First try to prevent #1566: kart chassis appears inside of terrain.

data/stk_config.xml

@@ -155,8 +155,15 @@
                  otherwise obstricts too much of the view.  -->
     <camera distance="1.0" forward-up-angle="15"
             backward-up-angle="5"/>
-    <!-- Additional offset to move graphical chassis with regards to the physics. -->
-    <graphics y-offset="0.0"/>
+    <!-- Options to affect the graphical chassis:
+         y-offset: Additional offset to move graphical chassis with regards
+                   to the physics, used to reduce frequencey of graphical
+                   chassis in terrain.
+         prevent-chassis-in-terrain: a hard flag to prevent nearly all instances
+                   of chassis in terrain. Can (atm) cause some stuttering. -->
+    <graphics y-offset="0.0"
+              prevent-chassis-in-terrain="true"
+    />
 
    <!-- Jump animation related values:
         animation-time: only if the estimated time for a jump is larger

src/karts/kart.cpp

@@ -2581,6 +2581,45 @@ void Kart::updateGraphics(float dt, const Vec3& offset_xyz,
     float xx = fabsf(m_speed)* getKartProperties()->getDownwardImpulseFactor()*0.0006f;
     Vec3 center_shift = Vec3(0, m_skidding->getGraphicalJumpOffset()
                               + lean_height +m_graphical_y_offset+xx, 0);
+    
+    // Try to prevent the graphical chassis to be inside of the terrain:
+    if(m_kart_properties->getPreventChassisInTerrain())
+    {
+        // Get the shortest suspension length (=closest point to terrain).
+        float min_susp_len = 99.9f;
+        for (int i = 0; i < getVehicle()->getNumWheels(); i++)
+        {
+            float susp_len = getVehicle()->getWheelInfo(i).m_raycastInfo
+                                                          .m_suspensionLength;
+            if (susp_len < min_susp_len)
+                min_susp_len = susp_len;
+        }   // for i<num_wheels
+
+        const btWheelInfo &w = getVehicle()->getWheelInfo(0);
+        // Recompute the default average suspension length, see 
+        // kartIsInRestNow() how to get from y-offset to susp. len.
+        float av_sus_len = -m_graphical_y_offset
+                         + w.m_chassisConnectionPointCS.getY()
+                         - w.m_wheelsRadius;
+
+        float delta = av_sus_len - min_susp_len;
+        // If the suspension length is so short, that it is less than the 
+        // lowest point of the kart, it indicates that the graphical chassis
+        // would be inside of the track:
+        if (delta > m_kart_model->getLowestPoint())
+        {
+            center_shift.setY(center_shift.getY() + delta - m_kart_model->getLowestPoint());
+        }
+
+        // FIXME: for now, debug output in case we have to debug it
+        //Log::verbose("kart", "min %f y off %f overall off %f lowest %f delta %f asl %f",
+        //    min_susp_len, m_graphical_y_offset, center_shift.getY(),
+        //    m_kart_model->getLowestPoint(),
+        //    delta,
+        //    av_sus_len
+        //    );
+    }
+
     center_shift = getTrans().getBasis() * center_shift;
 
     Moveable::updateGraphics(dt, center_shift,

src/karts/kart_properties.cpp

@@ -105,6 +105,7 @@ KartProperties::KartProperties(const std::string &filename)
     m_gravity_center_shift       = Vec3(UNDEFINED);
     m_bevel_factor               = Vec3(UNDEFINED);
     m_exp_spring_response        = false;
+    m_prevent_chassis_in_terrain = false;
     m_version                    = 0;
     m_color                      = video::SColor(255, 0, 0, 0);
     m_shape                      = 32;  // close enough to a circle.
@@ -618,6 +619,8 @@ void KartProperties::getAllData(const XMLNode * root)
     if(const XMLNode *graphics_node = root->getNode("graphics"))
     {
         graphics_node->get("y-offset",          &m_graphical_y_offset);
+        graphics_node->get("prevent-chassis-in-terrain",
+                                                &m_prevent_chassis_in_terrain);
     }
 
     if(m_kart_model)

src/karts/kart_properties.hpp

@@ -208,6 +208,9 @@ private:
      *  chassis. Useful for karts that don't have enough space for suspension
      *  compression. */
     float       m_graphical_y_offset;
+    /** A hard flag that moves the graphical chassis higher if it's insde
+      * the track. Might cause stuttering. */
+    bool        m_prevent_chassis_in_terrain;
     /** If the kart is supposed to have random wheel rotation at start. */
     bool        m_has_rand_wheels;
     /** Max. length of plunger rubber band. */
@@ -578,6 +581,13 @@ public:
      *  compression. */
     float getGraphicalYOffset() const {return m_graphical_y_offset; }
     // ------------------------------------------------------------------------
+    /** A hard flag that moves the graphical chassis higher if it's insde
+      * the track. Might cause stuttering. */
+    bool getPreventChassisInTerrain() const
+    {
+        return m_prevent_chassis_in_terrain;
+    }   // getPreventChassisInTerrain
+    // ------------------------------------------------------------------------
     /** Returns parameters for the speed-weighted objects */
     const SpeedWeightedObject::Properties& getSpeedWeightedObjectProperties() const
     {