Neil Oseman, director of photography and post-production supervisor on the indie fantasy series Ren: The Girl with the Mark, explains how impressive visual effects can be created by shooting simple elements against a black backdrop.
When it comes to shooting elements for VFX, green-screen gets all the press. But certain kinds of elements can be tricky to key well, and it’s not always the right look. For Season One of Ren: The Girl with the Mark, director Kate Madison and I needed to shoot last-minute elements for some important sequences, so we turned to black or white backgrounds.
Why? How does it work? Well certainly you can key out black or white just like you’d key out green, but the most powerful way to use these backgrounds is not with keying at all, but by a bit of basic maths. And don’t worry, the computer does the maths for you.
All major editing and VFX software has a menu somewhere called Compositing Modes or Blend Modes, and amongst these options are a few that particularly useful:
Add (or the similar mode called Screen) adds the brightness of each pixel of the layer to the layer underneath. Since black has a brightness of zero, your black screen disappears, and the element in front of it is blended seamlessly into the background image, with its apparent solidity determined by its brightness.
Subtract takes the brightness of the layer away from the brightness of the layer underneath. Again, black areas do nothing, because subtracting zero from any number results in that same number. But any brighter areas turn the background image dark.
Multiply, as you’ve probably guessed, multiplies the brightness of each pixel with the layer underneath. Since white has a brightness of one, and any number multiplied by one is that same number, your white screen vanishes. Whatever element is in front of your screen is blended into the background image, with darker parts of the element showing up more than lighter parts.
Examples of Add Mode
One of the elements Kate and I needed to shoot was a flame, to be comped onto a torch. We lit a torch and clamped it to a stand, shooting at night with the pitch black yard in the background. It was the work of moments to comp this element into the shot using Add mode.
Fire is the perfect partner for black-screen shooting, because it generates its own light and it’s not solid. Solid objects composited using Add or Multiply take on a ghostly appearance – perfect if the supernatural is what you’re aiming for, but not ideal in other situations. Because of the way Add mode works, anything that’s not peak white will be transparent to some degree.
We shot some fast-moving leaves and debris against black, but only the high level of motion blur allowed us to get away with it. In fact, if you know you’re going to have a lot of motion blur, black-screen might be the ideal method, because it will be tricky to get a clean key off a green-screen.
Other things that work well against black-screen are sparks, smoke, water and rain, again because they’re not solid. This is a great way to add artifical precipitation to a shot, by Adding an element of hosepipe water filmed against black.
Thinking Outside the Box
For another scene, Kate and I needed to shoot a whirlwind element. One of the VFX team suggested swirling sand in a vase of water. After a few experiments in the kitchen, we ended up using dirt from the garden. We placed fluorescent softboxes behind the vase, ensuring we got a bright white background, and made weird arrangements of white paper to eliminate as many of the dark reflections as we could. When I composited this footage in Multiply mode, the white surroundings disappeared and the dirt darkened the image of Ren underneath.
For yet another sequence, we needed to suggest wind blowing straight across camera. We ended up shooting backlit hosepipe water against black, and compositing it in Subtract mode. Again, the black background disappeared. The bright water was effectively turned into a negative by the Subtractive process, becoming dark. It now resembled fine particles of dirt or grit rushing past the camera – exactly what we needed.
With a little thinking outside the box, you can shoot all kinds of elements against white or black to meet your VFX needs. If you’re not confident with complex CGI like particle effects and 3D modelling, it’s an extremely useful trick to have up your sleeve. And even if you are, there’s something more organic and convincing about effects built from real elements.
If you’ve found this article helpful, please consider backing our Kickstarter for new episodes of Ren. It’s currently live at https://www.kickstarter.com/projects/mythica/ren2 and the rewards include vlogs from set, a Cinematic Lighting course, and a Director Experience.