Multiple exposure options have a cumulative effect. This allows the exposure to be controlled absolutely, without any averaging. Note that a single pass is much quicker and should be used whenever the desired exposure is known and star patterns are not required. Use two passes on the input. This is the default.
Box filtering averages the input pixels corresponding to each separate output pixel.
This option with a radius around 1 and a reduction in image width and height of 2 or 3 produces the highest quality pictures. A radius greater than 1 results in a defocused picture. This option may be used to mitigate the problems associated with inadequate image sampling, at the expense of a slightly blurred image. This option implies the -r option for Gaussian filtering, which defaults to a radius of 1. N Set the number of points on star patterns to N. A value of zero turns star patterns off.