Map in a cube #86

espinielli · 2016-12-29T15:45:37Z

I am trying to add another polyhedral map projection.
I took inspiration from Furuti's Cubic Globes.

I extended render-world accordingly and I get the following "piece of art" ;-)

If I take away the clipping part in render-world I see this other mess:

So things seems to be somewhat in place but maybe clipping needs special treatment...
Any helps?

First attempt to add a new polyhedral map projection, i.e. Reichard map projection as described by Furuti at http://www.progonos.com/furuti/MapProj/Normal/ProjPoly/projPoly2.html

mbostock · 2016-12-29T16:13:57Z

You will need clipping to an arbitrary spherical polygon (or at least the outline of the unfolded cube). Jason implemented this for his Airocean World reproduction, but alas he did not open-source the implementation so it is not available. (He also has a demonstration of great circle arc intersection.)

espinielli · 2016-12-31T15:34:44Z

It looks like the clip to an arbitrary spherical polygon is needed by other (requested) map projections, #67 , #25.
Out of curiosity, what did prevent Jason from contributing it to D3? Was the implementation not generic enough?

I think Jason outlined some issues and possible solutions in Geographic Clipping/Spherical Polygon Challenges.

PS: Speaking of the great circle arc intersection you mentioned, I did write a post to document my search of the "rationale" for the algorithm used by Jason (and exchange some emails w/ him).
By the way, I miss the point about you mentioning "great circle arc intersection" in this thread/PR...

espinielli · 2016-12-31T15:49:06Z

Is this code a D3's implementation? It looks like an old implementation...?

mbostock · 2016-12-31T17:11:09Z

That code looks like an old version of src/clip/polygon.js. D3 can clip spherical polygons to a great circle or to the antimeridian, but it cannot clip spherical polygons to another spherical polygon. If you want to implement the equivalent of Weiler–Atherton clipping on the sphere then you’re going to need to be able to compute the intersection of geodesics, which is why I mentioned it.

espinielli · 2017-01-15T22:04:27Z

From the code of Cahill-Keys (including polyhedron.js from Jason) I got a working Earth in a Cube.

It is a D3v3 version but it shows that polyhedron.js contains enough ingredients to make things work.
Again that code was from @jasondavies ...I do not know under which license it can be used/ported to D3v4 or if I should even take it away from my blocks...

mbostock · 2017-01-15T22:08:14Z

v4 supports polyhedral projections:

https://github.com/d3/d3-geo-projection/blob/master/README.md#polyhedral-projections
https://github.com/d3/d3-geo-projection/blob/master/src/polyhedral/index.js

Can you implementing it using the v4 API?

espinielli · 2017-01-15T22:36:03Z

Not possible. The modified D3v3 version from Jason had a d3.geo.pointInPolygon(point, polygon) which is not in D3v4.

mbostock · 2017-01-15T22:56:00Z

Isn’t that the polygonContains function? I’d be fine exposing that as d3.geoPolygonContains.

mbostock · 2017-01-15T22:58:28Z

Though that one requires the input coordinates are in radians, and it’d probably be worth generalizing it to d3.geoContains(feature, point), where feature is any GeoJSON feature (with coordinates in degrees).

espinielli · 2017-01-17T11:06:37Z

I tried by just generating a version of d3.geo that exposes polygonContains.
Using all v4 libraries I get a map without proper clipping:

So clipping spherical polygons is really needed...

Fil · 2017-01-17T13:12:04Z

very cute :)

on geoPolygonContains, see also d3/d3#2662 (comment)

espinielli · 2017-01-17T15:40:06Z

@Fil yes I saw it...but wanted to try the simple things first...not sure I am able to follow up on @mbostock's suggestion.
BTW polygonContains takes coordinates in degrees, see the relevant tests.

mbostock · 2017-01-17T15:42:12Z

So if an equivalent to d3.geo.pointInPolygon is available as d3.geoContains (or the internal polygonContains), what API is missing to make this work in v4? Or is it that the replacement for pointInPolygon is not working correctly?

mbostock · 2017-01-17T15:44:01Z

I have opened a feature request for d3.geoContains, d3/d3-geo#73, since that has come up twice now and seems like an obvious win.

espinielli · 2017-01-17T16:03:19Z

What I noticed is different is the implementation of recurse in polyhedral.js compared to the one in polyhedron.js.

The latter does something with ring... and outline uses it too

Fil · 2017-03-07T09:21:17Z

Hey I have put a lot of effort into this and I have to report I'm stuck at about the same point as @espinielli 😿

For fun I chose the third map in the Furuti page. Building the cube and faces is OK, and much simpler now that we have geoContains.
https://gist.github.com/Fil/49fc5cd4c6ba54465e23c2c6544ff1d3

However, I wasn't able to find how to avoid lines going straight from one face to the other (which is utterly disastrous when it's not going through a shared edge).

Here's what it gives with render-world.

