Demo of swinging string

draw.c

@@ -119,6 +119,19 @@ void draw_collision_poly(SDL_Renderer* ren, Body *body) {
         SDL_SetRenderDrawColor(ren, 0, 255, 0, 255);
     }
 
+    // draw strings
+    for (int i = 0; i < body->num_strings; i++) {
+        if (!body->strings[i].attached) {
+            continue;
+        }
+        printf("Draw string");
+        Vect B;
+        B = in_view(body->position);
+        Vect E;
+        E = in_view(body->strings[i].end_point);
+        SDL_RenderDrawLine(ren, B.x, B.y, E.x, E.y);
+    }
+
     if (body->collision_poly_size == 1) {
         SDL_Rect dot;
         Vect V;

draw.h

@@ -5,7 +5,7 @@
 #ifndef _DEFDRAW
 #define _DEFDRAW
 
-#define SHOWCOLLISION 0
+#define SHOWCOLLISION 1
 #define SHOWFORCES 0
 
 #include "garbo.h"

game.c

@@ -146,6 +146,31 @@ void get_new_physics(Body **phys) {
     /*physics->collision_poly[0].y = physics->y_pos;*/
 /*}*/
 
+
+int string_update_fixed(String *string) {
+    return 0;
+}
+
+int string_set_end(String *string, void *setting) {
+    string->end_point = *(Vect *)setting;
+    return 0;
+}
+
+String *get_fixed_strings(int number) {
+    String *strings = calloc(number, sizeof(String)); 
+    for (int i = 0; i < number; i++) {
+
+        strings[i].attached = false;
+
+        // takes self (String) and does nothing. 
+        strings[i].update_end_point = string_update_fixed; 
+
+        // takes self and vect to set string end point
+        strings[i].set_end_point = string_set_end; 
+    }
+    return strings;
+}
+
 player_st get_player(int x, int y) {
     /* creates player at given postion and zeroes physics */
 
@@ -176,6 +201,19 @@ player_st get_player(int x, int y) {
 
     player.physics->updateCollisionPoly = cast_update_collision_poly;
 
+    // swing rope
+    player.physics->num_strings = 1;
+    player.physics->max_strings = 1;
+    player.physics->strings = get_fixed_strings(1);
+    player.physics->strings->max_length = 890;
+
+    // testing
+    Vect end_pt;
+    end_pt.x = -100;
+    end_pt.y = 0;
+    player.physics->strings[0].set_end_point(player.physics->strings, &end_pt);
+    player.physics->strings[0].attached = true;
+
     Vect *rect = player.physics->collision_shape;
     rect[0].x = -10; rect[0].y = -10;
     rect[1].x = 10;  rect[1].y = -10;
@@ -183,7 +221,8 @@ player_st get_player(int x, int y) {
     rect[3].x = -10; rect[3].y = 10;
 
     // gravity
-    set_motor_newtons(player.physics, M_GRAVITY, 0.0, player.physics->obj_mass * 4);
+    set_motor_newtons(player.physics, M_GRAVITY, 0.0, 
+            player.physics->obj_mass * 4);
 
     // walking motor
     add_motor(player.physics, 0.0, player.physics->obj_mass * 9.81);
@@ -602,7 +641,6 @@ void advance_thing(Body * thing) {
         thing->updateCollisionPoly(thing);
     }
 
-
     thing->updateCollisionPoly(thing);
     Vect translation;
     Vect friction;
@@ -663,6 +701,63 @@ void advance_thing(Body * thing) {
         /*accel_thing(thing, rest.x, rest.y);*/
     }
 
