[Tile-serving] [osm2pgsql-dev/osm2pgsql] Flex: call `:get_bbox()` on geometry, not on parameter table (object) (Issue #2219)

[Marek Dorda]

This is not a bug report, but rather a feature request, or a request for advice.

According to your [documentation](https://osm2pgsql.org/doc/manual.html#processing-callbacks), method `:get_bbox()` is available for the parameter table (object). Which works great for the most cases. But I find it very limiting when I need to work with multipolygons.

I need to add centroid and bboxes to all (multi)polygons. It seems to be very straightforward:
```lua
function Place:get_geometry_bbox()
    if (self.object.type == 'node') then
        return nil
    end

    local xmin, ymin, xmax, ymax = self.object:get_bbox()
    if xmin == nil then
        return nil
    end
    return 'BOX(' .. tostring(xmin) .. ' ' .. tostring(ymin) .. ',' .. tostring(xmax) .. ' ' .. tostring(ymax) .. ')'
end

function Place:get_geometry_loc()
        return self.geometry:centroid()
end
```

and then just call it when writting:

```lua
function Place:write_row(k, v, save_extra_mains)

    -- some logic

    place_table:insert{
        -- more props
        geometry_bbox = self:get_geometry_bbox(),
        geometry_loc = self:get_geometry_loc()
    }

    -- some logic

    return 1
end
```

But it gets tricky when you need to work a bit more complex with multipolygons. Specifically: to get a centroid and bbox only from the largest part of the multipolygon.

It is easy for centroid:

```lua
function Place:get_geometry_loc()
    if not self.geometry:geometry_type() == 'MULTIPOLYGON' then
        return self.geometry:centroid()
    end

    -- for multipolygon return centroid of the biggest geometry part
    local max_geometry = nil
    local max_geometry_area = nil

    for g in self.geometry:geometries() do
        local geometry_area = g:spherical_area()
        if max_geometry_area == nil or max_geometry_area < geometry_area then
            max_geometry = g
            max_geometry_area = geometry_area
        end
    end

    return max_geometry:centroid();
end
```

But it cannot be done for bbox, becasue it is not a geometry method.

I admit that I have a very specific requirement, but purely from the logic of things, `:get_bbox()` seems to me to be a geometry thing and I would expect this method to be directly on geometry. Similar to `:centroid()` or `:area()` ([documentation](https://osm2pgsql.org/doc/manual.html#geometry-objects-in-lua)). It looks pretty inconsistent to me that way.

Did I miss something? Is there any way to achieve this?

Thank you so much!

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