What is the purpose of the XXX_3rd variables?
Rich asked: I'll have to study the code to remember the exact definition, but it's related to the fact that the zoom box can be rotated. It's probably a coordinate of what was the upper right corner of the zoom box before it was rotated. (It is sufficient to specify three corners of the zoom box to gain full generality, which includes non- rectangular zoom boxes.) I am sure some of the fractal artists will howl at any loss of functionality, but I think rectangular zoom boxes that may be rotated are sufficiently generalized - not sure we need non-rectangular parallelograms. I am sure there are some places in the code where this is made clear. Let me add that generality of zoom boxes also complicates calculations related to automatic transitions from one kind of precision to another. Tim