I want to create a scene which has several trees.
Maybe ,texture and Billboard is a good way,but if
we come closer to the trees,the tree is obviously
dummy.
Do you have a better way to render a real 3D tree?
1000 ways. Some algorithm use fractals to create trees geometry. This tpyically leads to huge amounts of data, so take care!
We simply use 3 pair-wise crossed, textured quads which are bundled to woods of 100 trees or so. This way we can display 10000 trees very quickly. But if you get nearer, you can clearly see how they are constructed.
If you want full 3d models you might just make 10 different tree models in a normal model editor - I believe this is what games like WC3 and Morrowind do. You could also perhaps find a happy medium by creating a bunch of branches and trunks in a model editor and then creating random trees by mixing the sections in different orders. Really the world is your oyster on this one- there are a whole lot of different ways to do it and no single “right” answer.
One technique I have found interesting was to prepare ‘slices’ of tree from various directions. As far as I remember, they have used 5 parallel slices, at 5-6 different angles. Then, when avatar approaches to tree, you display two 5-slice-sets most perpendicular to view direction. If you are exactly perpendicular to one of slicesets, you get almost perfect tree. Depending on angle, you alpha blend two slices - to 50/50 ratio, if two slice sets are equally good for displaying.
This allows you to display a tree with 10 quads, unfortunately taking considerable amount of texture space. Of course, this technique should be only used for leaves - trunk can be done by normal means.