The disassembly looks fine to me.  All the jumps calls and everything of that nature are aligned.  I tried 4 test cases with different combinations of sign values and they seemed okay.  Since the generated picture is relatively close to the original given that it was a chaotic system sensitive to errors, I would guess it is only a few special cases that cause it to return a wrong result.

EDIT: I made a program to run them side by side on random numbers and quit when the output is different.  Here is an output that gives different results between the routines:

$FFE0 ** $F5F1 (-0.125 ** -10.059)

Results in $0143 (1.26) in size optimized.
Results in $0239 (2.22) in speed optimized.