Fix some more warnings

[taylor/freespace2.git] / src / physics / physics.cpp

/*
 * Copyright (C) Volition, Inc. 1999.  All rights reserved.
 *
 * All source code herein is the property of Volition, Inc. You may not sell 
 * or otherwise commercially exploit the source or things you created based on
 * the source.
 */

/*
 * $Logfile: /Freespace2/code/Physics/Physics.cpp $
 * $Revision$
 * $Date$
 * $Author$
 *
 * Physics stuff
 *
 * $Log$
 * Revision 1.5  2002/07/13 19:47:02  theoddone33
 * Fix some more warnings
 *
 * Change demo building, edit Makefile if you want the demo.
 *
 * Revision 1.4  2002/06/17 06:33:10  relnev
 * ryan's struct patch for gcc 2.95
 *
 * Revision 1.3  2002/06/09 04:41:25  relnev
 * added copyright header
 *
 * Revision 1.2  2002/05/03 13:34:33  theoddone33
 * More stuff compiles
 *
 * Revision 1.1.1.1  2002/05/03 03:28:10  root
 * Initial import.
 *
 * 
 * 5     8/13/99 10:49a Andsager
 * Knossos and HUGE ship warp out.  HUGE ship warp in.  Stealth search
 * modes dont collide big ships.
 * 
 * 4     7/03/99 5:50p Dave
 * Make rotated bitmaps draw properly in padlock views.
 * 
 * 3     5/11/99 10:16p Andsager
 * First pass on engine wash effect.  Rotation (control input), damage,
 * shake.  
 * 
 * 2     10/07/98 10:53a Dave
 * Initial checkin.
 * 
 * 1     10/07/98 10:50a Dave
 * 
 * 125   8/26/98 4:00p Johnson
 * Removed max velocity assert so the new huge cap ship can warp in
 * safely.
 * 
 * 124   5/18/98 5:01p Comet
 * don't do displacement checks in multiplayer
 * 
 * 123   4/06/98 2:38p Duncan
 * AL: Fix potential negative sqrt due to numerical inaccuracy
 * 
 * 122   3/25/98 1:30p Andsager
 * comment out physics assert
 * 
 * 121   3/23/98 9:48a Andsager
 * comment out printf
 * 
 * 120   3/22/98 4:11p Andsager
 * Remove global Freespace_running
 * 
 * 119   3/20/98 5:15p Andsager
 * Revised calculation of rotation, shake, and velocity in response to
 * shockwave since capital ships were seen  to shake.
 * 
 * 118   3/19/98 1:06p Andsager
 * Fix bug checking excessive velocity with accelerated time.
 * 
 * 117   3/17/98 5:43p Andsager
 * Modify physics checks for very slow frame rates.
 * 
 * 116   3/17/98 9:54a Andsager
 * Temporary take out Int3()s in velocity checks
 * 
 * 115   3/17/98 9:51a Allender
 * temporarily commented out Int3 that was causing pain
 * 
 * 114   3/16/98 6:07p Johnson
 * DA:  check that ship velocity does not get too high from collisons
 * 
 * 113   3/16/98 4:39p Adam
 * change Dave's asserts so that displacement checks don't get used when
 * not "in mission"
 * 
 * 112   3/16/98 1:11p Andsager
 * Turn on velocity, displacement limit checks
 * 
 * 111   3/12/98 5:21p Andsager
 * Optimize physics for lasers and dumbfire missiles
 * 
 * 110   3/09/98 2:10p Andsager
 * Put in checks for debris (other) with excessive velocity.
 * 
 * 109   3/09/98 12:59p Mike
 * Put error checking in physics code to detect NANs.
 * 
 * 108   3/09/98 12:13a Andsager
 * Add code to help find jumps in position.
 * 
 * 107   3/03/98 10:39a Andsager
 * Fixed erroneous mprintf for log term in physics_apply_shock
 * 
 * 106   2/11/98 4:52p Mike
 * Better attacking by ships.
 * Fix stupidity in AI classes, which were in backward order!
 * 
 * 105   2/03/98 6:01p Andsager
 * Fix excessive rotvel in debris_create.  Check using physics function
 * check_rotvel_limit.
 * 
 * 104   2/03/98 10:45a Mike
 * Comment out mprintf that could occur very often.
 * 
 * 103   1/29/98 2:38p Andsager
 * Fix bug in physics_apply_shock so that large ships have a smaller
 * effect from shockwaves.
 * 
 * 102   1/23/98 11:31a Andsager
 * Added I_body_inv to phys_init.  Needed for shockwaves hitting ships in
 * descent style physics.
 * 
 * 101   1/23/98 9:02a John
 * Took out Dave's debugging Int3 since they aren't finding what he
 * thought they would and they broke Testcode and pofview.
 * 
 * 100   1/22/98 5:10p Mike
 * Fix bug with player's damp getting stuck off.
 * 
 * 99    1/20/98 3:13p Allender
 * check for Player_obj being valid before doing some other andsager
 * sanity check in physics_sim_vel
 * 
 * 98    1/20/98 10:08a Andsager
 * Remove uninitialized viariable warnings.
 * 
 * 97    1/19/98 3:46p Allender
 * fixed problem where a previous changed caused optimized builds to
 * break.  Don't do a quick out on velcoity_ramp() anymore
 * 
 * 96    1/19/98 12:00p Dave
 * DA:  Revise instantaneous velocity debug check to work with
 * multiplayer.
 * 
 * 95    1/16/98 3:03p Andsager
 * Fix debug info from int3() on player death.
 * 
 * 94    1/16/98 2:54p Andsager
 * Fix debug info.
 * 
 * 93    1/16/98 2:34p Andsager
 * Added debug code to find instantaneous acceleration.
 * 
 * 92    1/16/98 12:14p Andsager
 * Add error checking for the current brief stage
 * 
 * 91    12/29/97 5:10p Allender
 * fixed problems with speed not being reported properly in multiplayer
 * games.  Made read_flying_controls take frametime as a parameter.  More
 * ship/weapon select stuff
 * 
 * 90    12/29/97 12:58p Johnson
 * don't debug rotational velocity when Fred is running
 * 
 * 89    12/08/97 10:29a Andsager
 * Remove shockwave physics parameters from weapon_area_apply_blast and
 * move into physics
 * 
 * 88    12/05/97 3:31p Andsager
 * Added view shake if hit by a weapon or collisoin.
 * 
 * 87    12/03/97 5:47p Andsager
 * Changed reduced damping following collision or weapon to run off time
 * stamp. 
 * 
 * 86    11/24/97 1:54p Dan
 * Mike: Comment out Assert() in physics, debug_rotvel().
 * 
 * 85    11/24/97 8:46a Andsager
 * Added rotational velocity caps and debug info.
 * 
 * 84    11/20/97 4:01p Mike
 * Prevent divide overflow.
 * 
 * 83    11/20/97 12:34a Mike
 * Make ships coast to a stop when their engines have been destroyed.
 * Tricky because code doesn't use damp in forward dimension.
 * 
 * 82    11/19/97 5:57p Mike
 * Hmm, undid all my changes, except for the crucial removal of a blank
 * between "physics_sim" and the open paren.
 * 
 * 81    11/19/97 1:26a Andsager
 * Made shockwaves work again, including shake.
 * 
 * 80    11/17/97 5:15p Andsager
 * 
 * 79    11/13/97 6:01p Andsager
 * Improve comment in physic_collide_whack
 * 
 * 78    11/13/97 5:41p Andsager
 * Decreased the rotational whack after getting hit by lasers and
 * missiles.
 * 
 * 77    10/29/97 5:01p Andsager
 * fixed bug in collision physics (physics_collide_whack)
 * 
 * 76    9/16/97 5:28p Andsager
 * calculate velocity in physics_sim_vel
 * 
 * 75    9/11/97 5:25p Mike
 * Fix ! vs. & precedence bug in physics_sim_vel().
 * 
 * 74    9/09/97 10:14p Andsager
 * 
 * 73    9/04/97 5:09p Andsager
 * implement physics using moment of inertia and mass (from BSPgen).
 * Added to phys_info struct.  Updated ship_info, polymodel structs.
 * Updated weapon ($Mass and $Force) and ship ($Mass -> $Density) tables
 * 
 * 72    9/03/97 5:43p Andsager
 * fixed bug calculating ramp velocity after getting whacked
 * 
 * 71    9/02/97 4:19p Mike
 * Comment out code at end of physics_apply_whack() that made ships nearly
 * stop.
 * 
 * 70    8/29/97 10:13a Allender
 * work on server/client prediction code -- doesn't work too well.  Made
 * all clients simulate their own orientation with the server giving
 * corrections every so often.
 * 
 * 69    8/25/97 2:41p Andsager
 * collision code also changes ramp velocity to take account of collison.
 * some optimization of collision physics.
 * 
 * 68    8/19/97 9:56a John
 * new thruster effect fairly functional
 * 
 * 67    8/18/97 6:26p Andsager
 * preliminary version of collision physics
 * 
 * 66    8/17/97 9:19p Andsager
 * improvement to collision physics
 * 
 * 65    8/13/97 12:16p Andsager
 * intermediate level checkin for use with collision with extended objects
 * 
 * 64    8/05/97 3:13p Andsager
 * improved comments to apply_whack
 * 
 * 63    8/05/97 10:18a Lawrance
 * my_rand() being used temporarily instead of rand()
 * 
 * 62    7/31/97 12:44p Andsager
 * 
 * 61    7/25/97 5:05p John
 * fixed a potential bug in ramp_velocity when delta dist becomes 0, a
 * bunch of sideways 8 thingys appear :-)
 * 
 * 
 * 60    7/25/97 1:04p Andsager
 * Modified physics flag PF_REDUCED_DAMP for damping when object is hit.
 * Flag is now set in physics_apply_whack/shock and turned off in
 * physics_sim_vel.  Weapons should not directly set this flag.
 * 
 * 59    7/25/97 9:07a Andsager
 * modified apply_whack
 * 
 * 58    7/23/97 5:10p Andsager
 * Enhanced shockwave effect with shake based on blast force.  
 * 
 * 57    7/22/97 2:40p Andsager
 * shockwaves now cause proper rotation of ships
 * 
 * 56    7/21/97 4:12p Mike
 * Two ships move as one while docked, including physics whacks.  Spin of
 * objects when killed based on speed.
 * 
 * 55    7/21/97 2:19p John
 * made descent-style physics work for testcode. fixed bug in the
 * velocity_ramp code with t==0.0f
 * 
 * 54    7/17/97 8:02p Lawrance
 * improve comments to physics_apply_whack()
 * 
 * 53    7/16/97 4:42p Mike
 * Make afterburner shake viewer, not ship.
 * Shake for limited time.
 * Add timestamp_until() function to timer library.
 * 
 * 52    7/16/97 11:53a Andsager
 * 
 * 51    7/16/97 11:06a Andsager
 * Allow damping on z to support shockwaves
 * 
 * 50    7/15/97 12:28p Andsager
 * commented out some debug code.
 * 
 * 49    7/15/97 12:25p Andsager
 * More integration with new physics.
 * 
 * 48    7/15/97 12:03p Andsager
 * New physics stuff
 * 
 * 47    7/11/97 11:54a John
 * added rotated 3d bitmaps.
 * 
 * 46    6/25/97 12:22p Mike
 * Diminish bashing into of objects.  Make ships flying waypoints fly in
 * formation.
 * 
 * 45    6/17/97 10:59p Mike
 * Comment out irritating mprintf().
 * 
 * 44    6/11/97 4:27p Mike
 * Balance the whack ships take when they get hit.
 * 
 * 43    4/17/97 10:02a Lawrance
 * allow ship shaking for ABURN_DECAY_TIME after afterburners cut out
 * 
 * 42    4/16/97 10:48p Mike
 * Afterburner shake.
 * Made afterburner engagement a bit in physics flags, removed from ship
 * flags, removed a parameter to physics_read_flying_controls().
 * 
 * 41    4/11/97 3:17p Mike
 * Modify physics_sim() to use non quick version of vm_vec_mag().  Quick
 * version was causing cumulative errors in velocity of homing missiles.
 * 
 * 40    4/10/97 3:20p Mike
 * Change hull damage to be like shields.
 * 
 * 39    4/04/97 11:08a Adam
 * played with banking values
 * 
 * 38    4/04/97 12:19a Mike
 * Make ships bank when they turn.
 * 
 * 37    3/04/97 3:10p Mike
 * Intermediate checkin: Had to resolve some build errors.  Working on two
 * docked ships moving as one.
 * 
 * 36    2/25/97 7:39p Lawrance
 * added afterburner_on flag to physics_read_flying_controls() to account
 * for afterburner effects
 * 
 * 35    2/05/97 6:02p Hoffoss
 * Added heading rotation around universal Y axis to Fred when controlling
 * the camera.
 * 
 * 34    2/05/97 9:15a Mike
 * Partial implementation of new docking system, integrated so I could
 * update on John's new turret code.
 * 
 * 33    1/20/97 7:58p John
 * Fixed some link errors with testcode.
 * 
 * $NoKeywords: $
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

