The Stargate itself is nearly always filmed against a blue or green backdrop, not only making it easier to paste the vortex imagery onto the scene, but also facilitating the superimposition of the "event horizon ripple effect", which is entirely computer-generated. However, if a shot only involves an open wormhole without anyone stepping through it, the crew may choose to use a "practical puddle," which is simply a backlit screen placed in the gate displaying a video of the wormhole effect. This only works, however, on a darker set, as otherwise the projection will get washed out. On occasion, the Stargate itself is also completely swapped out for a computer generated model, usually in cases where it is being moved, or is depicted in space. Series producer Robert C. Cooper explained that it often costs a lot to erect a Stargate on location, and so in some cases offworld gates are also entirely a visual effect.