enforce scalar part is non-negative

Author: petercorke

UnitQuaternion.m

@@ -195,6 +195,11 @@
                     v = formula(v);
                 end
                 n = norm([s; v(:)]);
+                if s < 0
+                    % enforce non-negative scalar
+                    s = -s;
+                    v = -v;
+                end
                 uq.s = s/n;
                 uq.v = v/n;
             elseif isvec(s,4)
@@ -203,6 +208,10 @@
                 if ~isa(s, 'sym')
                     s = unit(s);
                 end
+                if s(1) < 0
+                    % enforce non-negative scalar
+                    s = -s;
+                end
                 uq.s = s(1);
                 uq.v = s(2:4);
             else
@@ -1189,6 +1198,7 @@ function animate(Q, varargin)
             uq = UnitQuaternion(s, qv);
         end
 
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %%%% OTHER STATIC METHODS, ALTERNATIVE CONSTRUCTORS
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%