something like ‘round_arch’ (for semi-eliptical eg semi-circular if height=width/2), ‘pointed_arch’ (for a Gothic one with 2 arcs meeting at a top centre corner point) and it seems flatter arches get called ‘segmented_arch’ (for a circular arc that has height <width/2).
(The diagrams in the proposal should reflect the semi-eliptical shape implemented for ‘round’, not a general parabolic-ish shape unless we more carefully define roof and base curve shapes. Actually, being able to explicitly choose a flatter arc not necessarily a semi-ellipse would be useful for roofs too, but that’s another matter).
Seems unnecessary to specify or invite use of the other roof shapes (incl onion etc) unless we really have examples? But fine if we do.
The presence and meaning of base:levels might need a little thought. roof:levels is a bit strange in being additive to the building:levels (but values of height are total with roof:height subtracted to get the walls-only height). Would base:levels also be additive? (not sure; probably yes if they are proper 1-storey arches). But base:height being subtractive like for roof makes sense. Haven’t got my head around this yet but we need to check what makes sense here. What if roof:levels and base:levels are both large so that the remaining (wall) levels is negative - this is wanted for the overall diagonally sloping building case. Also I guess same for heights for the case of the matching arched base and top, having height - roof:height - base:height as negative is a realistic case.
Another issue for the base that doesn’t occur for roof is interaction with the ground. Not an issue in many 3d renders, but already hurts peoples head with a flat building base and slopey ground where ground shape is implemented. May need to choose if we specify that the skillion/slope base is specifically meant for in-the-air undercut faces, or not. Again, needs some more thought to be sure we know what we want to do. Matching building shape to slopey ground is really another subject but it’s unavoidably linked once base shape can be set.
I’ve one other undercut shape for which this opens up only partial solutions - the lower part of ‘The Gherkin’ in London is a symmetrically undercut circle - which would either need a tall downward dome deeply embedded in the ground, a ring of undercut planar facets, or to stick with the current horizontal slicing. At least it’s good to have choices, even for unusal cases.
Thanks for your comments Cebderby, there’s some good suggestions here that will incorporate into the proposal, for example:
to not suggest all these roof shape equivalent values until someone finds a real life example where it would be useful
to change some of the roof-centric tag names to things like slope instead of skillion and arch (or round_arch) instead of round:inverted
proposing some entirely new, useful shapes like gothic arch and segmented arch (this one will need a bit of thought because it would be an arch shape that begins already at a very shallow angle, which isn’t something possible even with round roofs)
As for base:levels, I think in contrast to roof:levels it must be subtractive, say for example you have a 4 storey building where it has a sloped underside for the bottom 1 level, it must be tagged building:levels=4 and base:levels=1. I don’t think there’s any other way you could tag this. You can’t have building:levels=3 and base:levels=1 (for a total height of 4) because that’s really confusing in respect to existing building tagging.
I’m not really sure what point you are making in regards to how this could be affected by ‘slopey ground’, so I’d appreciate more info if you think there is a potential issue there.
As for the Gherkin, that’s going to be a tricky one to solve. I think it’s a similar problem to where the roof shape is a cone but there’s a multipolygon inner to create a flat central section for example (which isn’t official or supported well as far as I know). Certainly though we shouldn’t be encouraging editors to tag building shapes that go down underneath the ground just to provide the desired effect. Height=40 and base:height=100? No thanks
