keep track of initial radius, for deformation

spheroid.cpp

@@ -5,7 +5,7 @@
 #include "spheroid.hpp"
 
 Spheroid::Spheroid(Point& _center, size_t _p, size_t _q) :
-    ImplicitSurface(_center), p(_p), q(_q), stiffness(0) {
+    ImplicitSurface(_center), init_p(_p), p(_p), q(_q), stiffness(0) {
         _compute_volume();
     }
 
@@ -33,7 +33,7 @@ void Spheroid::update_center_pos(Point& _center) {
 
 void Spheroid::check_horizontal_plan_collision(double& height) {
     if ((center.z - p) <= height) {
-        p = center.z-height;
+        p = fmin(init_p, center.z-height);
         update_radius();
     }
 }

spheroid.hpp

@@ -31,7 +31,7 @@ class Spheroid : public ImplicitSurface {
          * V is the volume. Extremely useful to have a constant volume in the
          * ball
         **/
-        double p, q;
+        double init_p, p, q;
         size_t stiffness;
         double V;
         void _compute_volume();