Documentation Talk:Reference Section 2

rand(I) Returns the next pseudo-random number from the stream specified by the positive integer I. You must call seed() to initialize a random stream before calling rand(). The numbers are uniformly distributed, and have values between 0.0 and 1.0, inclusively. The numbers generated by separate streams are independent random variables.

seed(I) Initializes a new pseudo-random stream with the initial seed value A. The number corresponding to this random stream is returned. Any number of pseudo-random streams may be used as shown in the example below:

#declare R1 = seed(0);
#declare R2 = seed(12345);
sphere { <rand(R1), rand(R1), rand(R1)>, rand(R2) }

Multiple random generators are very useful in situations where you use rand() to place a group of objects, and then decide to use rand() in another location earlier in the file to set some colors or place another group of objects. Without separate rand() streams, all of your objects would move when you added more calls to rand(). This is very annoying.