ode gravity lost

main.cpp

#include <iostream>
#include <ode/ode.h>
#include <SDL.h>
#include <SDL2_gfxPrimitives.h>

dWorldID earth;
dJointGroupID contact_joint_group;
std::size_t const max_contacts = 10;
dContact contact[max_contacts];

void collision_callback(void*, dGeomID g1, dGeomID g2){
  for(std::size_t i= 0; i< max_contacts; ++ i){
    contact[i].surface.mode = dContactBounce;
    contact[i].surface.mu = 100.f;
    contact[i].surface.bounce = .5f;
    //contact[i].surface.bounce_vel = 0.f; 
  }
  int n_contacts= dCollide(g1, g2, max_contacts, &contact[0].geom, sizeof(dContact));
  if(!n_contacts)
    return;
  auto b1= dGeomGetBody(g1);
  auto b2= dGeomGetBody(g2);
  for(int i= 0; i< n_contacts; ++ i){
    auto contact_joint= dJointCreateContact(earth, contact_joint_group, &contact[i]);
    dJointAttach(contact_joint, b1, b2);
  }
}

int main(){
  int err = SDL_Init(SDL_INIT_VIDEO);
  if(err == -1){
    std::cerr<< "Error initializing SDL\n";
    return EXIT_FAILURE;
  }
  dInitODE();

  // Create earth with gravity
  earth = dWorldCreate();
  //dWorldSetERP(earth, 0.9);
  //dWorldSetCFM(earth, 0.00001);
  
  // Define ball and platform geometries
  auto collision_space= dSimpleSpaceCreate(0);
  auto ball_geom= dCreateSphere(collision_space, 40.f);
  auto platform_geom= dCreateBox(collision_space, 100.f, 40.f, 1.f);
  auto contact_joint_group = dJointGroupCreate(0);  

  // Define ball body and mass
  auto ball = dBodyCreate(earth);
  dGeomSetBody(ball_geom, ball);
  dMass ball_mass; 
  dMassSetSphereTotal(&ball_mass, 10.0, 40.0);
  dBodySetMass(ball, &ball_mass);
  dGeomSetPosition(ball_geom, 320.f, 50.f, 0.f);
  dGeomSetPosition(platform_geom, 320.f, 430.f, 0.f);

  // Prepare drawing window
  auto window = SDL_CreateWindow("test", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED, 640, 480, SDL_WINDOW_OPENGL);
  auto renderer = SDL_CreateRenderer(window, -1, 0);

  // Main Loop
  SDL_Event e;
  for(bool run = true; run;){
    dWorldSetGravity(earth, 0, 9.81, 0);
    // Handle Events
    while(SDL_PollEvent(&e)){
      switch(e.type){
        case SDL_QUIT:
          run = false;
      }
    }
    // Animate
    auto ball_pos = dBodyGetPosition(ball);
    auto platform_pos = dGeomGetPosition(platform_geom);
    dSpaceCollide(collision_space, 0, collision_callback);
    dWorldStep(earth, 0.01);
    dJointGroupEmpty(contact_joint_group);
    dBodySetGravityMode(ball, 1); // But I seem to be losing gravity attached to this object 
    
    // Draw
    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
    SDL_RenderClear(renderer);
    filledCircleRGBA(renderer, ball_pos[0], ball_pos[1], 40, 0, 255, 0, 255);
    boxRGBA(renderer, platform_pos[0]- 50, platform_pos[1]- 20, platform_pos[0] + 50, platform_pos[1]+ 20, 255, 0, 0, 255);
    SDL_RenderPresent(renderer);
  }

  // Cleanup ODE
  dBodyDestroy(ball);
  dWorldDestroy(earth);
  dCloseODE();

  // Cleanup SDL
  SDL_DestroyRenderer(renderer);
  SDL_DestroyWindow(window);
  SDL_Quit();
}