In the original daylight establishing shot of Cloud City, a matte painting of the city was combined with cloud elements. (This shot originally preceded the shot of Leia in the window of a Cloud City building waiting for Han Solo.)
When George Lucas requested that a Ralph McQuarrie-designed gas mine be added, ILM visual effects supervisor Dave Carson had a computer graphics version modelled so that a subtle rotation could be included.
In the final composite the computer graphics model was lit to match the cloud plate and a slight push-in was added.
