For that example, you could just have added the plane as child to the camera, with the appropriate offset. If you need a moving plane, but still want the plane to face the camera, you can do like I did for each sphere in my spheres example. The principle is to add the vertex position in view space. For just a single object, you could just add in a uniform position that you apply the modelViewMatrix to, and then add the vertex position before applying the projectionMatrix. (Or even projecting first and adding in the projected space.)