def sigmoid(axis, slope):
with float_error_ignore():<EOL><INDENT>return (<NUM_LIT> / (<NUM_LIT:1.0> + np.exp(-<NUM_LIT>*slope*axis))) - <NUM_LIT:1.0><EOL><DEDENT>
Sigmoid dividing axis into a positive and negative half,
with a smoothly sloping transition between them (controlled by the slope).

At default rotation, axis refers to the vertical (y) axis.