The following code snippet generates a one and a zero for my "vis" value shown in the DBG_MSG0 call (is_visible is an argument to the function declared as integer):

// force into a boolean 0/1 value

integer b4 = is_visible;

is_visible = is_visible != 0;

DBG_MSG0(["S1: change_piece_vis", full_piece_id, "vis", is_visible, b4, "link_num", link_num]);

b4 is printing out as zero, while is_visible ends up being one. The "skid marks" that are the result of this action in my scripts look like this:

set_body_flags 8 0 NEW body_vis_flags 7

S1: change_piece_vis 25 vis 1 0 link_num 57

The first skid mark is shows that body flags was indeed updated to have a zero at the relevant position confirming that the change_piece_vis code should be getting a zero value which is what b4 has stored.

So the "is_visible != 0" clause is not generating the right value.