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Ai Monty

Real-scale planet terrain implementation

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Hi! First post here, although I've been following this forums for years. I couldn't find anything clear on this topic though, so there it goes.
 
I'm planning my first "medium scale" 3d game as a hobbyist programmer, an sci-fi space sandbox game, and I'd love to implement "complex" real-scale procedural planets. I've previously done some work with 3d simplex noise and marching cubes, but this seems an step higher. The terrain would be created from a seed and wouldn't be modified so there's no need to save it on disk, generating it on-the-fly instead. But there are other challenges.
 
My current idea is to use an octree to subdivide the area up to a desired LOD and calculate the density at each point, discarding branches not visible or not belonging to any geometry. In practice, however, an earth-scale planet using a LOD of just 10 meters requires about 21 subdivisions for the most detailed branches. May performance and memory requirements go too high there? Also, mixing geometry from different branches and/or at different LOD levels sounds like a source of gaps and holes in the resulting geometry, putting aside I'm not sure how would LOD perform with tunnels and overhangs. Even worse, if I generate LOD using different frequencies, the branch discarding algorithm would become (even more) complex and error-prone.
 
As a side note, I'd prefer to do it with C# and XNA 4.0, but if technical requirements arise, I can perfectly develop it on C++. Any suggestions on how could this be done? (Or any related topics I that could have missed?)
 
Thanks anyway,
Adrián Montesinos
 
 
PD: I'm not a native English speaker, so please forgive me for any linguistic errors.

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Check out the project 'Infinity - The Quest for Earth'

 

 

 here's their screenshots page: http://www.infinity-universe.com/Infinity/index.php?option=com_zoom&Itemid=90&catid=4&PageNo=1

 

 A full procedural planet engine, where you can fly up to a planet, through its atmosphere, and to the landscape itself.. There are videos here:

 

http://www.infinity-universe.com/Infinity/index.php?option=com_content&task=blogcategory&id=17&Itemid=93

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One of the problems with generating near-realistic detailed terrain for a water/land planet - the erosion patterns (air or water), the rivers/water drainage is a complex  and high processing cyclic mutation of the base seed generated data.    All those continuous features that are the cumulative result of interaction across a usually large area.  You often have to calculate the whole (hard to do it ''in parts' because of use of intermediate data from outside the desired 'part'.  If you suddenly need an adjactent 'part' to be generated it may turn out disjoint with the existing 'part' and you can get trapped in a non-deterministic series of 'patchings' to try to resolve the inconsistancies.

 

The ecosystems (including weather) are  dependant on altitude/slopes/latitudes/proximity to water.

 

Take a look at a topo map of the earth to see the complex  patterns (some river systems drain land areas of a significant percentatge of the surface)

 

Depending on how often you come and go from a 'planet' (or a set of them) you may want to build the resulting data and cache that to avoid the huge processing cost/delays.

 

Of course if that kind of detail level or need for cohesion in the extended patterns isnt needed, then generating the final terrain in 'parts' is possible (you just have to decide how much adjacent recursive stepped data is needed.to be 'good enough'.

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Eric Bruneton has done a lot of work on procedural water and land, I think he also has freely available code and publications for a procedural planet renderer, you might want to google that. At least his ocean demo really stuck in my head.

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the cube to sphere transformation is a good startingpoint. all you have to do is create a cube with tessellated sides and normalize (vertexposition - planetcenter) in the vertexshader. multiply this with the planetradius and your done.

regarding c#/xna: its possible, have a look at this:
http://www.gamedev.net/blog/1302/entry-2250847-bow-shock-a-summary-of-work-done-so-far/
it should give you a great overview of things that can be done.

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Thanks to everyone for all the responses, really!
 
I don't need the terrain to look near-realistic, I'm ok with it being attractive and giving a good feel of scale. Anyway, you've all pointed me to enough resources to have a clearer idea for an starting-point, so I'll start prototyping as soon as posible. If this little project goes on, I may end up opening a developer journal, who knows. I like such things!

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