#include "physics.h"
#include "floating.h"
#include "player.h"
#include "freespace.h"
#include "linklist.h"
#include "timer.h"
#include "key.h"

// defines for physics functions
#define MAX_TURN_LIMIT  0.2618f         // about 15 degrees

#define ROT_DEBUG
#define ROTVEL_TOL              0.1                     // Amount of rotvel is decreased if over cap
#define ROTVEL_CAP              14.0                    // Rotational velocity cap for live objects
#define DEAD_ROTVEL_CAP 16.3                    // Rotational velocity cap for dead objects

#define MAX_SHIP_SPEED          300             // Maximum speed allowed after whack or shockwave
#define RESET_SHIP_SPEED        240             // Speed that a ship is reset to after exceeding MAX_SHIP_SPEED

#define SW_ROT_FACTOR                   5               // increase in rotational time constant in shockwave
#define SW_BLAST_DURATION               2000    // maximum duration of shockwave
#define REDUCED_DAMP_FACTOR     10              // increase in side_slip and acceleration time constants (scaled according to reduced damp time)
#define REDUCED_DAMP_VEL                30              // change in velocity at which reduced_damp_time is 2000 ms
#define REDUCED_DAMP_TIME               2000    // ms (2.0 sec)
#define WEAPON_SHAKE_TIME               500     //      ms (0.5 sec)    viewer shake time after hit by weapon (implemented via afterburner shake)
#define SPECIAL_WARP_T_CONST    0.651   // special warp time constant (loose 99 % of excess speed in 3 sec)

#define PHYS_DEBUG                                              // check if (vel > 500) or (displacement in one frame > 350)

void update_reduced_damp_timestamp( physics_info *pi, float impulse );

void physics_init( physics_info * pi )
{
        memset( pi, 0, sizeof(physics_info) );

        pi->mass = 10.0f;                                       // This ship weighs 10 units
        pi->side_slip_time_const = 0.05f;                                       
        pi->rotdamp = 0.1f;

        pi->max_vel.xyz.x = 100.0f;             //sideways
        pi->max_vel.xyz.y = 100.0f;             //up/down
        pi->max_vel.xyz.z = 100.0f;             //forward
        pi->max_rear_vel = 100.0f;      //backward -- controlled seperately

        pi->max_rotvel.xyz.x = 2.0f;            //pitch 
        pi->max_rotvel.xyz.y = 1.0f;            //heading
        pi->max_rotvel.xyz.z = 2.0f;            //bank

        pi->prev_ramp_vel.xyz.x = 0.0f;
        pi->prev_ramp_vel.xyz.y = 0.0f;
        pi->prev_ramp_vel.xyz.z = 0.0f;

        pi->desired_vel.xyz.x = 0.0f;
        pi->desired_vel.xyz.y = 0.0f;
        pi->desired_vel.xyz.z = 0.0f;

        pi->slide_accel_time_const=pi->side_slip_time_const;    // slide using max_vel.xyz.x & .xyz.y
        pi->slide_decel_time_const=pi->side_slip_time_const;    // slide using max_vel.xyz.x & .xyz.y

        pi->afterburner_decay = 1;      
        pi->forward_thrust = 0.0f;

        pi->flags = 0;

