As the application starts running on the phone, one can place the portal by tapping on the screen. The portal's size and position can be adjusted by pinching and retapping.
If the person is inside of the portal, the world will be rendered with effects; if being outside it will render the effects only in the portal window. The conditional rendering is achieved through a stencil buffer.
I built the effects with the particle system, skybox and shaders. The skybox is set transluscent so that the real world can be seen along with the special effects, which adds to the immersive feeling.