android min3d透视投影

Question

我有一些使用android 3dmin库的代码。我想要实现的是在触摸手指点绘制我的3dObject。基本功能已经完成，但我有透视投影的问题。我的3dObject被绘制到远离中心的距离。这是我的代码：

private float X = 0;
private float Y = 0;
private int width;
private int height;

private void calculateCoordinates(float x, float y)
{

    Log.d("touch", "x: "+Float.toString(((x-width/2)/5*0.01f)) +" " +
            " y: "+Float.toString(y));
    objModel.position().x=((x-width/2)/5*0.01f);
    objModel.position().y=-((y-height/2)/5*0.01f);

\\convert 2d toch point to Number3d 3dObject point. 5 is a objModel.position().z,
     \\0.01 is a constant. With setting scale at 0.09f and this constant at 0.03f i 
    \\achived desired effect, but why?

}

@Override
public void initScene() {

    width= getResources().getDisplayMetrics().widthPixels;
    height = getResources().getDisplayMetrics().heightPixels;


    scene.lights().add(new Light());
    scene.lights().add(new Light());

    IParser parser = Parser.createParser(Parser.Type.OBJ, getResources(),
            "com.example.min3dtest:raw/face_cube_obj", true);
    parser.parse();

    scene.lightingEnabled(true);

    objModel = parser.getParsedObject();
    objModel.scale().x = objModel.scale().y = objModel.scale().z = 0.03f;
    objModel.colorMaterialEnabled(true);
    objModel.texturesEnabled(true);
    scene.addChild(objModel);

    Log.d(TAG, "camera z = "+Float.toString(scene.camera().position.z));


    this.glSurfaceView().setOnTouchListener(new View.OnTouchListener() {

        @Override
        public boolean onTouch(View v, MotionEvent event) {

            int a = event.getAction();

            switch (a) {
            case MotionEvent.ACTION_DOWN:
            case MotionEvent.ACTION_MOVE:{

                X = event.getX();
                Y = event.getY();
                calculateCoordinates(X,Y);


                Log.d("sdfs", "down");
            }
                break;

            }

            return true;
        }
    });
}

@Override
public void updateScene() {

    objModel.rotation().x++;
    objModel.rotation().y++;
    objModel.rotation().z++;


}

}

它在某种程度上取决于我认为的规模。但我无法弄清楚为什么并找到好的方程式。

谢谢！