Merge pull request #7807 from Goober5000/fix/moment_of_inertia

fix the collision moment of inertia rotation

code/mod_table/mod_table.cpp

@@ -192,6 +192,8 @@ float Min_radius_for_persistent_debris;
 bool Zero_radius_explosions_skip_fireballs;
 bool Render_insignias_as_decals;
 bool Link_special_point_subsystems_to_destroyed_submodels;
+bool Fix_density_moment_of_inertia;
+bool Fix_collision_moment_of_inertia;
 
 
 #ifdef WITH_DISCORD
@@ -1696,6 +1698,24 @@ void parse_mod_table(const char *filename)
 				stuff_boolean(&Link_special_point_subsystems_to_destroyed_submodels);
 			}
 
+			if (optional_string("$Fix density moment of inertia:")) {
+				stuff_boolean(&Fix_density_moment_of_inertia);
+				if (Fix_density_moment_of_inertia) {
+					mprintf(("Game Settings Table: Using fixed density moment of inertia (higher density makes a ship harder to rotate)\n"));
+				} else {
+					mprintf(("Game Settings Table: Using retail density moment of inertia (higher density makes a ship easier to rotate)\n"));
+				}
+			}
+
+			if (optional_string("$Fix collision moment of inertia:")) {
+				stuff_boolean(&Fix_collision_moment_of_inertia);
+				if (Fix_collision_moment_of_inertia) {
+					mprintf(("Game Settings Table: Using fixed collision moment of inertia (collision rotation will use the object's actual orientation)\n"));
+				} else {
+					mprintf(("Game Settings Table: Using retail collision moment of inertia (collision rotation will use the inverse of the object's orientation)\n"));
+				}
+			}
+
 			// end of options ----------------------------------------
 
 			// if we've been through once already and are at the same place, force a move
@@ -1980,10 +2000,15 @@ void mod_table_reset()
 	Zero_radius_explosions_skip_fireballs = false;
 	Render_insignias_as_decals = false;
 	Link_special_point_subsystems_to_destroyed_submodels = false;
+	Fix_density_moment_of_inertia = false;
+	Fix_collision_moment_of_inertia = false;
 }
 
 void mod_table_set_version_flags()
 {
+	if (mod_supports_version(21, 0, 0)) {
+		Fix_density_moment_of_inertia = true;
+	}
 	if (mod_supports_version(22, 0, 0)) {
 		Fixed_turret_collisions = true;
 		Fixed_missile_detonation = true;
@@ -2012,4 +2037,7 @@ void mod_table_set_version_flags()
 		Zero_radius_explosions_skip_fireballs = true;
 		Render_insignias_as_decals = true;
 	}
+	if (mod_supports_version(26, 2, 0)) {
+		Fix_collision_moment_of_inertia = true;
+	}
 }

code/mod_table/mod_table.h

@@ -212,6 +212,8 @@ extern float Min_radius_for_persistent_debris;
 extern bool Zero_radius_explosions_skip_fireballs;
 extern bool Render_insignias_as_decals;
 extern bool Link_special_point_subsystems_to_destroyed_submodels;
+extern bool Fix_density_moment_of_inertia;
+extern bool Fix_collision_moment_of_inertia;
 
 void mod_table_init();
 void mod_table_post_process();

code/object/collideshipship.cpp

@@ -19,6 +19,7 @@
 #include "hud/hudshield.h"
 #include "io/joy_ff.h"
 #include "io/timer.h"
+#include "mod_table/mod_table.h"
 #include "network/multi.h"
 #include "network/multi_interpolate.h"
 #include "object/objcollide.h"
@@ -869,14 +870,17 @@ void calculate_ship_ship_collision_physics(collision_info_struct *ship_ship_hit_
 //
 static void get_I_inv (matrix* I_inv, matrix* I_inv_body, matrix* orient)
 {
-	matrix Mtemp1, Mtemp2;
+	matrix orient_t;
 	// I_inv = (Rt)(I_inv_body)(R)
 	// This is opposite to what is commonly seen in books since we are rotating coordianates axes 
 	// which is equivalent to rotating in the opposite direction (or transpose)
 
-	vm_matrix_x_matrix(&Mtemp1, I_inv_body, orient);
-	vm_copy_transpose(&Mtemp2, orient);
-	vm_matrix_x_matrix(I_inv, &Mtemp2, &Mtemp1);
+	vm_copy_transpose(&orient_t, orient);
+
+	if (Fix_collision_moment_of_inertia)
+		*I_inv = orient_t * (*I_inv_body) * (*orient);
+	else
+		*I_inv = (*orient) * (*I_inv_body) * orient_t;	// retail multiplied in the opposite order
 }
 
 #define	PLANET_DAMAGE_SCALE	4.0f

code/ship/ship.cpp

@@ -6734,9 +6734,10 @@ void physics_ship_init(object *objp)
 		pi->I_body_inv = pm->moment_of_inertia;
 
 	// scale inverse moment of inertia value by inverse density
-	vm_vec_scale( &pi->I_body_inv.vec.rvec, 1/sinfo->density );
-	vm_vec_scale( &pi->I_body_inv.vec.uvec, 1/sinfo->density );
-	vm_vec_scale( &pi->I_body_inv.vec.fvec, 1/sinfo->density );
+	float moi_scale = Fix_density_moment_of_inertia ? 1/sinfo->density : sinfo->density;	// retail scaled by density, not inverse density
+	vm_vec_scale( &pi->I_body_inv.vec.rvec, moi_scale );
+	vm_vec_scale( &pi->I_body_inv.vec.uvec, moi_scale );
+	vm_vec_scale( &pi->I_body_inv.vec.fvec, moi_scale );
 
 	pi->center_of_mass = pm->center_of_mass;
 	pi->side_slip_time_const = sinfo->damp;