        // default values for moment of inetaia
        (void) vm_vec_make( &pi->I_body_inv.v.rvec, 1e-5f, 0.0f, 0.0f );
        (void) vm_vec_make( &pi->I_body_inv.v.uvec, 0.0f, 1e-5f, 0.0f );
        (void) vm_vec_make( &pi->I_body_inv.v.fvec, 0.0f, 0.0f, 1e-5f );

}


//==========================================================================
// apply_physics - This does correct physics independent of frame rate.
//
// Given:
//    damping = damping factor.  Setting this to zero make the object instantly
//              go to the target velocity.  Increasing it makes the object ramp
//              up or down to the target velocity.
//    desired_vel = the target velocity
//    initial_vel = velocity at last call
//    t = elapsed time since last call
//
// Returns:
//    new_vel = current velocity
//    delta_pos = delta position (framevec)
// You can extend this to 3d by calling it 3 times, once for each x,y,z component.

void apply_physics( float damping, float desired_vel, float initial_vel, float t, float * new_vel, float * delta_pos )
{
        if ( damping < 0.0001f )        {
                if ( delta_pos )
                        *delta_pos = desired_vel*t;
                if ( new_vel )
                        *new_vel = desired_vel;
        } else {
                float dv, e;
                dv = initial_vel - desired_vel;
                e = (float)exp( -t/damping );
                if ( delta_pos )
                        *delta_pos = (1.0f - e)*dv*damping + desired_vel*t;
                if ( new_vel )
                        *new_vel = dv*e + desired_vel;
        }
}



float Physics_viewer_bank = 0.0f;
int Physics_viewer_direction = PHYSICS_VIEWER_FRONT;
static physics_info * Viewer_physics_info = NULL;

// If you would like Physics_viewer_bank to be tracked (Which is needed
// for rotating 3d bitmaps) call this and pass it a pointer to the
// viewer's physics info.
void physics_set_viewer( physics_info * p, int dir )
{
        if ( (Viewer_physics_info != p) || (Physics_viewer_direction != dir) )  {
                Viewer_physics_info = p;
                Physics_viewer_bank = 0.0f;
                Physics_viewer_direction = dir;
        }
}



//      -----------------------------------------------------------------------------------------------------------
// add rotational velocity & acceleration



void physics_sim_rot(matrix * orient, physics_info * pi, float sim_time )
{
        angles  tangles;
        vector  new_vel;
        matrix  tmp;
        float           shock_amplitude;
        float           rotdamp;
        float           shock_fraction_time_left;

        Assert(is_valid_matrix(orient));
        Assert(is_valid_vec(&pi->rotvel));
        Assert(is_valid_vec(&pi->desired_rotvel));

        // Handle special case of shockwave
        shock_amplitude = 0.0f;
        if ( pi->flags & PF_IN_SHOCKWAVE ) {
                if ( timestamp_elapsed(pi->shockwave_decay) ) {
                        pi->flags &= ~PF_IN_SHOCKWAVE;
                        rotdamp = pi->rotdamp;
                } else {
                        shock_fraction_time_left = timestamp_until( pi->shockwave_decay ) / (float) SW_BLAST_DURATION;
                        rotdamp = pi->rotdamp + pi->rotdamp * (SW_ROT_FACTOR - 1) * shock_fraction_time_left;
                        shock_amplitude = pi->shockwave_shake_amp * shock_fraction_time_left;
                }
        } else {
                rotdamp = pi->rotdamp;
        }

        // Do rotational physics with given damping
        apply_physics( rotdamp, pi->desired_rotvel.xyz.x, pi->rotvel.xyz.x, sim_time, &new_vel.xyz.x, NULL );
        apply_physics( rotdamp, pi->desired_rotvel.xyz.y, pi->rotvel.xyz.y, sim_time, &new_vel.xyz.y, NULL );
        apply_physics( rotdamp, pi->desired_rotvel.xyz.z, pi->rotvel.xyz.z, sim_time, &new_vel.xyz.z, NULL );

        /*
#ifdef ROT_DEBUG
        if (check_rotvel_limit( pi )) {
                nprintf(("Physics", "rotvel reset in physics_sim_rot\n"));
        }
#endif
*/
        Assert(is_valid_vec(&new_vel));

        pi->rotvel = new_vel;

        tangles.p = pi->rotvel.xyz.x*sim_time;
        tangles.h = pi->rotvel.xyz.y*sim_time;
        tangles.b = pi->rotvel.xyz.z*sim_time;

        // If this is the viewer_object, keep track of the
        // changes in banking so that rotated bitmaps look correct.
        // This is used by the g3_draw_rotated_bitmap function.
        if ( pi == Viewer_physics_info )        {
                switch(Physics_viewer_direction){
                case PHYSICS_VIEWER_FRONT:                              
                        Physics_viewer_bank -= tangles.b;
                        break;

                case PHYSICS_VIEWER_UP:
                        Physics_viewer_bank -= tangles.h;
                        break;

                case PHYSICS_VIEWER_REAR:
                        Physics_viewer_bank += tangles.b;
                        break;

                case PHYSICS_VIEWER_LEFT:
                        Physics_viewer_bank += tangles.p;
                        break;

                case PHYSICS_VIEWER_RIGHT:
                        Physics_viewer_bank -= tangles.p;
                        break;

                default:
                        Physics_viewer_bank -= tangles.b;
                        break;
                }

                if ( Physics_viewer_bank < 0.0f ){
                        Physics_viewer_bank += 2.0f * PI;
                }

                if ( Physics_viewer_bank > 2.0f * PI ){
                        Physics_viewer_bank -= 2.0f * PI;
                }
        }

/*      //      Make ship shake due to afterburner.
        if (pi->flags & PF_AFTERBURNER_ON || !timestamp_elapsed(pi->afterburner_decay) ) {
                float   max_speed;

                max_speed = vm_vec_mag_quick(&pi->max_vel);
                tangles.p += (float) (rand()-RAND_MAX/2)/RAND_MAX * pi->speed/max_speed/64.0f;
                tangles.h += (float) (rand()-RAND_MAX/2)/RAND_MAX * pi->speed/max_speed/64.0f;
                if ( pi->flags & PF_AFTERBURNER_ON ) {
                        pi->afterburner_decay = timestamp(ABURN_DECAY_TIME);
                }
        }
*/

        // Make ship shake due to shockwave, decreasing in amplitude at the end of the shockwave
        if ( pi->flags & PF_IN_SHOCKWAVE ) {
                tangles.p += (float) (myrand()-RAND_MAX/2)/RAND_MAX * shock_amplitude;
                tangles.h += (float) (myrand()-RAND_MAX/2)/RAND_MAX * shock_amplitude;
        }


        vm_angles_2_matrix(&pi->last_rotmat, &tangles );
        vm_matrix_x_matrix( &tmp, orient, &pi->last_rotmat );
        *orient = tmp;

        vm_orthogonalize_matrix(orient);
                
