Android min3D使用触摸旋转对象

Question

I'm trying to rotate the camera around an object by using the touchscreen. The rotation around Y axis works fine (The X axis is disabled). Rotation around the X axis is really weird. when the Object (its a rocket) gets higher rocket.position().y++ & scene.camera().position.y++ , the rotation around the x axis gets bigger and weird. If the rockets stops rocket.position().y = 500; & scene.camera().position.y = 500; rocket.position().y = 500; & scene.camera().position.y = 500; , I can't rotate around x axis, I zoom in or out the object instead. With both axis enabled its weird as hell.

In initScene I set the camera to look at the center of the rocket.

Here's my code:

initScene:
    scene.camera().position.z = 90;
    scene.camera().target = raketeOBJ.position();

onTouchEvent:
    public boolean onTouchEvent(MotionEvent me) {
     if (me.getAction() == MotionEvent.ACTION_DOWN) {
        xpos = me.getX();
        ypos = me.getY();
        return true;
    }

    if (me.getAction() == MotionEvent.ACTION_UP) {
        xpos = -1;
        ypos = -1;
        touchTurn = 0;
        touchTurnUp = 0;
        return true;
    }

    if (me.getAction() == MotionEvent.ACTION_MOVE) {
        float xd = me.getX() - xpos;
        float yd = me.getY() - ypos;

        xpos = me.getX();
        ypos = me.getY();

        touchTurn = xd / -200f;
        touchTurnUp = yd / -200f;
        return true;
    }

    try {
        Thread.sleep(15);
    } catch (Exception e) {

    }

    return super.onTouchEvent(me);
}

UpdateScene:

    if (touchTurn != 0) {
        scene.camera().position.rotateY(touchTurn);
        touchTurn = 0;
    }

    if (touchTurnUp != 0) {
        scene.camera().position.rotateX(touchTurnUp);
        touchTurnUp = 0;
    }

Answer 1

I think it is a view-orientation-issue. try basic android opengl example "rotating cubes", i think it is a similar behaviour.

To avoid this you have to transform display-coordinates into min3D object rotation-coordinates.

Think about moving your finger 1cm on display to the left, with 3D-object following your finger. This means x_display = x_object. If you rotate your object, there is still a fixed connection between display and object. (x_display = - 1cm) equals (x_object = -1cm)
at 90° & 270° your object will move xy-inverted, at 180° your object will move x-inverted.

use some sin/cos to overcome this.

edit: also rotationangle (object) = - rotationangle (camera)

Answer 2

One option is to rotate the container itself instead of the camera. In min3d you often work with an Object3dContainer as a class variable in your activity that displays the model. When you get your MotionEvent you can use the method rotation() from your Object3dContainer class and set x or y properties as appropriate.

Answer 3

You keep saying wierd, but you know that it doesn't describe what you see. Rotate is actually method of Number3d. It doesn't know about the target , so I guess rotation is done according the center, not according the rocket (Didn't quite check that, just a guess). There might be 2 solutions:

1. The easiest thing is to set the rocket to (0,0,0) and instead moving the rocket, move evertything else. Well that would be a stupid suggestion. Forget it.
2. The other solution is to calculate camera rotation traectory using sin() and cos()
3. There might be a third solution, but I didn't think aa lot about it and it might be wrong. What you have to do is move the camera like target is at (0,0,0), rotate and then return the camera.

Would look like something like this:

Number3d target = scene.camera.target;
Number3d cp = scene.camera.position.clone();
// move position like target is (0,0,0)
cp.x -= target.x;
cp.y -= target.y;
cp.z -= target.z;

cp.roateX(angle);

// restore offset
cp.x += target.x;
cp.y += target.y;
cp.z += target.z;
scene.camera.position.setAllFrom(cp);

You can try 3 but don't rely it'll do the job.