+    for (int i = 0; i < thing->num_strings; i++) {
+        if (!thing->strings[i].attached) {
+            continue;
+        }
+
+        double st_len = vect_distance(thing->next_position, thing->strings[i].end_point);
+        double max_len = thing->strings[i].max_length;
+        if (st_len > max_len) {
+
+            printf("STRING: %f, %f\n", st_len, max_len);
+
+            Vect end = thing->strings[i].end_point;
+            Vect position = thing->next_position;
+
+            Vect string;
+            string.x = end.x - position.x;
+            string.y = end.y - position.y;
+
+            double disp = st_len - max_len;
+            double angle = atan2(string.y, string.x);
+            double scale_factor =  max_len / vect_mag(string);
+
+            Vect corrected_string;
+            corrected_string.x = cos(angle) * max_len;
+            corrected_string.y = sin(angle) * max_len;
+            printf("corrected string : %f %f\n", corrected_string.x, corrected_string.y);
+            thing->next_position = vect_scalar(thing->next_position, 
+                    scale_factor);
+
+            /*thing->next_position.x += cos(angle) * disp;*/
+            /*thing->next_position.y += sin(angle) * disp;*/
+
+            // move the thing back to the right position
+            /*Vect adj_pos = vect_add(vect_scalar(corrected_string, -1),*/
+                    /*string);*/
+            /*adj_pos = vect_scalar(adj_pos, -1);*/
+            /*thing->position = vect_add(thing->position, adj_pos);*/
+
+            // set velocity to 0 in direction of string
+            Vect vel_correction = project_vect(thing->vel, string);
+            double dir;
+            double corr = vect_scalar_projection(thing->vel, string);
+            if (corr < 0) {
+                corr *= -1;
+            } 
+            Vect v_corr;
+            v_corr.y = corr * sin(angle);
+            v_corr.x = corr * cos(angle);
+
+            thing->vel = vect_add(v_corr, thing->vel);
+
+        }
+
+
+
+    }
+
     uint32_t now = SDL_GetTicks();
 
     uint32_t time_delta = now - thing->last_advance_time ;
@@ -799,9 +894,7 @@ void add_to_world(world_thing thing) {
     memcpy(world + things_in_world, &thing, sizeof(world_thing));
 
     things_in_world += 1;
-
     //printf("Added to world: %d\n", thing.kind);
-
 }
 
 void get_room(void) {
@@ -835,7 +928,7 @@ void startgame(SDL_Renderer * ren) {
     memset(world, 0, sizeof(world_thing) * 100);
     SDL_GetRendererOutputSize(ren, &width, &height);
 
-    player = get_player(500,500);
+    player = get_player(100,1000);
 
     world_thing player_world;
 

game.h

@@ -45,8 +45,22 @@ typedef struct {
     int y;
 } Point;
 
-typedef struct BodyStruct{
+/* String structure, used as a sub-element of a Body, compressible, but not
+ * stretchable. */
+struct String {
+    bool attached;
+    double max_length;
+    Vect end_point;
+    int (*update_end_point)(struct String*); // method to update the end pt
+                                              // say if string is attached
+                                              // to another object
+    int (*set_end_point)(struct String*, void *); // manually set the end point of 
+                                           // string
+};
 
+typedef struct String String;
+
+typedef struct BodyStruct{
     // turn on dynamic physics
     // For moving objects.
     // eg. on for payer, off for walls
@@ -78,7 +92,6 @@ typedef struct BodyStruct{
     //float y_acc;
     /*------------------*/
 
-
     // constants
     double elasticity;
 
@@ -89,7 +102,6 @@ typedef struct BodyStruct{
     int collision_shape_size;
     void (*updateCollisionPoly)(struct BodyStruct *);
 
-
     // fields
     double glob_friction;
     bool glob_gravity; // t/f
@@ -100,6 +112,11 @@ typedef struct BodyStruct{
     int num_motors;
     int max_motors;
     Motor *motors;
+
+    int num_strings;
+    int max_strings;
+    String *strings;
+
 } Body;
 
 typedef struct {

vect.c

@@ -47,3 +47,12 @@ double vect_scalar_projection(Vect V, Vect P) {
 double vect_dir(Vect V) {
     return atan2(V.y, V.x);
 }
+
+// distance between two points
+double vect_distance(Vect A, Vect B) {
+    double y = (B.y - A.y);
+    double x = (B.x - A.x);
+    y = y * y;
+    x = x * x;
+    return sqrt(x + y);
+}

vect.h

@@ -30,4 +30,6 @@ Vect project_vect(Vect one, Vect two);
 /* Return the scalar projection of V onto P */
 double vect_scalar_projection(Vect V, Vect P); 
 
+/* Return the distance between two point vectors A and B */
+double vect_distance(Vect A, Vect B); 
 #endif