}

//      -----------------------------------------------------------------------------------------------------------
// add rotational velocity & acceleration

void physics_sim_rot_editor(matrix * orient, physics_info * pi, float sim_time)
{
        angles  tangles;
        vector  new_vel;
        matrix  tmp;
        angles  t1, t2;

        apply_physics( pi->rotdamp, pi->desired_rotvel.xyz.x, pi->rotvel.xyz.x, sim_time, 
                                                                 &new_vel.xyz.x, NULL );

        apply_physics( pi->rotdamp, pi->desired_rotvel.xyz.y, pi->rotvel.xyz.y, sim_time, 
                                                                 &new_vel.xyz.y, NULL );

        apply_physics( pi->rotdamp, pi->desired_rotvel.xyz.z, pi->rotvel.xyz.z, sim_time, 
                                                                 &new_vel.xyz.z, NULL );
        
        pi->rotvel = new_vel;

        tangles.p = pi->rotvel.xyz.x*sim_time;
        tangles.h = pi->rotvel.xyz.y*sim_time;
        tangles.b = pi->rotvel.xyz.z*sim_time;

        t1 = t2 = tangles;
        t1.h = 0.0f;  t1.b = 0.0f;
        t2.p = 0.0f; t2.b = 0.0f;

        // put in p & b like normal
        vm_angles_2_matrix(&pi->last_rotmat, &t1 );
        vm_matrix_x_matrix( &tmp, orient, &pi->last_rotmat );
        
        // Put in heading separately
        vm_angles_2_matrix(&pi->last_rotmat, &t2 );
        vm_matrix_x_matrix( orient, &pi->last_rotmat, &tmp );

        vm_orthogonalize_matrix(orient);
                
}

// Adds velocity to position
// finds velocity and displacement in local coords
void physics_sim_vel(vector * position, physics_info * pi, float sim_time, matrix *orient)
{
        vector local_disp;              // displacement in this frame
        vector local_v_in;              // velocity in local coords at the start of this frame
        vector local_desired_vel;       // desired velocity in local coords
        vector local_v_out;             // velocity in local coords following this frame
        vector damp;

        //      Maybe clear the reduced_damp flag.
        //      This fixes the problem of the player getting near-instantaneous acceleration under unknown circumstances.
        //      The larger problem is probably that PF_USE_VEL is getting stuck set.
        if ((pi->flags & PF_REDUCED_DAMP) && (timestamp_elapsed(pi->reduced_damp_decay))) {
                pi->flags &= ~PF_REDUCED_DAMP;
        }

        // Set up damping constants based on special conditions
        // ie. shockwave, collision, weapon, dead
        if (pi->flags & PF_DEAD_DAMP) {
                // side_slip_time_const is already quite large and now needs to be applied in all directions
                (void) vm_vec_make( &damp, pi->side_slip_time_const, pi->side_slip_time_const, pi->side_slip_time_const );

        } else if (pi->flags & PF_REDUCED_DAMP) {
                // case of shock, weapon, collide, etc.
                if ( timestamp_elapsed(pi->reduced_damp_decay) ) {
                        (void) vm_vec_make( &damp, pi->side_slip_time_const, pi->side_slip_time_const, 0.0f );
                } else {
                        // damp is multiplied by fraction and not fraction^2, gives better collision separation
                        float reduced_damp_fraction_time_left = timestamp_until( pi->reduced_damp_decay ) / (float) REDUCED_DAMP_TIME;
                        damp.xyz.x = pi->side_slip_time_const * ( 1 + (REDUCED_DAMP_FACTOR-1) * reduced_damp_fraction_time_left );
                        damp.xyz.y = pi->side_slip_time_const * ( 1 + (REDUCED_DAMP_FACTOR-1) * reduced_damp_fraction_time_left );
                        damp.xyz.z = pi->side_slip_time_const * reduced_damp_fraction_time_left * REDUCED_DAMP_FACTOR;
                }
        } else {
                // regular damping
                (void) vm_vec_make( &damp, pi->side_slip_time_const, pi->side_slip_time_const, 0.0f );
        }

        // Note: CANNOT maintain a *local velocity* since a rotation can occur in this frame. 
        // thus the local velocity of in the next frame can be different (this would require rotate to change local vel
        // and this is not desired

        // get local components of current velocity
        vm_vec_rotate (&local_v_in, &pi->vel, orient);

        // get local components of desired velocity
        vm_vec_rotate (&local_desired_vel, &pi->desired_vel, orient);

        // find updated LOCAL velocity and position in the local x direction
        apply_physics (damp.xyz.x, local_desired_vel.xyz.x, local_v_in.xyz.x, sim_time, &local_v_out.xyz.x, &local_disp.xyz.x);

        // find updated LOCAL velocity and position in the local y direction
        apply_physics (damp.xyz.y, local_desired_vel.xyz.y, local_v_in.xyz.y, sim_time, &local_v_out.xyz.y, &local_disp.xyz.y);

        // find updated LOCAL velocity and position in the local z direction
        // for player ship, damp should normally be zero, but may be altered in a shockwave
        //  in death, shockwave,etc. we want damping time const large for all 3 axes
        // warp in test - make excessive speed drop exponentially from max allowed
        // become (0.01x in 3 sec)

        int special_warp_in = FALSE;
        float excess = local_v_in.xyz.z - pi->max_vel.xyz.z;
        if (excess > 5 && (pi->flags & PF_SPECIAL_WARP_IN)) {
                special_warp_in = TRUE;
                float exp_factor = float(exp(-sim_time / SPECIAL_WARP_T_CONST));
                local_v_out.xyz.z = pi->max_vel.xyz.z + excess * exp_factor;
                local_disp.xyz.z = (pi->max_vel.xyz.z * sim_time) + excess * (float(SPECIAL_WARP_T_CONST) * (1.0f - exp_factor));
        } else if (pi->flags & PF_SPECIAL_WARP_OUT) {
                float exp_factor = float(exp(-sim_time / SPECIAL_WARP_T_CONST));
                vector temp;
                vm_vec_rotate(&temp, &pi->prev_ramp_vel, orient);
                float deficeit = temp.xyz.z - local_v_in.xyz.z;
                local_v_out.xyz.z = local_v_in.xyz.z + deficeit * (1.0f - exp_factor);
                local_disp.xyz.z = (local_v_in.xyz.z * sim_time) + deficeit * (sim_time - (float(SPECIAL_WARP_T_CONST) * (1.0f - exp_factor)));
        } else {
                apply_physics (damp.xyz.z, local_desired_vel.xyz.z, local_v_in.xyz.z, sim_time, &local_v_out.xyz.z, &local_disp.xyz.z);
        }

        // maybe turn off special warp in flag
        if ((pi->flags & PF_SPECIAL_WARP_IN) && (excess < 5)) {
                pi->flags &= ~(PF_SPECIAL_WARP_IN);
        }

        // update world position from local to world coords using orient
        vector world_disp;
        vm_vec_unrotate (&world_disp, &local_disp, orient);
#ifdef PHYS_DEBUG
        // check for  excess velocity or translation
        // GET DaveA.
        if ( (Game_mode & GM_IN_MISSION) && (Game_mode & GM_NORMAL) ) {
                // Assert( (sim_time > 0.5f) || (vm_vec_mag_squared(&pi->vel) < 500*500) );
                // Assert( (sim_time > 0.5f) || (vm_vec_mag_squared(&world_disp) < 350*350) );
        }
#endif
        vm_vec_add2 (position, &world_disp);

        // update world velocity
        vm_vec_unrotate(&pi->vel, &local_v_out, orient);

        if (special_warp_in) {
                vm_vec_rotate(&pi->prev_ramp_vel, &pi->vel, orient);
        }
}

//      -----------------------------------------------------------------------------------------------------------
// Simulate a physics object for this frame
void physics_sim(vector* position, matrix* orient, physics_info* pi, float sim_time)
{
        // check flag which tells us whether or not to do velocity translation
        if (pi->flags & PF_CONST_VEL) {
                vm_vec_scale_add2(position, &pi->vel, sim_time);
        } else {
                physics_sim_vel(position, pi, sim_time, orient);

                physics_sim_rot(orient, pi, sim_time);

                pi->speed = vm_vec_mag(&pi->vel);                                                       //      Note, cannot use quick version, causes cumulative error, increasing speed.
                pi->fspeed = vm_vec_dot(&orient->v.fvec, &pi->vel);             // instead of vector magnitude -- use only forward vector since we are only interested in forward velocity
        }

