Deck.gl: Polygon Layer documentation - coordinate specification

[SymbolixAU]

Background

Polygon layer coordinate specification

The documentation for polygon layer specifies the following data format

/**
   * Data format:
   * [
   *   {
   *     // Simple polygon (array of coords)
   *     contour: [[-122.4, 37.7], [-122.4, 37.8], [-122.5, 37.8], [-122.5, 37.7], [-122.4, 37.7]],
   *     zipcode: 94107,
   *     population: 26599,
   *     area: 6.11
   *   },
   *   {
   *     // Complex polygon with holes (array of coords)
   *     contour: [
   *       [[-122.4, 37.7], [-122.4, 37.8], [-122.5, 37.8], [-122.5, 37.7], [-122.4, 37.7]],
   *       [[-122.45, 37.73], [-122.47, 37.76], [-122.47, 37.71], [-122.45, 37.73]]
   *     ],
   *     zipcode: 94107,
   *     population: 26599,
   *     area: 6.11
   *   },
   *   ...
   * ]
   */

I've just generated an example which has coordinates in the form of

{
  "geometry":{
    "type":"Polygon",
    "coordinates":[
      [
        [-162.96333331843185, 23.963333318432139,-94860.0,1.0],
        [-162.89693422519478, 23.963333318432139,-95135.0,1.0]
        ... etc
     ]
   ]
}
}

Where the elevation is specified as the 3rd value in each coordinate array (as per GeoJSON).

Which gives

Should the documentation reflect that you can specify the elevation in the coordinate array, as well as in the getElevation prop?

And is this a valid way to supply the elevation?

---

Related issue from my mapdeck library

Update

The same works for pathLayer too, where you can specify the elevation in the coordinates [ ] array

---

To Do List

• [ ] Add label and assign to milestone
• [ ] Coding
• [ ] Doc update
• [ ] Whats new update
• [ ] Test

[Pessimistress]

It is totally valid. Think of z values in the polygon array the "base" elevation and getElevation the amount of extrusion. This question has came up before and we should clarify it in the docs.

[SymbolixAU]

Excellent.

Think of z values in the polygon array the "base" elevation and getElevation the amount of extrusion.

This is a key point of clarification and would be useful in the docs.

[mdsumner]

@Pessimistress is extrusion then relative to each base vertex, so also tited at the top for a triangle? And a bit nonsensical for non triangle polygons, or does triangulation get applied implicitly?

[SymbolixAU]

@mdsumner I had the same question - hopefully this helps

Setting up some data (in R)

poly1<- matrix(c(0,0,0,0,1,1000,1,1,50000,1,0,25000,0,0,0), ncol = 3, byrow = T)
poly2<- matrix(c(0,0,0,0,-1,1000,-1,-1,50000,-1,0,25000,0,0,0), ncol = 3, byrow = T)

poly1 <- sf::st_sfc( sf::st_polygon( x = list( poly1 ) ) )
poly2 <- sf::st_sfc( sf::st_polygon( x = list( poly2 ) ) )

poly1 <- sf::st_sf( geometry = poly1 )
poly2 <- sf::st_sf( geometry = poly2 )

sf <- rbind( poly1, poly2 )
sf$id <- 1:2

The polygons have a Z attribute. I'm making two plots, one using the extrusion, and one without

Non-extruded

Just using the Z attribute on the data

mapdeck() %>%
    add_polygon(
        data = sf
        , fill_colour = "id"
    )

Extruded

uses getElevation to extrude the polygon from the map

mapdeck() %>%
    add_polygon(
        data = sf
        , elevation = 100000
        , fill_colour = "id"
    )

So the Z is applied for each vertex. The extrusion is applied to the whole polygon, while respecting the initial Z elevation (as both the base and top surfaces are tilted)

[mdsumner]

Thanks! (nice!!!)

Those top-faces do look like implicitly triangulated quads to me, see how it's kind of saddle-looking? I bet there's an earcut triangulation going on, I'll try with more complex polys

[Pessimistress]

This is a key point of clarification and would be useful in the docs.

@SymbolixAU May the documentation use the lovely example image in your original issue?

[SymbolixAU]

@Pessimistress of course it can. Would you like me to send a higher-res version, or are you happy grabbing the gif directly from the original post? And would you like the underlying data?

(@mdsumner - assuming this is OK with you as it's your example originally)

[mdsumner]

No problem!

[Pessimistress]

Docs updated http://deck.gl/#/documentation/deckgl-api-reference/layers/polygon-layer