At our Photography Subgroup Meeting in May we held a workshop on Green screen techniques. The workshop consisted of a short talk on the history of the techniques, followed by a demonstration of OBS software and a live green screen systems with webcam. We then tried out lighting a much smoother backdrop in our studio with SLR cameras.
What is Chroma Key / Green Screen?
Compositing (layering) two images or video streams together based on colour hues is a technique used in video/photo production and post-production as shown in the picture above. Chroma keying can be done with backgrounds of any colour that are uniform and distinct. Green and blue are commonly chosen as they differ most from most skin colours.
Compositing was originally done on (chemical) film. The blue emulsion layer of film has comparable grain and detail compared to red and green layers. The genie escaping from a bottle was the first use of a proper bluescreen process for The Thief of Bagdad (1940). With electronic chroma keying, lossy compression algorithms used in TV distribution retain more detail in the green channel so Green tends to be a more common option. Other colours can be used and in a single fight screen for a recent film, Spider-Man was shot in front of a green screen and Green Goblin against a blue screen.
What did we learn?
- Subject lighting should be slightly brighter than background.
- Subject should be away from background to avoid green spill.
- A smooth, uniformly lit, background screen, is critical.
- A vertical green screen (rather than draped on the floor) is more likely to be uniform.
- Cameras with auto white light balance are an issue.
- OBS software is powerful free program for video chromakey
- Use OBS “Colour Key” rather than “Chroma Key” to select colour of background.
For still photography the question of whether green screen video setup helps photoshop green screen compositing is still a question we will come back to. It has potential to allow complex setup and to define ideal colour selection for background removal.