}

//      -----------------------------------------------------------------------------------------------------------
// Simulate a physics object for this frame.  Used by the editor.  The difference between
// this function and physics_sim() is that this one uses a heading change to rotate around
// the universal Y axis, rather than the local orientation's Y axis.  Banking is also ignored.
void physics_sim_editor(vector *position, matrix * orient, physics_info * pi, float sim_time )
{
        physics_sim_vel(position, pi, sim_time, orient);
        physics_sim_rot_editor(orient, pi, sim_time);
        pi->speed = vm_vec_mag_quick(&pi->vel);
        pi->fspeed = vm_vec_dot(&orient->v.fvec, &pi->vel);             // instead of vector magnitude -- use only forward vector since we are only interested in forward velocity
}

// function to predict an object's position given the delta time and an objects physics info
void physics_predict_pos(physics_info *pi, float delta_time, vector *predicted_pos)
{
        apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.x, pi->vel.xyz.x, delta_time, 
                                                                 NULL, &predicted_pos->xyz.x );

        apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.y, pi->vel.xyz.y, delta_time, 
                                                                 NULL, &predicted_pos->xyz.y );

        apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.z, pi->vel.xyz.z, delta_time, 
                                                                 NULL, &predicted_pos->xyz.z );
}

// function to predict an object's velocity given the parameters
void physics_predict_vel(physics_info *pi, float delta_time, vector *predicted_vel)
{
        if (pi->flags & PF_CONST_VEL) {
                predicted_vel = &pi->vel;
        } else {
                apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.x, pi->vel.xyz.x, delta_time, 
                                                                         &predicted_vel->xyz.x, NULL );

                apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.y, pi->vel.xyz.y, delta_time, 
                                                                         &predicted_vel->xyz.y, NULL );

                apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.z, pi->vel.xyz.z, delta_time, 
                                                                         &predicted_vel->xyz.z, NULL );
        }
}

// function to predict position and velocity of an object
void physics_predict_pos_and_vel(physics_info *pi, float delta_time, vector *predicted_vel, vector *predicted_pos)
{

        apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.x, pi->vel.xyz.x, delta_time, 
                              &predicted_vel->xyz.x, &predicted_pos->xyz.x );

        apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.y, pi->vel.xyz.y, delta_time, 
                              &predicted_vel->xyz.y, &predicted_pos->xyz.y );

        apply_physics( pi->side_slip_time_const, pi->desired_vel.xyz.z, pi->vel.xyz.z, delta_time, 
                              &predicted_vel->xyz.z, &predicted_pos->xyz.z );
}

// physics_read_flying_controls() 
//
// parmeters:  *orient  ==>
//                                      *pi             ==>
//                                      *ci             ==> 
//      Adam: Uncomment-out this define to enable banking while turning.
#define BANK_WHEN_TURN



// function looks at the flying controls and the current velocity to determine a goal velocity
// function determines velocity in object's reference frame and goal velocity in object's reference frame
void physics_read_flying_controls( matrix * orient, physics_info * pi, control_info * ci, float sim_time, vector *wash_rot)
{
        vector goal_vel;                // goal velocity in local coords, *not* accounting for ramping of velcity
        float ramp_time_const;          // time constant for velocity ramping

        float velocity_ramp (float v_in, float v_goal, float time_const, float t);

//      if ( keyd_pressed[KEY_LSHIFT] ) {
//              keyd_pressed[KEY_LSHIFT] = 0;
//              Int3();
//      }

        ci->forward += (ci->forward_cruise_percent / 100.0f);

//      mprintf(("ci->forward == %7.3f\n", ci->forward));

        // give control imput to cause rotation in engine wash
        extern int Wash_on;
        if ( wash_rot && Wash_on ) {
                ci->pitch += wash_rot->xyz.x;
                ci->bank += wash_rot->xyz.z;
                ci->heading += wash_rot->xyz.y;
        }

        if (ci->pitch > 1.0f ) ci->pitch = 1.0f;
        else if (ci->pitch < -1.0f ) ci->pitch = -1.0f;

        if (ci->vertical > 1.0f ) ci->vertical = 1.0f;
        else if (ci->vertical < -1.0f ) ci->vertical = -1.0f;

        if (ci->heading > 1.0f ) ci->heading = 1.0f;
        else if (ci->heading < -1.0f ) ci->heading = -1.0f;

        if (ci->sideways > 1.0f  ) ci->sideways = 1.0f;
        else if (ci->sideways < -1.0f  ) ci->sideways = -1.0f;

        if (ci->bank > 1.0f ) ci->bank = 1.0f;
        else if (ci->bank < -1.0f ) ci->bank = -1.0f;

        if ( pi->flags & PF_AFTERBURNER_ON )
                ci->forward = 1.0f;

        if (ci->forward > 1.0f ) ci->forward = 1.0f;
        else if (ci->forward < -1.0f ) ci->forward = -1.0f;

        pi->desired_rotvel.xyz.x = ci->pitch * pi->max_rotvel.xyz.x;
        pi->desired_rotvel.xyz.y = ci->heading * pi->max_rotvel.xyz.y;

        float   delta_bank;

#ifdef BANK_WHEN_TURN
        //      To change direction of bank, negate the whole expression.
        //      To increase magnitude of banking, decrease denominator.
        //      Adam: The following statement is all the math for banking while turning.
        delta_bank = - (ci->heading * pi->max_rotvel.xyz.y)/2.0f;
#else
        delta_bank = 0.0f;
#endif

        pi->desired_rotvel.xyz.z = ci->bank * pi->max_rotvel.xyz.z + delta_bank;
        pi->forward_thrust = ci->forward;

        if ( pi->flags & PF_AFTERBURNER_ON ) {
                goal_vel.xyz.x = ci->sideways*pi->afterburner_max_vel.xyz.x;
                goal_vel.xyz.y = ci->vertical*pi->afterburner_max_vel.xyz.y;
                goal_vel.xyz.z = ci->forward* pi->afterburner_max_vel.xyz.z;
        }
        else {
                goal_vel.xyz.x = ci->sideways*pi->max_vel.xyz.x;
                goal_vel.xyz.y = ci->vertical*pi->max_vel.xyz.y;
                goal_vel.xyz.z = ci->forward* pi->max_vel.xyz.z;
        }

        if ( goal_vel.xyz.z < -pi->max_rear_vel ) 
                goal_vel.xyz.z = -pi->max_rear_vel;


        if ( pi->flags & PF_ACCELERATES )       {
                //
                // Determine *resultant* DESIRED VELOCITY (desired_vel) accounting for RAMPING of velocity
                // Use LOCAL coordinates
                // if slide_enabled, ramp velocity for x and y, otherwise set goal (0)
                //    always ramp velocity for z
                // 

                // If reduced damp in effect, then adjust ramp_velocity and desired_velocity can not change as fast.
                // Scale according to reduced_damp_time_expansion.
                float reduced_damp_ramp_time_expansion;
                if ( pi->flags & PF_REDUCED_DAMP ) {
                        float reduced_damp_fraction_time_left = timestamp_until( pi->reduced_damp_decay ) / (float) REDUCED_DAMP_TIME;
                        reduced_damp_ramp_time_expansion = 1.0f + (REDUCED_DAMP_FACTOR-1) * reduced_damp_fraction_time_left;
                } else {
                        reduced_damp_ramp_time_expansion = 1.0f;
                }

