Video over background — [post] — Keying with a black screen and dual video masking is a technique which allows to show a moving element (our raising stair in this case) traversing a background, even though the original film does contain black parts which must be maintained.
The technique involves layered identical, but inverted, masked video — as well as alpha channel cut-outs of the background in photo-shop.
The video keying zone has no precise delimiters: It serves only to constrain, avoiding keying other black such as the side of the raising stair. Inverted mask as separate layers allow to display the critical keyed part of the background as well as the non-keyed portion.
At this moment in time, it is interesting to look back at our colour-keying experience, of the ‘green screen’ era, and then experimenting with other colours of screen, including very positive results from a white wall. At that time we did not think about a ‘black screen’, however, with high-contrast situations, as with our mostly white raising stairways, we are pushing the idea a bit further, and it has worked. (See our 2013 post about those earlier tests.)