Not enough building merging & simplification

[zerebubuth]

Building heights and extra properties (e.g: roof_shape or roof_colour) are preventing merging between buildings. For example, tile 14/6066/9294 in the north-west of Sao Paulo:

(Coloured uniquely by feature.) There is some merging going, but still leaving us with over 9,000 unique multipolygons. Partly, this is because we're not quantising height enough:

(There are 5,406 unique height values.) Even when we zoom out, and are quantising values (although not all?), there's still a lot of merging inhibited by neighbouring building heights differing by more than 5m.

It also looks like we're not quantising heights for buildings (should they be building:parts?) with min_height or layer != 0.

Also, we have quite a lot of small courtyards within buildings which probably aren't visible at this zoom level.

Finally, in this zoomed-in block from the 13/3033/4647 tile, you can see there's not a lot of merging going on between building geometries (even if they might be merged into the same multipolygon feature), so we're wasting a lot of bytes to describe each individual polygon.

In summary:

• Drop superfluous properties, e.g: roof_material, and quantize height more aggressively at lower zooms for smaller buildings.
• Make sure we're quantising heights for buildings with layer or min_height.
• Drop small courtyards within buildings.
• Dilate or buffer building geometry slightly while merging to try and merge adjacent buildings with very small gaps between them.

[nvkelso]

Looks like the Mapzen house styles hide many buildings at zooms 13 and 14, indicating to me we can drop scale_rank 3,4,5 at zoom 13 and scale_rank 4,5 at zoom 14.

        # building footprints, pre-extrusion
        footprints:
            filter:
                # show footprints for buildings at least one zoom level before they will be extruded
                - { $zoom: 13, scale_rank: [1,2] }
                - { $zoom: 14, scale_rank: [1,2,3] }
                - { $zoom: 15, height: { min: 100 } }
                - { $zoom: 15, area: { min: 500 } }
                - { $zoom: 15, volume: { min: 100000 } }
                - { $zoom: 16, area: { min: 100 } }
                - { $zoom: 16, volume: { min: 50000 } }
                - { $zoom: { min: 17 }, area: true }

[nvkelso]

Connects to #1689.

[nvkelso]

zoom 13

Smattering of building removed (okay), much more merging (great!)

SF: 13/37.7607/-122.4357

zoom 14

Tons of tiny building removed (sad but okay), much more merging (great!), tiny file size (great!)

SF: 14/37.7822/-122.4300

zoom 15

All the tiny building kept (good), much more merging (great!), smaller file size (great!)

SF: 15/37.7835/-122.4275

zoom 16

All the buildings, with their IDs (no merging, good), similar file size (okay, improve another day possibly thru geometry generalization)

SF: 16/37.7796/-122.4229

Future self:

All using this color lookup function:

                        color: |
                            function() {
                              if( feature.id ) {
                                  var letters = '0123456789ABCDEF';
                                  var color = '#';
                                  for (var i = 0; i < 6; i++ ) {
                                      color += letters[Math.floor(Math.random() * 16)];
                                  }
                                  return color;
                              } else {
                                  if( feature.scale_rank == 6 ) {
                                      return [1.,0.,0.,1];
                                  } else if( feature.scale_rank == 5 ) {
                                      return [0.,1.,0.,1];
                                  } else if( feature.scale_rank == 4 ) {
                                      return [0.,0.,1.,1];
                                  } else if( feature.scale_rank == 3 ) {
                                      return [0.,0.,0.5,1];
                                  } else if( feature.scale_rank == 2 ) {
                                      return [0.,0.5,0.,1];
                                  } else if( feature.scale_rank == 1 ) {
                                      return [0.,0.5,0.,1];
                                  } else if( feature.scale_rank == 0 ) {
                                      return [0.5,0.,0.,1];
                                  } else {
                                      return [0.5,0.5,0.5,1];
                                  }
                              }
                            }

[nvkelso]

A few comments:

First: this change looks visually great for Bubble Wrap and hugely reduces file size. Huzzah!

But there seem to be 2 issues:

• scale_rank is always 1 now instead of range of 1 to 5. -- regression we should fix
• zoom 14 missing some buildings -- not a ship block issue, but we'd likely fix it fixing the regression, and it's important for a style like Refill which we've compromised a bit.

To clarify, I think zoom 13 looks better in this new setup (dropping more buildings), but zoom 14 looks worse (drops slightly too many buildings).

I've filed #1732 to dig into that, and we can close this original issue which is huge step forward :)