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At Page 707,5th paragrah.it write:If a normal vector is stored as three 32-bit floats,it has enough accuracy to point from Earth to a rock on Mars with sub-centimeter precision.
I think the float vector can represent the distance from Earth to Mars,but it can not in sub-centimeter precision.
Eric Hanies给我的回复：
We‘re quoting from this work:
[1207] Sowizral, Henry, Kevin Rushforth, and Michael Deering, The Java 3D API Specification, Addison Wesley, 1997.
This is online here:
B.8
Normal Representation and Quantization
Probably the most innovative concept in compressed geometry is the method of compressing surface normals. Traditionally, 96-bit normals (three 32-bit IEEE floating-point numbers) are used in calculations to determine 8-bit color intensities. Theoretically, 96 bits of information could be used to represent 296 different normals, spread evenly over the surface of a unit sphere. This is a normal every 2-46 radians in any direction. Such angles are so exact that spreading out angles evenly in every direction from earth, you could point out any rock on Mars with subcentimeter accuracy.
There are typos on this page, it should say "2^96", not "296", and "2^(-46)", not "2-46". See the original paper http://michaelfrankdeering.com/Projects/HardWare/CompressedGeometry/SigGeometryCompression.pdf for the proper passage.
Note that he is not specifying the distance to Mars, he is saying that you could point in a direction from the Earth to a rock on Mars with subcentimeter accuracy. If you can show that this is incorrect, let us know and also write Michael Deering and discuss it with him.
By the way, a good recent article about normal compression methods is http://jcgt.org/published/0003/02/01/
总结：原文的意思是说从地标发出的射线夹角，即使到达火星后，也能在火星表面形成不到半厘米的扩散。我一开始理解成以地心为坐标原点，建立笛卡尔坐标系，在火星表面按坐标指示石块位置了。
At Page 818，Equation 17.12，the last parenthesis didn‘t match the first brack.Did it have another mean or only a mistake?
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原文地址：http://www.cnblogs.com/ptolemaeus/p/RealTime_Rendering_Understood_QA.html
