Panda3D Manual: Using Intervals to move the Panda (CXX)
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The next step is to cause the panda to actually move back and forth. Add the following lines of code:

#include "pandaFramework.h"
#include "pandaSystem.h"

#include "cIntervalManager.h"
#include "cLerpNodePathInterval.h"
#include "cMetaInterval.h"

PandaFramework framework;

int main(int argc, char *argv[]) {
    //open a new window framework and set title
  framework.open_framework(argc, argv);
  framework.set_window_title("My Panda3D Window");
  
    //open the window
  WindowFramework *window = framework.open_window();
  NodePath cam = window->get_camera_group(); //get the camera and store it
 
    //load the environment model
  NodePath environ = window->load_model(framework.get_models(),"models/environment");
  environ.reparent_to(window->get_render());
  environ.set_scale(0.25,0.25,0.25);
  environ.set_pos(-8,42,0);

    //load our panda
  NodePath pandaActor = window->load_model(framework.get_models(),"panda-model");
  pandaActor.set_scale(0.005,0.005,0.005);
  pandaActor.reparent_to(window->get_render());
  
    //load the walk animation
  window->load_model(pandaActor,"panda-walk4");
  window->loop_animations(0);

    //create the lerp intervals needed to walk back and forth
  PT(CLerpNodePathInterval) pandaPosInterval1;
  pandaPosInterval1 = new CLerpNodePathInterval("pandaPosInterval1",13.0,CLerpInterval::BT_no_blend,
                              true,false,pandaActor,window->get_render());
  pandaPosInterval1->set_start_pos(LPoint3f(0,10,0));
  pandaPosInterval1->set_end_pos(LPoint3f(0,-10,0));
  PT(CLerpNodePathInterval) pandaPosInterval2;
  pandaPosInterval2 = new CLerpNodePathInterval("pandaPosInterval2",13.0,CLerpInterval::BT_no_blend,
                              true,false,pandaActor,window->get_render());
  pandaPosInterval2->set_start_pos(LPoint3f(0,-10,0));
  pandaPosInterval2->set_end_pos(LPoint3f(0,10,0));
  PT(CLerpNodePathInterval) pandaHprInterval1;
  pandaHprInterval1 = new CLerpNodePathInterval("pandaHprInterval1",3.0,CLerpInterval::BT_no_blend,
                              true,false,pandaActor,window->get_render());
  pandaHprInterval1->set_start_hpr(LPoint3f(0,0,0));
  pandaHprInterval1->set_end_hpr(LPoint3f(180,0,0));
  PT(CLerpNodePathInterval) pandaHprInterval2;
  pandaHprInterval2 = new CLerpNodePathInterval("pandaHprInterval2",3.0,CLerpInterval::BT_no_blend,
                              true,false,pandaActor,window->get_render());
  pandaHprInterval2->set_start_hpr(LPoint3f(180,0,0));
  pandaHprInterval2->set_end_hpr(LPoint3f(0,0,0));
  
    //create and play the sequence that coordinates the intervals
  PT(CMetaInterval) pandaPace;
  pandaPace = new CMetaInterval("pandaPace");
  pandaPace->add_c_interval(pandaPosInterval1,0,CMetaInterval::RS_previous_end);
  pandaPace->add_c_interval(pandaHprInterval1,0,CMetaInterval::RS_previous_end);
  pandaPace->add_c_interval(pandaPosInterval2,0,CMetaInterval::RS_previous_end);
  pandaPace->add_c_interval(pandaHprInterval2,0,CMetaInterval::RS_previous_end);
  pandaPace->loop();

    //do the main loop:
  ClockObject* clock (ClockObject::get_global_clock()); //get the global clock object
  Thread *current_thread = Thread::get_current_thread();
  while(framework.do_frame(current_thread)) {
    CIntervalManager::get_global_ptr()->step(); //step the interval manager
    double time = clock->get_real_time(); //get the time in seconds
    double angledegrees = time * 6.0; //the angle of the camera in degrees
    double angleradians = angledegrees * (3.14 / 180.0); //in radians
    cam.set_pos(20*sin(angleradians),-20.0*cos(angleradians),3); //set the position
    cam.set_hpr(angledegrees, 0, 0); //set the hpr
  }

    //close the window framework
  framework.close_framework();
  return (0);
}

Intervals are tasks that change a property from one value to another over a specified period of time. Starting an interval effectively starts a background process that modifies the property over the specified period of time. For example, consider the pandaPosInterval1 above. When that interval is started, it will gradually adjust the position of the panda from (0,10,0) to (0,-10,0) over a period of 13 seconds. Similarly, when the pandaHprInterval1 is started, the orientation of the panda will rotate 180 degrees over a period of 3 seconds.

MetaIntervals are tasks that execute one interval after another. The pandaPace sequence above causes the panda to move in a straight line, then turn, then in the opposite straight line, then to turn again. The code pandaPace->loop() causes the MetaInterval to be started in looping mode.

The result of all this is to cause the panda to pace back and forth from one tree to the other.

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