Black-screen & White-screen: The Best Kept Secrets in Compositing

Accessing the compositing modes in Final Cut Pro 7
Accessing the compositing modes in Final Cut Pro 7

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 sometimes it’s not the right look. In the last few days Kate Madison and I have needed to shoot last-minute elements for some shots in Ren: The Girl with the Mark, and we turned to monochromatic 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.

If you’ve ever used Photoshop, you’ll have noticed some layer modes called Screen and Multiply. Final Cut Pro has the same modes (it also has Add, which to most intents and purposes is the same as Screen) and so do all the major editing and FX packages.

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.

Multiply, as the name suggests, 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.

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 garden in the background. It was the work of moments to comp this element into the shot using Screen mode.

The flame element, shot at night in the garden to ensure a seamless black background
The flame element, shot at night in the garden to ensure a seamless black background
I adjusted the flame's size and used Screen mode to composite it over the background.
I adjusted the flame’s size and used Screen mode to composite it over the background.

Fire is the perfect partner for black-screen shooting, because it generates its own light and it’s not solid. Solid objects composited using Screen/Add or Multiply take on a ghostly appearance – perfect for, er, ghost effects – but not ideal in other situations; because of the way Screen 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.

A smoke element shot against a black drape and backlit so that the smoke is visible but the drape is not
A smoke element shot against a black drape and backlit so that the smoke is visible but the drape is not
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Shooting dirt in a vase of water against white

Other things that work well against black-screen are sparks, smoke and water/rain, again because they’re not solid. If you want to add rain or snow to a shot, black-screen is the way to go – check out my post about that here.

Yesterday 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 used fluorescent softboxes for the background, ensuring we got a bright white background, and made weird arrangements of white paper to eliminate as many of the dark reflections in the vase as we could.

One of the tornado elements shot with the set-up pictured above. We let the dirt settle in the bottom of the water, then swirled the water with a spoon (which had to kept out of frame).
One of the tornado elements shot with the set-up pictured above. We let the dirt settle in the bottom of the water, then swirled the water with a spoon (which had to be kept out of frame).

A few weeks back we shot hosepipe water against black, inverted it and used Multiply to superimpose it as blowing dirt.

With a little thinking outside the box, you can shoot all kinds of elements against white or black to meet your VFX needs. I’ll leave you with this featurette I made in 2006, breaking down the various low-tech FX – many of them black-screen – that I employed on my feature film Soul Searcher.

Black-screen & White-screen: The Best Kept Secrets in Compositing

4 Cunning Substitution Effects in Labyrinth

After countless viewings on VHS and DVD over my lifetime, I finally got to see Labyrinth on the big screen today. The imagination and detail in this film are just astonishing. Every scene has little puppet creatures wandering or flying about in the background to bring the sets to life. In today’s screening I noticed, for the first time, that there are two bottles of milk – presumably delivered by the Goblin Milkman – outside the door of Jareth’s castle. How brilliant is that?

"Where did she learn that rubbish? It doesn't even start with 'I wish'!"
“Where did she learn that rubbish? It doesn’t even start with ‘I wish’!”

Anyway, while there are many awesome things about Labyrinth, one of the techniques that I think is put to particularly good effect in the film is in-camera substitution. Typically this involves one type of puppet leaving frame briefly, and a second puppet – of the same character – reappearing in its place. Puppets are often limited in the actions that they can perform, and while scenes will commonly use different versions of the puppet in different shots to cover the full range of actions, Henson sometimes uses different versions of the puppet in the same shot to sell the illusion of a single, living creature. And though many of these effects are fairly obvious to a modern audience, you can still admire their ingenious design and perfect timing.

Skip through the movie to the timecodes listed below to see some of the best substitution effects.

1. Goblin Under the Bedclothes – 11:40

In the film’s first puppet scene, Sarah’s parent’s bedroom becomes infested with goblins, building up to David Bowie’s big oh-so-eighties entrance. One goblin crawls along the bed, under the sheets, before emerging. It looks like the initial crawling is achieved by pulling a rough goblin shape along on a wire under the sheets. The shape then drops out of the end of the bedclothes, behind a chest, and a moment later a puppet pops up from behind the same chest. This substitution effect obviates the need for a custom-built or chopped-up bed, which would have been necessary to permit the passage of the proper puppet and its puppeteer under the bedclothes.

Sirdidymus
Sir Didymus

2. Sir Didymus’ Acrobatics – 58:35

This shot appears to employ three different models of Sir Didymus, the honourable but fighting-crazed guardian of the bridge over the Bog of Eternal Stench. The first is a floppy version which is thrown behind some rocks by Ludo. After a practical puff of dusk, a second Sir Didymus – this one in a more rigid, leaping position – is launched from some kind of catapult hidden behind the rocks. He flies out of frame, to be replaced a moment later by the Muppet-style hand- and rod-puppet which is used for the majority of Sir Didymus’ shots.

3. Cowardly Ambrosius – 1:16:25

To his infinite chagrin, Sir Didymus’ bravery is not matched by that of his canine steed, Ambrosius. During the battle with Humongous, the petrified pooch rears up, throwing off his valiant rider, and retires shamelessly into hiding. The rearing up is accomplished with a rather unconvincing puppet dog. After he drops back down out of frame (aided by a slight zoom in to help lose him), a real dog enters in the background, running into hiding.

"I can't live within you." Not at all creepy, Dave.
“I can’t live within you.” Not at all creepy, Dave.

4. Double David – 1:27:53

In the film’s finale number, “Within You”, David Bowie’s Goblin King messes with our sense of direction as he jumps and flips around the disorientating Escher artwork brought to life. Early in the sequence he jumps off a ledge, only to reappear simultaneously in a background doorway, now seemingly obeying a pull of gravity at 90° to that which acted on his leap. A shot of Bowie jumping off the ledge cuts to another of him coming through the doorway. The doorway is filmed with the camera on its side, and to finish the action of the first Bowie’s leap, a body double is pulled across frame on a dolly. This can be seen at 25:36 in the behind-the-scenes documentary:

This kind of low-tech but ingenious filmmaking is in danger of dying, as CGI is perceived as the only tool to create illusions. But with a little thought, a little planning, cunning framing, and a knowledge of how to use editing (or lack thereof) to your advantage, very effective illusions can still be created in camera.

If you enjoyed this post, you may also like:

The 10 Greatest Movie Puppets of All Time – including the aforementioned Humongous

Double Vision – five ways of having one actor play two characters in the same scene

Top Five Low Tech Effects – tipping my hat to the cheekiest in-camera effects used in big Hollywood movies

Five Simple But Effective Camera Tricks – revealing some simple camera tricks I’ve used in my own films

4 Cunning Substitution Effects in Labyrinth