//      if ( !use_descent && (Player_obj->phys_info.forward_accel_time_const < 0.1) && !(Ships[Player_obj->instance].flags & SF_DYING) && (Player_obj->type != OBJ_OBSERVER) )
//                      Int3(); // Get dave A

                if (pi->flags & PF_SLIDE_ENABLED)  {
                        // determine the local velocity
                        // deterimine whether accelerating or decleration toward goal for x
                        if ( goal_vel.xyz.x > 0.0f )  {
                                if ( goal_vel.xyz.x >= pi->prev_ramp_vel.xyz.x ) 
                                        ramp_time_const = pi->slide_accel_time_const;
                                else
                                        ramp_time_const = pi->slide_decel_time_const;
                        } else  {  // goal_vel.xyz.x <= 0.0
                                if ( goal_vel.xyz.x <= pi->prev_ramp_vel.xyz.x )
                                        ramp_time_const = pi->slide_accel_time_const;
                                else
                                        ramp_time_const = pi->slide_decel_time_const;
                        }
                        // If reduced damp in effect, then adjust ramp_velocity and desired_velocity can not change as fast
                        if ( pi->flags & PF_REDUCED_DAMP ) {
                                ramp_time_const *= reduced_damp_ramp_time_expansion;
                        }
                        pi->prev_ramp_vel.xyz.x = velocity_ramp(pi->prev_ramp_vel.xyz.x, goal_vel.xyz.x, ramp_time_const, sim_time);

                        // deterimine whether accelerating or decleration toward goal for y
                        if ( goal_vel.xyz.y > 0.0f )  {
                                if ( goal_vel.xyz.y >= pi->prev_ramp_vel.xyz.y ) 
                                        ramp_time_const = pi->slide_accel_time_const;
                                else
                                        ramp_time_const = pi->slide_decel_time_const;
                        } else  {  // goal_vel.xyz.y <= 0.0
                                if ( goal_vel.xyz.y <= pi->prev_ramp_vel.xyz.y )
                                        ramp_time_const = pi->slide_accel_time_const;
                                else
                                        ramp_time_const = pi->slide_decel_time_const;
                        }
                        // If reduced damp in effect, then adjust ramp_velocity and desired_velocity can not change as fast
                        if ( pi->flags & PF_REDUCED_DAMP ) {
                                ramp_time_const *= reduced_damp_ramp_time_expansion;
                        }
                        pi->prev_ramp_vel.xyz.y = velocity_ramp( pi->prev_ramp_vel.xyz.y, goal_vel.xyz.y, ramp_time_const, sim_time);
                } else  {
                        // slide not enabled
                        pi->prev_ramp_vel.xyz.x = 0.0f;
                        pi->prev_ramp_vel.xyz.y = 0.0f;
                }

                // find ramp velocity in the forward direction
                if ( goal_vel.xyz.z >= pi->prev_ramp_vel.xyz.z )  {
                        if ( pi->flags & PF_AFTERBURNER_ON )
                                ramp_time_const = pi->afterburner_forward_accel_time_const;
                        else
                                ramp_time_const = pi->forward_accel_time_const;
                } else
                        ramp_time_const = pi->forward_decel_time_const;

                // If reduced damp in effect, then adjust ramp_velocity and desired_velocity can not change as fast
                if ( pi->flags & PF_REDUCED_DAMP ) {
                        ramp_time_const *= reduced_damp_ramp_time_expansion;
                }
                pi->prev_ramp_vel.xyz.z = velocity_ramp( pi->prev_ramp_vel.xyz.z, goal_vel.xyz.z, ramp_time_const, sim_time);


                // this translates local desired velocities to world velocities

                vm_vec_zero(&pi->desired_vel);
                vm_vec_scale_add2( &pi->desired_vel, &orient->v.rvec, pi->prev_ramp_vel.xyz.x );
                vm_vec_scale_add2( &pi->desired_vel, &orient->v.uvec, pi->prev_ramp_vel.xyz.y );
                vm_vec_scale_add2( &pi->desired_vel, &orient->v.fvec, pi->prev_ramp_vel.xyz.z );
        } else  // object does not accelerate  (PF_ACCELERATES not set)
                pi->desired_vel = pi->vel;
}



//      ----------------------------------------------------------------
//      Do *dest = *delta unless:
//                              *delta is pretty small
//              and     they are of different signs.
void physics_set_rotvel_and_saturate(float *dest, float delta)
{
        /*
        if ((delta ^ *dest) < 0) {
                if (abs(delta) < F1_0/8) {
                        // mprintf((0, "D"));
                        *dest = delta/4;
                } else
                        // mprintf((0, "d"));
                        *dest = delta;
        } else {
                // mprintf((0, "!"));
                *dest = delta;
        }
        */
        *dest = delta;
}


// ----------------------------------------------------------------------------
// physics_apply_whack applies an instaneous whack on an object changing
// both the objects velocity and the rotational velocity based on the impulse
// being applied.  
//
//      input:  impulse         =>              impulse vector ( force*time = impulse = change in momentum (mv) )
//                              pos                     =>              vector from center of mass to location of where the force acts
//                              pi                              =>              pointer to phys_info struct of object getting whacked
//                              orient          =>              orientation matrix (needed to set rotational impulse in body coords)
//                              mass                    =>              mass of the object (may be different from pi.mass if docked)
//
#define WHACK_LIMIT     0.001f
#define ROTVEL_WHACK_CONST 0.12
void physics_apply_whack(vector *impulse, vector *pos, physics_info *pi, matrix *orient, float mass)
{
        vector  local_torque, torque;
//      vector  npos;

        //      Detect null vector.
        if ((fl_abs(impulse->xyz.x) <= WHACK_LIMIT) && (fl_abs(impulse->xyz.y) <= WHACK_LIMIT) && (fl_abs(impulse->xyz.z) <= WHACK_LIMIT))
                return;

        // first do the rotational velocity
        // calculate the torque on the body based on the point on the
        // object that was hit and the momentum being applied to the object

        vm_vec_crossprod(&torque, pos, impulse);
        vm_vec_rotate ( &local_torque, &torque, orient );

        vector delta_rotvel;
        vm_vec_rotate( &delta_rotvel, &local_torque, &pi->I_body_inv );
        vm_vec_scale ( &delta_rotvel, (float) ROTVEL_WHACK_CONST );
        vm_vec_add2( &pi->rotvel, &delta_rotvel );

#ifdef ROT_DEBUG
        if (check_rotvel_limit( pi )) {
                nprintf(("Physics", "rotvel reset in physics_apply_whack\n"));
        }
#endif

        //mprintf(("Whack: %7.3f %7.3f %7.3f\n", pi->rotvel.xyz.x, pi->rotvel.xyz.y, pi->rotvel.xyz.z));

        // instant whack on the velocity
        // reduce damping on all axes
        pi->flags |= PF_REDUCED_DAMP;
        update_reduced_damp_timestamp( pi, vm_vec_mag(impulse) );

        // find time for shake from weapon to end
        int dtime = timestamp_until(pi->afterburner_decay);
        if (dtime < WEAPON_SHAKE_TIME) {
                pi->afterburner_decay = timestamp( WEAPON_SHAKE_TIME );
        }

        vm_vec_scale_add2( &pi->vel, impulse, 1.0f / pi->mass );
        if (!(pi->flags & PF_USE_VEL) && (vm_vec_mag_squared(&pi->vel) > MAX_SHIP_SPEED*MAX_SHIP_SPEED)) {
                // Get DaveA
                nprintf(("Physics", "speed reset in physics_apply_whack [speed: %f]\n", vm_vec_mag(&pi->vel)));
//              Int3();
                vm_vec_normalize(&pi->vel);
                vm_vec_scale(&pi->vel, (float)RESET_SHIP_SPEED);
        }
        vm_vec_rotate( &pi->prev_ramp_vel, &pi->vel, orient );          // set so velocity will ramp starting from current speed
                                                                                                                                                                        // ramped velocity is now affected by collision
}

