New projections

New projections

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Sun Sep 8 21:35:00 2019   by   terkuavy
new map projections should be added (van der griten, larivee, eisenlohr)
 
Mon Sep 9 10:40:10 2019   by   Torben
Adding new projections is quite a bit of work, so it is not something I am likely to do.  But you are welcome to download the source code and add them yourself. I will be happy to merge your code into the public code when you do so.

Note that the planet generator is (mostly) a hobby project, so any extensions that are not publishable in peer-reviewed scientific forums are done in my free time.  Adding more projections is not scientifically interesting. Adding rivers is, and I have used spent many hours to try do do so, without success.  But if I do succeed, this will probably be publishable.
 
Mon Sep 9 15:43:25 2019   by   overlord
In regard to rivers, I presume the complications go beyond simply making the rivers flow from higher to lower elevations? What kind of problems are you running into?
 
Tue Sep 10 10:04:01 2019   by   Torben
One problem is the ability to zoom in on a detail without generating the full planet map at this resolution.  This means that a river might flow into and out of a detail map, and you need to know exactly where without generating a detailed  river for parts outside the shown map.

This means that you need to generate the rivers using the same method as for generating the map: Recursive subdivision. I have a method that works for flat map generated by recursively subdividing triangles: A triangle edge may or may not have a river, and when dividing one triangle into two, a river on the split edge moves to one or the other half edge (using a deterministic algorithm), and it is decided whether a river should cross the new edge (that splits the triangle).  Whether or not there is a river affects the altitude of the new vertex, and the altitudes of vertices affect whether or not a river crosses the new edge.  This works reasonably well.

The planet generator works by subdividing tetrahedra. The natural extension is that rivers cross faces of a tetrahedron, but the way the tetrahedron subdivision works means that this can not affect the altitude of the new vertex.  The reasons are complicated, but they are there.  I have tried putting rivers on edges instead, but this has two problems: One is that an edge crosses the planet surface in one point only (or not at all), and the other is that one edge is shared between many tetrahedra, where and edge in the flat case is shared only by two triangles.  This means that the direction of flow is not obvious.  I have tried various fixes to this, but none of them give convincing rivers -- either they disappear underground, or they turn into extremely complex networks that cover most of the surface.  I haven't given up, but I need a new idea to make it work.  It may require a complete change in the basic subdivision algorithm.
 

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