physics: fix assertions when particles get very large positions

Fixes #822

doc/ReleaseNotes

@@ -8,6 +8,7 @@ This is a recommended bugfix release, especially for macOS users.
 * macOS thirdparty packages are now linked with libc++ (#584)
 * Homebrew Python should now locate Panda libraries correctly (#755)
 * Work around Tk bug cancelling Load Params in Particle Panel on macOS (#811)
+* Fix NaN assertions when particles get very large positions (#822)
 * Add support for Autodesk Maya 2020
 * Fix maya2egg erroring when running from a pip installation (#709)
 * Support .pz and .gz compressed models in deployment system

panda/src/physics/linearFrictionForce.cxx

@@ -61,8 +61,7 @@ get_child_vector(const PhysicsObject* po) {
   physics_debug(" v "<<v<<" len "<<v.length()
       <<" friction "<<friction<<" len "<<friction.length()
       <<" dot "<<(normalize(v).dot(normalize(friction))));
-  assert(friction.almost_equal(LVector3::zero())
-      || IS_NEARLY_EQUAL(normalize(v).dot(normalize(friction)), -1.0f));
+  assert(friction.almost_equal(LVector3::zero()) || v.dot(friction) < 0.0f);
   // cary said to cap this at zero so that friction can't reverse your
   // direction, but it seems to me that if you're computing: v + (-v * _coef),
   // _coef in [0, 1] that this will always be greater than or equal to zero.

panda/src/physics/linearNoiseForce.cxx

@@ -89,17 +89,12 @@ get_child_vector(const PhysicsObject *po) {
   LPoint3 p = po->get_position();
 
   // get all of the components
-  int int_x, int_y, int_z;
+  PN_stdfloat int_x, int_y, int_z;
   PN_stdfloat frac_x, frac_y, frac_z;
 
-  int_x = (int) p[0];
-  frac_x = p[0] - int_x;
-
-  int_y = (int) p[1];
-  frac_y = p[1] - int_y;
-
-  int_z = (int) p[2];
-  frac_z = p[2] - int_z;
+  frac_x = std::modf(p[0], &int_x);
+  frac_y = std::modf(p[1], &int_y);
+  frac_z = std::modf(p[2], &int_z);
 
   // apply the cubic smoother to the fractional values
   PN_stdfloat cubic_x, cubic_y, cubic_z;