add a test solution for #33

examples/testbed/framework/test_entries.cpp

@@ -63,8 +63,8 @@
 #include <testbed/tests/spring_cloth_collision_test.h>
 #include <testbed/tests/cloth_dragger.h>
 #include <testbed/tests/rope_test.h>
-//#include <testbed/tests/tree_test.h>
 #include <testbed/tests/mass_spring.h>
+#include <testbed/tests/shift_center.h>
 
 TestEntry g_tests[] =
 {
@@ -113,7 +113,7 @@ TestEntry g_tests[] =
 	{ "Spring Cloth Collision", &SpringClothCollision::Create },
 	{ "Spring Cloth Dragging", &ClothDraggerTest::Create },
 	{ "Rope", &Rope::Create },
-	//{ "Tree", &Tree::Create },
 	{ "Mass-Spring System", &MassSpring::Create },
+	{ "Shift Center", &ShiftCenter::Create },
 	{ NULL, NULL }
 };

examples/testbed/tests/shift_center.h

@@ -0,0 +1,104 @@
+/*
+* Copyright (c) 2016-2016 Irlan Robson http://www.irlan.net
+*
+* This software is provided 'as-is', without any express or implied
+* warranty.  In no event will the authors be held liable for any damages
+* arising from the use of this software.
+* Permission is granted to anyone to use this software for any purpose,
+* including commercial applications, and to alter it and redistribute it
+* freely, subject to the following restrictions:
+* 1. The origin of this software must not be misrepresented; you must not
+* claim that you wrote the original software. If you use this software
+* in a product, an acknowledgment in the product documentation would be
+* appreciated but is not required.
+* 2. Altered source versions must be plainly marked as such, and must not be
+* misrepresented as being the original software.
+* 3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef SHIFT_CENTER_H
+#define SHIFT_CENTER_H
+
+#include <testbed/tests/quickhull_test.h>
+
+extern DebugDraw* g_debugDraw;
+extern Camera g_camera;
+extern Settings g_settings;
+
+class ShiftCenter : public Test
+{
+public:
+	ShiftCenter()
+	{
+		g_camera.m_center.Set(2.0f, -2.0f, 0.0f);
+		g_camera.m_zoom = 20.0f;
+		g_settings.drawCenterOfMasses = true;
+
+		{
+			b3BodyDef bd;
+			b3Body* ground = m_world.CreateBody(bd);
+
+			b3HullShape hs;
+			hs.m_hull = &m_groundHull;
+
+			b3ShapeDef sd;
+			sd.shape = &hs;
+
+			ground->CreateShape(sd);
+		}
+
+		{
+			b3BodyDef bd;
+			bd.type = e_dynamicBody;
+			bd.position.Set(0.0f, 5.0f, 0.0f);
+
+			b3Body* body = m_world.CreateBody(bd);
+
+			b3SphereShape sphere;
+			sphere.m_center.SetZero();
+			sphere.m_radius = 1.0f;
+
+			b3ShapeDef sd;
+			sd.density = 0.1f;
+			sd.friction = 0.3f;
+			sd.shape = &sphere;
+
+			body->CreateShape(sd);
+
+			// Retrieve the local center of mass and inertia about the 
+			// local center of mass
+			b3MassData massData;
+			body->GetMassData(&massData);
+
+			// Shift the inertia to the local origin
+			massData.I += massData.mass * b3Steiner(massData.center);
+
+			// Make a copy of the old local center of mass
+			b3Vec3 oldCenter = massData.center;
+			
+			// Pick a center of mass of choice
+			massData.center.z += 10.0f;
+
+			// Measure the displacement from the old local center of mass 
+			// to the new local center of mass.
+			b3Vec3 d = massData.center - oldCenter;
+
+			// Move the inertia at the local origin to the new local origin
+			massData.I += massData.mass * b3Steiner(d);
+
+			// Update local center of mass and inertia
+			body->SetMassData(&massData);
+		}
+	}
+
+	~ShiftCenter()
+	{
+	}
+
+	static Test* Create()
+	{
+		return new ShiftCenter();
+	}
+};
+
+#endif