/**
 screen position change to World 3D position

 @param screenPos normalized screen position 0~1
 @param cam change world to screen camera
 @return world 3d position vec3
 */
vec3 DrwrApp::worldFromScreen( const vec2 screenPos, const CameraPersp *cam ) {
    // generate a ray from the camera into our world
    float u = screenPos.x;
    float v = screenPos.y;
    // because OpenGL and Cinder use a coordinate system
    // where (0, 0) is in the LOWERleft corner, we have to flip the v-coordinate
    Ray ray = cam->generateRay(u , 1.0f - v, cam->getAspectRatio() );

    float result = 0.0f;
    //vec3 planePos = this->mCamera.getEyePoint();
    vec3 planePos = vec3(0.f,0.f,0.f);
    vec3 normal = cam->getViewDirection();
    if ( ray.calcPlaneIntersection( planePos, normal, &result ) ) {
        return ray.calcPosition( result );
    }
    return vec3(0.f,0.f,0.f);
}