Hi everyone,
next step of my project is to make the game look a little realistic! I want my player to think he is in the middle of the rainforest on the amazonas!
Now I am looking for the best way to do this! I heard and read something about billboards. Do I just simply create walls and put a texture with a rainforest on it? And then use some 3D Models of trees in front of them to make it more realistic!
So I thought about this:
Do you think this is the best way to create a realistic feeling? Or is there something different I should look at?
If this is the right way of doing it I got one more question: What is the easiest way to place them? Can I use an algorithm or do I have to place every wall manually?
Thanks 
I think the easiest way how to place them, depends on your level editor. Will you use a tile editor, stay with the hightmap or combine both?
There is a test called TestTerrainTrees, which uses SharedMeshes as optimization. Perhaps this could be also interessting for you? Probably not for all your trees, but perhaps the first row?
Well the concept you point out is not really billboarding. Often in games, a forest area that is enclosed employs your idea - the player is "roped" off by 3D trees and just beyond that to give an illusion that there's an expansive forest, there are walls (with varying degrees of complexity) of vegetation textures.
It's a simple solution in my opinion, I bet it could probably be procedurally done rather easily, but it probably would take a lot of custom work to make it look more than just a barrier. A more expansive feeling you'd probably use this in combination with other effects.
Personally I don't like it, because I don't like being boxed in…BUT it depends on the game. For an RPG I don't like it…for simple games like an arena death match in the heart of the amazon…doesn't really matter. I like to be able to roam in an RPG…but if you're wanting to get Oblivion (or the like) quality, that's a lot of work.
I would say unless you are generating your river and map programatically, then place things manually. You are likely to get better results and it will very probably take you less time too in the end.
As Starnick suggests, you have a trade off with quads - you should get more coverage for a smaller performance hit than modelled vegetation. But the illusion will not hold up very well at close distance. If your view is constrained to the water then using quads may be a good option.
In those tight bends it's likely that quads won't look too realistic… you're getting right up close and going all the way around them. You might be better using some modelled trees in those areas, or at least some quads arranged in a +.
The quads will look best when they're off to the side a bit and you're going past them at speed, so they are in your peripheral vision. A feeling of depth will be enhanced with some parallax, so I would try some modelled vegetation close to the shore, some quads at medium distance, and also perhaps bake some distant trees into your skybox. This could give you a decent feeling of motion, a good illusion of density and not too much of a performance hit.