'Normal Mapping'
by Misja Baas

原文链接：http://www.3dtotal.com/team/Tutorials_3/normal_mapping/normal_mapping01.asp

Normal mapping is a great way of showing more detail on a 3D model by simulating the way surface detail responds to light. It is a 2D effect so it will not change the shape of an object, but inside the profile outlines it can simulate a tremendous amount of extra detail. This is very useful for real time game engines where processing power is a limiting factor or animations where it’s render time that can be the limiting factor.
（法线贴图是一种显示三维模型更多细节的重要方法，它解算了模型表面因为灯光而产生的细节。这是一种2维的效果，所以它不会改变模型的形状，但是它计算了轮廓线以内的极大的额外细节。在处理能力受限的情况下，这对实时游戏引擎是非常有用的，另外当你渲染动画受到时间限制时，它也是及其有效的解决办法。）

Original and normal mapped object.(原始物体和法线贴图物体）
Normal mapping is not new but with the latest graphics cards it’s become more and more affordable. It’s very similar to bump mapping in that they both achieve the same effect.However, bump mapping simulates vertical offsets relative to the face direction it’s projected on, new directions are a result of the height differences between neighboring pixels. Normal mapping on the other hand uses colors to indicate directional offsets, making it far more efficient. It doesn’t use it’s neighbors to determine it’s angle, a single pixel is enough. The red channel in a normal map encodes the left-right axis of normal blue channel encodes vertical depth.
（法线贴图不是什么新事物，但依赖与最新的显卡，它正变得越来越重要。要达到上图的效果，通过凹凸贴图也是很容易的事。但是，凹凸贴图仅仅解算在它相对于面投射方向的垂直偏移，新的方向是两个相邻像素之间高度差异的结果。法线贴图使用颜色来表述在其他方向的偏移，这使它更有效。它无需使用它相邻的像素来测定它的角度，一个单一的像素就足够了。法线贴图中的红色通道编码了法线的左右轴，蓝色通道编码了垂直深度。）
This makes for a pretty interesting image that is, unfortunately, impossible to paint properly by hand (unlike bump maps). Which means you have to generate a normal map, this is done in several ways, you can bake it, render it or convert a height/bump map.
（这些创造了一幅漂亮的图片，但不幸的是你不大可能用手来绘制合适的图（跟凹凸贴图不同）。这意味着你不得不生成一张法线贴图，有好几种方法可以做到，你可以烘培它，渲染它，或是从一张凹凸贴图转换而来。）

Source object and the target object partially enclosing it.
(源物体和目标物体要尽量靠近）

Baking is done by casting rays from the low poly surface (also called target since it will ultimately receive the normal map)  onto the high poly surface (also called source) , the direction of the rays is determined by the vertex normals, they can be cast in both directions, forward and backwards. If  you have a smooth surface (or one smoothing group) the normals are “averaged”, when you have a hard edged surface the normals point in the direction of the face normal, i.e. straight up from the face.
（烘培是通过从低多边形物体表面（也叫目标物体，因为它最终将接受法线贴图）投射光线到高多边形表面（即源物体）来进行的，光线的方向通过定点的法线来确定，他们能被投射到多个方向，向前或向后。如果你有一个平滑的表面（或是一个光滑组），法线将被平均化。当你有一个硬边缘的表面，法线方向是垂直于表面向上的。）

This is an important difference to be aware of when you start baking because when your edges are hard the vertex normals for one face point in a different direction than the vertex normals of it’s neighbors meaning you get “blind spots” in the projection.
（当你开始烘培时，需要意识到这是很重要的区别，当你的边是硬边是，对一个面而言，点的法线是指向不同的方向的，而不是指向它相邻的点的法线，这意味着在光线发射的过程中你得到了“盲点”。）

Another thing to be aware of is in order to bake a map properly every single polygon that is going to be included in the bake needs to have it’s own space in the UV layout. You cannot have overlapping UV’s since the software will not know which of the faces you want to be used in the calculation and instead will draw them all at once.
（另一个需要注意的是，为了烘培出一张完整的贴图，每一个在烘培中包含进来单一的多边形都必须有自己独立的UV。不能出现UV重叠，因为软件不知道哪个面是你想用在解算中的，因而会一次性的绘制出来。）

Since the target surface is one big, very complex  projector  you should treat every face on it as if it where an inverse UV projector, sucking up the image instead of projecting it. By looking through a face on the target mesh, as if it where a window, you should be able to see every polygon on your source mesh pointing more or less in your direction. If a source poly is perpendicular to you, or pointing away from you, it won’t be captured on the normal map. Another thing to take into account is that you need to see enough of each source poly so it can be given enough pixels to clearly define it on the normal map.
（因为目标表面是一个大的，非常复杂的放射体，你需要处理它上面的每一个面，例如在那些uv投影反转的地方，会使图像凹进去而不是凸起来。通过察看目标网格上的一个面，比如窗户那里的面，你将可以看到在你源网格的每一个多边形或多或少的指向你的方向。如果源多边形和你的目光垂直相交，或是指向远离你的方向，此时是无法捕捉到法线贴图的。还有一个需要考虑的是对每个源多边形你需要看的足够大，这样才能获取足够的像素在法线贴图上定义它。）


Check projection directions and visible surface detail.
（检察发射方向和表面细节的可见性。）

All in all baking is the most time consuming method but it will also give you the best results. It’s worth it!
（总之，烘培是最耗时的方法，但它将给最好的效果。这是值得的！）

World space: （世界空间）
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World space normal maps are great for objects that don’t ever move or deform like, say, the world.
（世界空间法线贴图适用于那些既不移动也不变形的东西，比如世界环境）


Tangent space (left) and object space (right) normal maps of the same cube.
（同一个方盒子物体的相切空间（左）和物体空间（右）法线贴图）

When the high poly version was finally ready I remodeled the entire low poly model from scratch snapping it’s vertices directly on the high poly model, of course you need to make sure you place them strategically, this can be a bit challenging when you also need to worry about your target mesh’s layout for deformation later. Another good reason to rough it out first.


Finished low poly model, 6611 polys

Anyway, back to baking: this is when the errors start to appear, look out for extreme and hard edged color differences, these can be fixed by painting them away in Photoshop (use the “normalize only” option in Nvidia’s Photoshop plugin
before saving) or pushing the target mesh in or outward a bit, globally or locally. Don’t worry too much about ruining your low poly objects shape, you can always fix this later when the normal map is finished, it may not be entirely
accurate but no one’s gonna notice. Besides you are an Artiste right? Whatever you made was intentional, even the unintentional.


Final tangent space normal map for the character.
（最终得到的角色的相切空间的法线贴图。）

Low poly character (left) and high poly character models.
(低多边形角色（左）和高多边形角色模型）