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Tree billboards

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Haj, I'm thinking of a terrain engine. But I always bump my brain against a problem. Rendering trees I've read a couple of threads on this subject. They tell me to dynamicly update billboards of the tree. But if how do update them? Are the textures for the billboard pre created? Or do I render them once or more to a texture in-game? GBas

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Have you read all the articles in VTP.org (Virtual Terrain Project) ?

The state of art can't be easilly explained here. Take a look at SpeedTree to see the current state of art. It's not something you'll code in a few weeks.

The systems for tree rendering are very complex. They use a lot of dynamic caching, CLOD, 'seamless' transitions between various models from bilboards far to full meshes near, etc...

The main idea to bring power to tree rendering is use far less power and update rate for far objects. This is done by using CLOD and frame to frame coherency to the max.

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No, I really want to make a 'realistic' forest. I want to be able to walk tru it and say "Hey, look at that tree!". I want to make an interactive landscape. Water-sea, day - night, portals for houses and caves.
I know it's very (I'm sorry very very very!) difficult, but I'm still in highschool so I still have the time of my live ;).

If somebody knows good articels, books or whatever, please tell me. Or if you want to help me...

GBas

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The only thing I recall seeing was this report from the 2002GDC which talks in a high level manner of how to acheive what you're wanting.

Quote:

At this year's GDC we finally discovered that gone are the days of billboarded trees. Some very good ways of doing forests have been discovered and, judging by the results, technology is finally advancing in the right direction. Again, tree rendering can be a real problem if you consider the inherent complexity of a real tree. It is getting hard to do a realistic tree below 10,000 triangles, and you would need quite a few of those to create a forest scene.

A first approach (glimpsed at last year's conference) is to use layers of animated billboards. The key idea is to get very-high quality pictures of trees, branches and leaves, and then arrange them in Z-order so that, animating them, it looks like a complete, waving tree. This method requires very low triangle counts, and the results can be absolutely stunning. The downside is, as usual with billboard approaches, that the user will see the same tree regardless of the direction he's looking from. This can somehow be reduced by using some parallax trickery, but in the end billboards tend to look pretty bad when you need walk around them or, even worse, stay in the shadow of the tree.

A second interesting method was explained at one of the Xbox conferences. It basically combines alpha-blended LODs with some good old volume rendering concepts. The key idea is to have a very high quality tree model in memory (the demo used a 30,000 triangle tree) and then pre-compute slices of the tree, which are stored in texture maps. These slices resemble a CAT scan, and have to be axis-aligned. In the demo, 24 slices (8 in each axis) were computed at 256x256, 32 bit. Then a mipmapped version at 128x128 was also calculated for distant versions.

Then, at rendering time the model is only used in closer trees, so an average of 2-3 real trees are drawn. The remaining trees are rendered by drawing the stacked texture images pre-computed before, so a mid resolution tree requires only 24 quads. If the LOD threshold is set properly, the transition is unnoticeable, and the results are really convincing. Still, some interesting issues arise. First, it remains to be seen how animation can be incorporated into this model. Remember the sliced maps are pre-computed, and with a high-density model you simply cannot afford to re-calculate them per frame to accommodate animation. Second, this method delivers great results at the cost of high memory footprint. Even if we forget about the 30,000-triangle tree model, some fast math yields that a single tree would take up approximately 6 Mb of uncompressed texture data. Luckily, the scans have large transparent areas, making them ideal for on-the-fly compression.


Here's a bonus review of RealNAT for inspiration ;)

I'm hoping that all that was helpful in some small way, it's something I recall reading a while back.

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CLOD stands for continuous level of detail. Draw more near and less far. Replace polygonal models of leaves by impostors of several leaves for instance. Then ensure the transition is smooth (morphing) to avoid pop-up artefacts. This last issue is in fact the most subtle to handle.

The first thing to do is learn the common vocabulary used in outdoor rendering. It helps thinking faster and be more comfortable with some quite abstract and complex algorithms. The same underlying concepts appear in many fields, terrain, vegetation, water, etc... But in details the can be implemented very differently.

VTP is definitely the place to start. There are dozens of pdfs available on the web.

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You posted simultaneously probably.

It's a general concept. For instance, all these technologies use CLOD :

- A view dependent progressive mesh. For instance a human model with 50000 polygons near and 10 far. In between each time it gets further a few polygons are removed.

- the smooth transition between triangles then impostors is CLOD.

- mip mapping with trilinear filtering is CLOD. Each mip map is a lod. And the z filtering brings continuity.

- geomipmapping, ROAM, etc ... are all CLOD.

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