The keyword is followed by 4D vector and a float separated by a comma. You can get a good feel for the four dimensional nature of a julia fractal by using POV-Ray's animation feature to vary a slice's Distance parameter. You can make the julia fractal appear from nothing, grow, then shrink to nothing as Distance changes, much as the cross section of a 3-D object changes as it passes through a plane. The precision parameter is a tolerance used in the determination of whether points are inside or outside the fractal object.
Larger values give more accurate results but slower rendering.
Use as low a value as you can without visibly degrading the fractal object's appearance but note values less than 1.0 are clipped at 1.0. The default if none is specified is precision 20 . The presence of the keywords quaternion or hypercomplex determine which 4-D algebra is used to calculate the fractal. The default is quaternion . Both of these algebras were discovered in the 19th century.