Thanks for your patient reply,  Maybe   I    find the reason for this issue,    because the car  looks invert ,  I  change the direction(Z) vector as following :

                                SimpleVector posAhead = Interp(lookAheadAmount+percent);
            SimpleVector poscurrent = cube.getTransformedCenter();
            posAhead.sub(poscurrent);
            /*
            the car is invert , if I use   posAhead.scalarMul(-1f); the car looks ok,
            I think  if here is the reason for issue
            */
            posAhead.scalarMul(-1f);
            Matrix rotY=posAhead.getRotationMatrix( new SimpleVector(0,-1,0));
            cube.setRotationMatrix(rotY);


So   I    remove this line : 
                               
                                posAhead.scalarMul(-1f);

In this case , the camera is behind the car , of course ,the car looks invert.  but  a new issue appears,  the distance between car and camera becomes  far and far, and the car out of view at last ,   here is camera postion set code:


                      Camera camera = world.getCamera();
            SimpleVector oldCamPos=camera.getPosition();
            SimpleVector oldestCamPos=new SimpleVector(oldCamPos);
            oldCamPos.scalarMul(9f);

            SimpleVector carCenter=cube.getTransformedCenter();
            SimpleVector camPos=new SimpleVector(carCenter);
            SimpleVector zOffset=cube.getZAxis();
            SimpleVector yOffset=new SimpleVector(0, -10, 0); //-100
            zOffset.scalarMul(-20); //-250

            camPos.add(zOffset);
            camPos.add(yOffset);

            camPos.add(oldCamPos);
           
            camPos.scalarMul(0.1f);

            SimpleVector delta=camPos.calcSub(carCenter);
            //distance between camera and car
            float len=delta.length();

            Logger.log("-->distance between camera and  car --->" +  len ); // the len becomes  biger and biger and  the car out of  view at last
           
                      camera.setPosition(camPos);
            camera.lookAt(cube.getTransformedCenter());


I try to change the  ZOffset  , but  no effect  , distance between camera and car should constant  and  a little  change. Could you help me? Thanks a lot.




I captured  some pictures help me to explain clearn.

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Maybe, but when camera behind the car , it follow the car indeed

Yes ,  it take from the car example of desktop jPCT,  The issue is  , if the car's speed is slow,  the carmera is  closer to the car , but  if   the speed of car is high , the camera is far   to the car ,   in other words , the distance between camera  and car  is becomes very big , close and far , I hope you  can see  my explain

hello , I have a new question ,  camera follow the car use the code as show before ,  if the car speed slow, the camera close to the car ,but if the car speed is  high  ,  the camera is become very far to the  car ,   this is not my expect ,  I  want  the distance between camera and car is suitable, not so far ,  I change the   zOffset.scalarMul(Xf)  , but it seems no effect , How should I do ?  Thanks very much

Thanks EgonOlsen, Now , the car can run along the road by my desinged path , the camera follow behind the car .

But it  seems the car is static , because the  camera position  is set by car position, I pick  code  from other topic is :

      SimpleVector oldCamPos = camera.getPosition();
      oldCamPos.scalarMul(9f);
      SimpleVector carCenter = car.getTransformedCenter();
      SimpleVector zOffset = car.getZAxis();
      zOffset.scalarMul(-250f);
      SimpleVector camPos = new SimpleVector(carCenter);
      camPos.add(camYOffset);
      camPos.add(zOffset);
      camPos.add(oldCamPos);
      camPos.scalarMul(0.1f);
      camera.setPosition(camPos);
      camera.lookAt(carCenter);

      Is there  some way  to let camera  smooth follow the car ?   Camera have little movement up and down or right to left  ,  thanks a lot ~

thanks for your help , I  change the code like this


//get the car current  position
SimpleVector poscur = car.getTransformedCenter();

//get the car lookat waypoint , the  nav point is ahead of car
SimpleVector posLookat = getLookatwaypoint();

posLookat .scalarMul(-1f);
posLookat .add(poscur );
Matrix rotY=posLookat .getRotationMatrix(new SimpleVector(0,-1,0));
car.setRotationMatrix(rotY);

the car  rotation  and lookat seems  ok , Thank again ~

And  I   have another question ,    I  put some object on my road to use as "waypoint",  I load the model  and  get the waypoint position like this:

Object3D [] obj = Loader.loadOBJ(res.getAssets().open("road.obj"),null,100f);
int k =0;
 for(int i = 0; i < obj.length; i++){
 if(obj.getName().contains("w.0")){
obj.build();
 waypoint[k++] = obj.getTransformedCenter();                        
}
else if(obj.getName().contains("Plane"))
{
saidao = obj;
Logger.log("get the road!!!!!!");
}
}

saidao.rotateX((float) (Math.PI) / 1f);
....

actually, the waypoint get by this method is not correct , because  the road  rotateX  late !  So the waypoint  must do  some actions  to location correct position, but  I don't know how to to do this , please help me ,  Thank you very much !!


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