// function generates a velocity ramp with a given time constant independent of frame rate
// uses an exponential approach to desired velocity and a cheat when close to improve closure speed
float velocity_ramp (float v_in, float v_goal, float ramp_time_const, float t)
{
        float delta_v;
        float decay_factor;
        float dist;

        // JAS: If no time elapsed, change nothing
        if ( t==0.0f )
                return v_in;

        delta_v = v_goal - v_in;
        dist = (float)fl_abs(delta_v);

        // hack to speed up closure when close to goal
        if (dist < ramp_time_const/3)
                ramp_time_const = dist / 3;

        // Rather than get a divide by zero, just go to the goal
        if ( ramp_time_const < 0.0001f )        {
                return v_goal;
        }

        // determine decay factor  (ranges from 0 for short times to 1 for long times)
        // when decay factor is near 0, the velocity in nearly unchanged
        // when decay factor in near 1, the velocity approaches goal
        decay_factor = (float)exp(- t / ramp_time_const);

        return (v_in + delta_v * (1.0f - decay_factor) );
}


// ----------------------------------------------------------------------------
// physics_apply_shock applies applies a shockwave to an object.  This causes a velocity impulse and 
// and a rotational impulse.  This is different than physics_apply_whack since a shock wave is a pressure
// wave which acts over the *surface* of the object, not a point.
//
// inputs:      direction_vec           =>              a position vector whose direction is from the center of the shock wave to the object
//                              pressure                                =>              the pressure of the shock wave at the object
//                              pi                                              =>              physics_info structure
//                              orient                          =>              matrix orientation of the object
//                              min                                     =>              vector of minimum values of the bounding box
//                              max                                     =>              vector of maximum values of the bounding box
//                              radius                          =>              bounding box radius of the object, used for scaling rotation
//
// outputs:     makes changes to physics_info structure rotvel and vel variables
//                              
#define STD_PRESSURE            1000            // amplitude of standard shockwave blasts
#define MIN_RADIUS                      10                      // radius within which full rotvel and shake applied
#define MAX_RADIUS                      50                      // radius at which no rotvel or shake applied
#define MAX_ROTVEL                      0.4             // max rotational velocity
#define MAX_SHAKE                       0.1             // max rotational amplitude of shake
#define MAX_VEL                         8                       // max vel from shockwave 
void physics_apply_shock(vector *direction_vec, float pressure, physics_info *pi, matrix *orient, vector *min, vector *max, float radius)
{
        vector normal;
        vector local_torque, temp_torque, torque;
        vector impact_vec;
        vector area;
        vector sin;

        if (radius > MAX_RADIUS) {
                return;
        }

        vm_vec_normalize_safe ( direction_vec );

        area.xyz.x = (max->xyz.y - min->xyz.z) * (max->xyz.z - min->xyz.z);
        area.xyz.y = (max->xyz.x - min->xyz.x) * (max->xyz.z - min->xyz.z);
        area.xyz.z = (max->xyz.x - min->xyz.x) * (max->xyz.y - min->xyz.y);

        normal.xyz.x = vm_vec_dotprod( direction_vec, &orient->v.rvec );
        normal.xyz.y = vm_vec_dotprod( direction_vec, &orient->v.uvec );
        normal.xyz.z = vm_vec_dotprod( direction_vec, &orient->v.fvec );

        sin.xyz.x = fl_sqrt( fl_abs(1.0f - normal.xyz.x*normal.xyz.x) );
        sin.xyz.y = fl_sqrt( fl_abs(1.0f - normal.xyz.y*normal.xyz.y) );
        sin.xyz.z = fl_sqrt( fl_abs(1.0f - normal.xyz.z*normal.xyz.z) );        

        (void) vm_vec_make( &torque, 0.0f, 0.0f, 0.0f );

        // find the torque exerted due to the shockwave hitting each face
        //  model the effect of the shockwave as if the shockwave were a plane of projectiles,
        //  all moving in the direction direction_vec.  then find the torque as the cross prod
        //  of the force (pressure * area * normal * sin * scale * mass)
        //  normal takes account the fraction of the surface exposed to the shockwave
        //  the sin term is not technically needed but "feels" better
        //  scale factors out the increase in area with larger objects
        //  more massive objects get less rotation

        // find torque due to forces on the right/left face
        if ( normal.xyz.x < 0.0f )              // normal < 0, hits the right face
                vm_vec_copy_scale( &impact_vec, &orient->v.rvec, max->xyz.x * pressure * area.xyz.x *  normal.xyz.x * sin.xyz.x / pi->mass );
        else                                                            // normal > 0, hits the left face
                vm_vec_copy_scale( &impact_vec, &orient->v.rvec, min->xyz.x * pressure * area.xyz.x * -normal.xyz.x * sin.xyz.x / pi->mass );

        vm_vec_crossprod( &temp_torque, &impact_vec, direction_vec );
        vm_vec_add2( &torque, &temp_torque );

        // find torque due to forces on the up/down face
        if ( normal.xyz.y < 0.0f )
                vm_vec_copy_scale( &impact_vec, &orient->v.uvec, max->xyz.y * pressure * area.xyz.y *  normal.xyz.y * sin.xyz.y / pi->mass );
        else
                vm_vec_copy_scale( &impact_vec, &orient->v.uvec, min->xyz.y * pressure * area.xyz.y * -normal.xyz.y * sin.xyz.y / pi->mass );

        vm_vec_crossprod( &temp_torque, &impact_vec, direction_vec );
        vm_vec_add2( &torque, &temp_torque );

        // find torque due to forces on the forward/backward face
        if ( normal.xyz.z < 0.0f )
                vm_vec_copy_scale( &impact_vec, &orient->v.fvec, max->xyz.z * pressure * area.xyz.z *  normal.xyz.z * sin.xyz.z / pi->mass );
        else
                vm_vec_copy_scale( &impact_vec, &orient->v.fvec, min->xyz.z * pressure * area.xyz.z * -normal.xyz.z * sin.xyz.z / pi->mass );

        vm_vec_crossprod( &temp_torque, &impact_vec, direction_vec );
        vm_vec_add2( &torque, &temp_torque );

        // compute delta rotvel, scale according to blast and radius
        float scale;
        vector delta_rotvel;
        vm_vec_rotate( &local_torque, &torque, orient );
        vm_vec_copy_normalize(&delta_rotvel, &local_torque);
        if (radius < MIN_RADIUS) {
                scale = 1.0f;
        } else {
                scale = (MAX_RADIUS - radius)/(MAX_RADIUS-MIN_RADIUS);
        }
        vm_vec_scale(&delta_rotvel, (float)(MAX_ROTVEL*(pressure/STD_PRESSURE)*scale));
        // nprintf(("Physics", "rotvel scale %f\n", (MAX_ROTVEL*(pressure/STD_PRESSURE)*scale)));
        vm_vec_add2(&pi->rotvel, &delta_rotvel);

        // set shockwave shake amplitude, duration, flag
        pi->shockwave_shake_amp = (float)(MAX_SHAKE*(pressure/STD_PRESSURE)*scale);
        pi->shockwave_decay = timestamp( SW_BLAST_DURATION );
        pi->flags |= PF_IN_SHOCKWAVE;

        // set reduced translational damping, set flags
        float velocity_scale = (float)MAX_VEL*scale;
        pi->flags |= PF_REDUCED_DAMP;
        update_reduced_damp_timestamp( pi, velocity_scale*pi->mass );
        vm_vec_scale_add2( &pi->vel, direction_vec, velocity_scale );
        vm_vec_rotate(&pi->prev_ramp_vel, &pi->vel, orient);    // set so velocity will ramp starting from current speed

        // check that kick from shockwave is not too large
        if (!(pi->flags & PF_USE_VEL) && (vm_vec_mag_squared(&pi->vel) > MAX_SHIP_SPEED*MAX_SHIP_SPEED)) {
                // Get DaveA
                nprintf(("Physics", "speed reset in physics_apply_shock [speed: %f]\n", vm_vec_mag(&pi->vel)));
//              Int3();
                vm_vec_normalize(&pi->vel);
                vm_vec_scale(&pi->vel, (float)RESET_SHIP_SPEED);
        }

