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karwosts

Normal Map for an Impostor?

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So I'm currently playing around with trying to model/render a tree. Using Blender at the moment with its tree plugin, which will generate the tree trunk and instantiate a bunch of leaf-objects. I've got a nice looking result at around a few hundred thousand polys, but I need to start drastically reducing the polygon count. I'm going to replace my real leaf with just a textured quad of many leaves. I can generate this color texture by just arranging a bunch of real leaf meshes and taking a render shot. Now I'd like to try to generate a normal map for this impostor as well, so that each texel will have a normal associated with it for more realistic lighting. However I'm having trouble imagining how to create this. Normally the process for normal mapping involves UV unwrapping the object and baking to a texture, for something like baking a high poly mesh to a low poly one. I don't really think its appropriate to UV unwrap this multi-leaf object though, because the final object doesn't have any vertices or texcoords, its just a single quad. What I'm imagining is that the normal map will look exactly like a color render of an object, except that instead of the object color for each texel it will have normal information. I'd appreciate it if anyone can share any ideas for how to create a normal map for an impostor like this. I'm sorry if this doesn't make much sense, I'm still trying to figure out in my head exactly what I need. I'll try to clarify if anyone has questions. Thanks for reading.

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Well, first I would take a look at the render options of blender, maybe you can render the normals to the screen, in this case it would be quite easy.

If this is not possible, blender is able to render the normals to a low-poly model, in this case it is really low poly (just a quad). You only need to unwrap the target model, not the source model. Try to do something like this:
1. Create high poly model (your tree).
2. Create low poly model (quad of the size of your tree).
3. Unwrap low poly model.
4. Assign empty texture to low poly model.
5. First select the high poly model, then the low poly (low poly will be the active one) and bake the normals to the active model.

Good luck.

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There is a little program by nvidia, think its called Melody. It takes a high res model and a low res model and creates a normal map that makes the low res look like the high res model (or pretty good anyway). I can't remember what model format it accepts/accepted, think it might have been .3ds and maybe .obj. You don't have to use the models afterwards, just the normal map it creates, I think it'll use the existing texture coordinates.

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