www.jpct.net

Hi !

I wanted a turret (mounted on a spaceship) to aim at a target.
So i have a turretBody only rotating around y axis and a turretBarrel rotating around x axis.

I tried to do it so:

public void targetAt(){
// Targetdirection
SimpleVector directionD = target.getTransformedCenter().calcSub(turretBody.getTransformedCenter()).normalize();

//The object's initial direction
SimpleVector directionI = turretBarrel.getWorldTransformation().getYAxis().normalize();

//Axis to rotate around
SimpleVector axis = new SimpleVector( directionD.calcCross( directionI ) ).normalize();

float turnrate = .03f;
if (axis.x < 0)
turretBarrel.rotateX(-turnrate);
else
turretBarrel.rotateX(turnrate);

if (axis.z < 0)
turretBody.rotateY(-turnrate);
else
turretBody.rotateY(turnrate);
}


Works fine if ship is not rotating.
But when the ship rotates at a specific position the turret aims exactly in the direction away from target. I think because the cross product of the vectors changes direction on different angle of the vectors.
But how can i get always the right rotateaxis ?

Here the complete code with objectgeneration:

public void init(){
world.removeAll();
createObjects();
}


public void gameloop(){
targetAt();
ship.rotateX(.005f);
target.translate(1 * translatedirection,0,0);
if (Math.abs(target.getTransformedCenter().x) > 80)
translatedirection *= -1;
}

public void createObjects(){
target = Primitives.getBox(5, 3);
target.setAdditionalColor(RGBColor.BLUE);
target.translate(0,-40,0);
world.addObject(target);

ship = Primitives.getBox(7, 1);
ship.setAdditionalColor(RGBColor.WHITE);
ship.translate(50,100,0);
world.addObject(ship);

turretBody =  Primitives.getBox(3, 1);
turretBody.setAdditionalColor(RGBColor.GREEN);
turretBody.translate(0,-10,0);
world.addObject(turretBody);

turretBarrel = Primitives.getBox(1, 7);
turretBarrel.build();
turretBarrel.rotateX(Manager.degreesToRadians(180));
turretBarrel.setRotationPivot(new SimpleVector(0,-7,0));
turretBarrel.setAdditionalColor(RGBColor.RED);
turretBarrel.translate(0,5,0);
world.addObject(turretBarrel);

ship.addChild(turretBody);
turretBody.addChild(turretBarrel);
ship.rotateX(Manager.degreesToRadians(90));
}

public void targetAt(){
// Targetdirection
SimpleVector directionD = target.getTransformedCenter().calcSub(turretBody.getTransformedCenter()).normalize();

//The object's initial direction:
SimpleVector directionI = turretBarrel.getWorldTransformation().getYAxis().normalize();

//Axis to rotate around should be:
SimpleVector axis = new SimpleVector( directionD.calcCross( directionI ) ).normalize();

float turnrate = .03f;
if (axis.x < 0)
turretBarrel.rotateX(-turnrate);
else
turretBarrel.rotateX(turnrate);

if (axis.z < 0)
turretBody.rotateY(-turnrate);
else
turretBody.rotateY(turnrate);
}


Thanks for any help.
Greetings
Dinin

Maybe you can somehow detect that this flip happens and correct it? I was digging in my sources to find something similar that i once made. It works a little different, but it suffers from the same problem. It only works if the turret's orientation doesn't change. Once it rotates too far, everything flips.