#ifdef ROT_DEBUG
        if (check_rotvel_limit( pi )) {
                nprintf(("Physics", "rotvel reset in physics_apply_shock\n"));
        }
#endif

                                                                                                                                                                // ramped velocity is now affected by collision
}

// ----------------------------------------------------------------------------
// physics_collide_whack applies an instaneous whack on an object changing
// both the objects velocity and the rotational velocity based on the impulse
// being applied.  
//
//      input:  impulse                                 =>              impulse vector ( force*time = impulse = change in momentum (mv) )
//                              world_delta_rotvel      =>              change in rotational velocity (already calculated)
//                              pi                                                      =>              pointer to phys_info struct of object getting whacked
//                              orient                                  =>              orientation matrix (needed to set rotational impulse in body coords)
//

// Warning:  Do not change ROTVEL_COLLIDE_WHACK_CONST.  This will mess up collision physics.
// If you need to change the rotation, change  COLLISION_ROTATION_FACTOR in collide_ship_ship.
#define ROTVEL_COLLIDE_WHACK_CONST 1.0
void physics_collide_whack( vector *impulse, vector *world_delta_rotvel, physics_info *pi, matrix *orient )
{
        vector  body_delta_rotvel;

        //      Detect null vector.
        if ((fl_abs(impulse->xyz.x) <= WHACK_LIMIT) && (fl_abs(impulse->xyz.y) <= WHACK_LIMIT) && (fl_abs(impulse->xyz.z) <= WHACK_LIMIT))
                return;

        vm_vec_rotate( &body_delta_rotvel, world_delta_rotvel, orient );
//      vm_vec_scale( &body_delta_rotvel, (float)       ROTVEL_COLLIDE_WHACK_CONST );
        vm_vec_add2( &pi->rotvel, &body_delta_rotvel );

#ifdef ROT_DEBUG
        if (check_rotvel_limit( pi )) {
                nprintf(("Physics", "rotvel reset in physics_collide_whack\n"));
        }
#endif

        update_reduced_damp_timestamp( pi, vm_vec_mag(impulse) );

        // find time for shake from weapon to end
        int dtime = timestamp_until(pi->afterburner_decay);
        if (dtime < WEAPON_SHAKE_TIME) {
                pi->afterburner_decay = timestamp( WEAPON_SHAKE_TIME );
        }

        pi->flags |= PF_REDUCED_DAMP;
        vm_vec_scale_add2( &pi->vel, impulse, 1.0f / pi->mass );
        // check that collision does not give ship too much speed
        // reset if too high
        if (!(pi->flags & PF_USE_VEL) && (vm_vec_mag_squared(&pi->vel) > MAX_SHIP_SPEED*MAX_SHIP_SPEED)) {
                // Get DaveA
                nprintf(("Physics", "speed reset in physics_collide_whack [speed: %f]\n", vm_vec_mag(&pi->vel)));
//              Int3();
                vm_vec_normalize(&pi->vel);
                vm_vec_scale(&pi->vel, (float)RESET_SHIP_SPEED);
        }
        vm_vec_rotate( &pi->prev_ramp_vel, &pi->vel, orient );          // set so velocity will ramp starting from current speed
                                                                                                                                                                        // ramped velocity is now affected by collision
        // rotate previous ramp velocity (in model coord) to be same as vel (in world coords)
}


int check_rotvel_limit( physics_info *pi )
{
        if ( 0 == pi->flags )           // weapon
                return 0;

        if ( Fred_running )
                return 0;

        int change_made = 0;
        if ( !(pi->flags & PF_DEAD_DAMP) ) {
                // case of normal, live ship
                // -- Commented out by MK: Assert( vm_vec_mag_squared(&pi->max_rotvel) > ROTVEL_TOL );
                // Assert( (pi->max_rotvel.xyz.x <= ROTVEL_CAP) && (pi->max_rotvel.xyz.y <= ROTVEL_CAP) && (pi->max_rotvel.xyz.z <= ROTVEL_CAP) );
                //              Warning(LOCATION,"Excessive rotvel (wx: %f, wy: %f, wz:%f)\n", pi->rotvel.xyz.x, pi->rotvel.xyz.y, pi->rotvel.xyz.z);
                if ( fl_abs(pi->rotvel.xyz.x) > pi->max_rotvel.xyz.x ) {
                        pi->rotvel.xyz.x = (pi->rotvel.xyz.x / fl_abs(pi->rotvel.xyz.x)) * (pi->max_rotvel.xyz.x - (float) ROTVEL_TOL);
                        change_made = 1;
                }
                if ( fl_abs(pi->rotvel.xyz.y) > pi->max_rotvel.xyz.y ) {
                        pi->rotvel.xyz.y = (pi->rotvel.xyz.y / fl_abs(pi->rotvel.xyz.y)) * (pi->max_rotvel.xyz.y - (float) ROTVEL_TOL);
                        change_made = 1;
                }
                if ( fl_abs(pi->rotvel.xyz.z) > pi->max_rotvel.xyz.z ) {
                        pi->rotvel.xyz.z = (pi->rotvel.xyz.z / fl_abs(pi->rotvel.xyz.z)) * (pi->max_rotvel.xyz.z - (float) ROTVEL_TOL);
                        change_made = 1;
                }
        } else { 
                // case of dead ship
                if ( fl_abs(pi->rotvel.xyz.x) > DEAD_ROTVEL_CAP ) {
                        pi->rotvel.xyz.x = (pi->rotvel.xyz.x / fl_abs(pi->rotvel.xyz.x)) * (float) (DEAD_ROTVEL_CAP - ROTVEL_TOL);
                        change_made = 1;
                }
                if ( fl_abs(pi->rotvel.xyz.y) > DEAD_ROTVEL_CAP ) {
                        pi->rotvel.xyz.y = (pi->rotvel.xyz.y / fl_abs(pi->rotvel.xyz.y)) * (float) (DEAD_ROTVEL_CAP - ROTVEL_TOL);
                        change_made = 1;
                }
                if ( fl_abs(pi->rotvel.xyz.z) > DEAD_ROTVEL_CAP ) {
                        pi->rotvel.xyz.z = (pi->rotvel.xyz.z / fl_abs(pi->rotvel.xyz.z)) * (float) (DEAD_ROTVEL_CAP - ROTVEL_TOL);
                        change_made = 1;
                }
        }
        return change_made;
}

// ----------------------------------------------------------------------------
// update_reduced_damp_timestamp()
//
void update_reduced_damp_timestamp( physics_info *pi, float impulse )
{

        // Compute duration of reduced damp from present
        // Compare with current value and increase if greater, otherwise ignore
        int reduced_damp_decay_time;
        reduced_damp_decay_time = (int) (REDUCED_DAMP_TIME * impulse / (REDUCED_DAMP_VEL * pi->mass));
        if ( reduced_damp_decay_time > REDUCED_DAMP_TIME )
                reduced_damp_decay_time = REDUCED_DAMP_TIME;

        // Reset timestamp if larger than current (if any)
        if ( timestamp_valid( pi->reduced_damp_decay ) ) {
                int time_left = timestamp_until( pi->reduced_damp_decay );
                if ( time_left > 0 ) {
                        // increment old time, but apply cap
                        int new_time = reduced_damp_decay_time + time_left;
                        if ( new_time < REDUCED_DAMP_TIME ) {
                                pi->reduced_damp_decay = timestamp( new_time );
                        }
                } else {
                        pi->reduced_damp_decay = timestamp( reduced_damp_decay_time );
                }
        } else {
                // set if not valid
                pi->reduced_damp_decay = timestamp( reduced_damp_decay_time );
        }

}