Q: how to extract building parts?

That definition is written (and intended) as a rule, though, not an observation of things people usually do.

So right now, the S3DB page is self-contradicting: It requires that the outline surrounds all building parts (in the outline section), but also mentions the possibility of building parts outside the outline (in the relation section). Mixing the two statements makes no sense, and to maintain internal consistency, we need to either change both sections at the same time – or none of them. For the record, I currently prefer not changing them.

That’s a problem if 2D building areas are supposed to show the ground-level footprint, but I’m not sure that’s actually what people expect.

Many ordinary mappers use aerial imagery for buildings, where drawing the projection often feels more natural. And in the case of “special” buildings with bridges and overhangs, I find that people do in fact often intuitively expect the above-ground sections to be included in the area.

One example from Germany that was discussed on the forums in the past is this restaurant building spanning a motorway:

That’s now mapped as a rectangular area spanning the road, rather than just as a few tiny dots where the load-bearing pillars connect to the ground.

Likewise, bridges connecting multiple building parts tend to be included in building outlines, to the point where some mappers have been complaining that Mapnik renders roads on top of them.

Interesting situation. I’d also consider this a separate building, but as far as I know, there’s no rule that building outlines aren’t allowed to overlap? In fact, I’ve always considered overlapping building outlines the one situation where building relations are necessary at all. (Before the changes being discussed here, that is.)

But they are. If building outlines represent vertical projection, they can be inside one of other, because vertical projections of [non-vertical] buildings can overlap.

I also do not quite understand what is the problem with 2d rendering.
If you use top-down view of a 3D render or just sattelite images, you wil get exactly the same thing.

I am not sure is it relevant or not, but just what I found:

http://constructioncosts.eu/glossary/building-footprint/

From the beginning of the same page:

Definitions:

  1. Building footprint
    The ground area utilized by a building.
    Pay attention to the word “footprint”. It isn’t “bodyprint”, but “footprint”. It’s about the area used by the foot of the building, rather than body of the building.

  2. Building outline
    The area within projection of all building parts onto a horizontal plane.

What should be presented on a 2D map: building outline or building footprint?

I’d like to have both like 2D representation of the Villa Mediterranee (mentioned above) on the image below:

The dotted area on the image represents building overhang.

However all 2D renderers don’t make a distinction between the building outline and building footprint.

A recommendation should be given for a mapper what to provide as a fallback for 2D renderers: building footprint or building outline.

I’m in favour of building footprint rather than building outline. Why?

Imagine that there are benches, trash bins, vending machines and other stuff on the public area beneath the overhang of the Villa Mediterranee. In the case of the building footprint, one would immediately see the public area with all the stuff on it.

Obviously there will be exceptions from this recommendation like the example with the suspended restaurant mentioned by Tordanik.

Exactly.

Let’s consider two building outlines B1 and B2 which have an overlapping area A.

Let’s assume the building B1 has a building part P1 which is entirely located within the area A.

It means that the building part P1 is located within both outlines B1 and B2!!!

The section Building outlines of the Simple 3D Buildings specification isn’t applicable for this case. A mapper should opt to the relation type=building.

How should we edit the the Simple 3D Buildings specification?

Basicallly, I propose to keep it as is right now. I’d add a disclaimer to the Building relations section, that it’s an advanced concept for experienced mappers. If a mapper is new to the Simple 3D buildings, he can safely skip that section. Actualy the concept of relations in OSM requires some mental efforts and practice time to grasp.