I suppose the answer is some kind of polygon clipping to the set of faces, as Enrico demonstrated in his d3v3 version, but it escapes me how to implement it :(

(After all this time I've spent in d3-geo, I must say its code and operation are still sometimes quite difficult to grasp.)

mbostock · 2017-03-07T16:54:37Z

Have you looked at Jason’s implementation of clipping to an arbitrary spherical polygon? It’s in the clip-polygon branch of d3. It’s four years old and the code has since moved to d3-geo, and as you say the code can be difficult to follow, but it might be helpful in understanding what is necessary.

Fil · 2017-03-07T23:21:15Z

I've spent a few more hours on this, now I think I'm beginning to understand a few more things, but still no success. The first step would be to expose the old clipPolygon in d3-geo in a way that would allow to use projection.clipPolygon([polygon]).

But all the names have been changed, it's a strange maze.

If I summarize we now have:

d3_geo_clip => exposed as clip
d3_geo_clipPolygonRejoin => exposed as clipPolygon
d3_geo_clipPolygon => unported (hard); it needs d3_geo_clipPolygonSort d3_geo_clipPolygonDistance and more ; would have to be renamed as clipPolygonSet or something
projection.clipPolygon => unported (relatively easy)

I've begun some babbling here
https://gist.github.com/Fil/dd1383ddd8669c8c1b97d5ca5dce1035

good night

mbostock · 2017-03-08T00:29:50Z

If it helps, here’s a compare view for those two commits. The main new things are:

src/geo/clip-polygon.js - to clip spherical geometry to an arbitrary spherical polygon.
src/geo/intersect.js - to compute the intersection of two geodesics (spherical line segments).

v4’s src/clip/polygon.js corresponds to the clip-polygon branch’s src/geo/clip-polygon-rejoin.js. A bunch of stuff was renamed in the great port to ES modules, but you can see that the two still have similar structure.

One confusion is that in v4 the “polygon” refers to what is being clipped, whereas in the clip-polygon branch the “polygon” refers to how the geometry is clipped. That is, in v4 it’s an abstract algorithm for clipping polygons which is used to implement both circle and antimeridian clipping, whereas in clip-polygon it’s an implementation for clipping spherical geometry to an arbitrary spherical polygon.

mbostock · 2017-03-08T00:40:56Z

One of the difficulties in understand this code is moving between the concrete (e.g., src/clip/circle.js) and abstract (e.g., src/clip/index.js) clipping algorithms. I wonder whether things could be simplified, at the cost of some code duplication, by foregoing any generalized abstract clipping algorithm and having separate implementations for different clipping methods (antimeridian, circle, spherical polygon). That is, a more compositional approach rather than trying to subclass an abstract implementation.

mbostock · 2017-03-08T00:44:35Z

Related to the preceding comment, one of the changes Jason made was to require an intersection sorting function be passed to the abstract clipping algorithm: the one in v4 uses a sorting function that only works for circle or antimeridian clipping.

Fil · 2017-03-08T09:25:26Z

I've made a block to help visualise the type of polygons we're dealing with.
https://bl.ocks.org/Fil/694ba0d0bc1fc4c24eb257dc210eb01a

PS: maybe it would be necessary to open a new issue for this, in d3-geo?

mbostock · 2017-03-08T15:04:51Z

There’s already an issue for clipping to an arbitrary spherical polygon, d3/d3-geo#46. I think that’s the primary blocking feature for more polyhedral projections.

Fil · 2017-07-23T16:37:36Z

I've managed to incorporate Jason Davies' old clip-polygon branch into d3v4, and it ~~sort-of works in some cases~~ works.

more later…

espinielli · 2017-07-24T15:27:26Z

This opens the flood's gates!

Fil · 2017-07-24T21:27:10Z

Pull-request for clip-polygon opened at d3/d3-geo#108.

For this projection and others of the Furuti collection, let's maybe create a family rather than create them one by one?

Fil · 2017-09-30T20:19:37Z

Closing this in favor of #129 — will be solved after d3-geo/clip (d3/d3-geo#115) is merged and published to npm.

Note that d3-geo-polygon is still unstable. When we stream points that belong to the clipping polygon itself, it sometimes bleeds out -- for example on the Waterman projection with a default aspect -- rotate [0,0]). I have been as conservative as possible: d3-geo-projection works as usual without d3-geo-polygon, and checks that it has projection.preclip (d3-geo > 1.8.1). Examples and issues at [d3-geo-polygon](https://github.com/d3/d3-geo-polygon) Solves #129 Solves #124 Solves d3/d3-geo#108 Solves #86

espinielli added 2 commits December 17, 2016 19:41

first (naive) attempt

9863d33

Reichard "Earth-in-a-box" map projection

d04afb9

First attempt to add a new polyhedral map projection, i.e. Reichard map projection as described by Furuti at http://www.progonos.com/furuti/MapProj/Normal/ProjPoly/projPoly2.html

Fil mentioned this pull request Aug 5, 2017

Clip polygon #124

Closed

Fil closed this Sep 30, 2017

Fil mentioned this pull request Oct 1, 2017

[Obsolete] d3.geoPolyhedral() now uses d3-geo-polygon if the plugin is available. #130

Closed

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Map in a cube #86

Map in a cube #86

espinielli commented Dec 29, 2016

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Map in a cube #86

Map in a cube #86

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