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3. Texture Baking and Surface Transfer
5. Links and More Useful Stuff
If you made your high resolution model using edge looping then creating your low resolution model is as easy as removing the edge loops you don't need. I need to write up some general rules for the low poly model and I am also writing a section on Headus UV. But for now, I want to cover the areas that give people the most trouble when setting up real-time game models for surface transfer, baking modular pieces and dealing with mirrored UVs.
Mirrored UV's and Texture Baking
Mirrored UVs are generally problematic when you're texture baking because you get seams or flipped normal maps etc. The information below shows you how to avoid these problems using Maya (and it probably pertains to most other 3D applications as well). In general if you have the space on your sheet to layout all of your UV's without mirroring your job is easy. However, in games they tell me that efficiency is key (sarcasm). So inevitably you'll need to mirror UV's or build modular components.
Geometry Mirroring, Seam and Winding Order
If you are mirroring your geometry you need to make sure that you have the mirrored piece in place during the bake. If you don't you'll get a nasty seam because the normal map won't bake correctly. The reason you need both pieces in place is so the normals can smooth across the seam and not appear "hard". Check this example of the wrong way:
Now check out this example of the right way:
But that's not all you have to do to make sure your seams bake correctly! Winding order also matters a great deal. If your winding order is wrong, your normal map will bake out wrong. If both halves match but are reversed, your normal map will also be wrong. If the halves do not match, wrong again. Both halves need to match and be correct or front facing. Winding order can be viewed (and matching checked) in the UV texture editor. Here's how to get it right:
Here are examples. The image below is incorrect. Red means "back faces" which will get you in trouble.
This image is also incorrect. The winding order is inconsistant from one side to the other.
This winding order is correct! Both sides match and they are front facing.
If your UV's are set up as above it should work great. Keep in mind that I'm showing both halves of the UV's here which is what you need to get a good bake. Later, once baked, you delete the half you dont need then mirror the geometry to get your identical and overlapping UVs. That saves you a bunch of space and your resolution will be twice as good.
Modular Components and Ambient Occlusion
This is pretty straight forward but takes forthought and a tad of strategy. Basically, if you're going to build things modularly, you need to bake the pieces together so you don't get seams (like above) and so the pieces have the correct occlusion. I'm going to build a simple wall here to illustrate the point. A wall might not be the best example, but it's fast for me to build...)
Say your wall is made up of a cap and a front piece that you want to use to build a big old wall section in the manner shown here. The black piece is the part I want to be modular:
If you bake it as illustrated in the image below (separate) you will get artifacts because the ambient occlusion will not match and because (if this were a more detailed piece) the normals would be hard along that edge.
The image below shows that there is no ambient occlusion under the overhang and that the edges don't match due to light leaks around the edge. I'll be the first to admit this isn't much of a problem here, but as your modular components get more complex it becomes harder to retouch in Photoshop. Placing your pieces to be baked as intended in the level solves these issues.
Sooooo...simply bake your pieces out putting the components together before baking like this:
The when you bake all of your occlusion works and you won't get artifacts. Simple right?
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