In my previous article, we discussed the whys and wherefores of shooting people in green suits or not and touched on the issue of how to shoot greenscreen.
Making a greenscreen shot look convincing is very challenging even for experienced VFX Artists and post houses – the challenge should not be underestimated or taken lightly. However I will attempt to give some guidelines to give you the best possible results with the least amount of pain.
The first and most important point for all is to have a VFX Supervisor on set. Even on a low-budget production you can afford to get the Compositor on set to supervise (many Compositors are actively looking for on-set experience). Should ideally be the same person who will deal with the greenscreen shots in post.
This is me and the team and a lovely flat and even greenscreen shot at Broadley Studios, Marleybone
Shoot or make the background first – if it’s footage or a still you need to shoot on location shoot it before the greenscreen shoot. If it is CG or a Digital Matte Painting (a still composite usually prepared in PhotoShop), have at least a mockup or work in progress version ready before shooting the greenscreen.
The reason is that you need to match the lighting and shadows on your foreground actors to the lighting and shadows of the background. It is almost impossible to change the lighting, beyond subtle tweaks, convincingly in post.
If your background is exterior daylight then shoot your greenscreen exterior in daylight. The sun is 1,390,000 km in diameter and is 149,600,000 km away. Unless your budget allows you to place lights that size and distance away (requiring at least a Type 2 Civilisation on the Kardashev Scale) then using actual sunlight is your best option. Studio lights never ever look like sunlight.
Shoot RAW or as close to RAW as you can
Many DSLRs use h264 compression which may look great in a standard shot but the compression will severely hurt. ProRes 422 means the resolution of your chroma (which the key is largely derived from) is half that of your full camera resolution. ProRes 4444, DNG, BlackMagicRAW or ArriRaw are all good options if you can get them.
Another trick is to shoot at a higher resolution than your final picture so you have more pixels to key detail from.
Make your screen as flat and even as possible
Make your greenscreen as flat and even as possible. Flat and even in every way: material, consistent-texture, even-colour, free-from wrinkles, clean, free from shadows, evenly lit, free from bright highlights. You will never get it 100% perfect but the close you get the better it will be.
The bigger the greenscreen the more certain issues become inevitable, in this epic short film “Trollbridge” the sheer scale and confined location meant that there were difficulties in keeping a perfectly smooth surface. This made the greenscreens challenging but I am sure you will agree the final results were amazing.
Place your greenscreen as far away from the subject as possible
Right so you want the foreground lit the way the background plate is lit. But you want the greenscreen evenly lit. So there needs to be enough distance for the actors to be lit with proper cinematic lighting and in a way that matches the intended background; and for the greenscreen to be lit in a completely flat and even way.
You also don’t want contamination from one to the other – shadows cast by your key-light on the greenscreen and the dreaded green bounce light.
Green bounce is horrible, it means that the colour of the edges will be green, making it hard to get a matte and when you do, the edge is green and needs to be recoloured convincingly (which is tricky). If the subject is backlit, that can help as the backlight will kill some of the green light bouncing from the screen but don’t, for the Love of All That is Sacred, backlight a subject who needs to sit in front of a dark background. It will always look ridiculous (“if she’s standing in front of a black spaceship in the dark where is that light on her hair coming from”)
Shave everyone’s head
Keying hair is difficult. Shave everyone’s head and make it part of the story if your screenplay and cast will permit. I will be the first to admit that this option is not always practical.
Avoid Foreground Elements that are highly reflective or the same colour as the Greenscreen
Obviously if your main character is a green alien then use a bluescreen, likewise if they are in jungle camouflage or something. A reflective material will become the same colour as what it reflects – probably meaning it will become green or blue ot match your screen. A show involving a green alien and a guy in a shiny suit might want to come up with a different approach.
Don’t Use Greenscreen
If you don’t need it don’t use it. Really think about whether a shot needs to be greenscreen at all.
A really good example is screen shots. You will not believe how many greenscreen shots are basically just phone screens, computer screens, tv-screens etc… The reason for these is that many productions cannot decide ahead of time what exactly will be in the screen. As indie-filmmakers your super-power is that you can make decisions without needing to wait for 15 different Executives to sign it off. So use your super-power, decide what will be on those screens and have it ready before you shoot! It’s the low-budget equivalent of those 360 LED environments used on the Mandalorian (see the link above).
BTW if you do shoot your screens live please please please do not go changing your mind later – it will be nigh on impossible (believe me I’ve had to do it!) to replace a filmed graphic with something else especially if the graphic is animated or people pass in front of the screen – in that case, then yes fill it with green or possibly simply shoot them switched off (check with your on-set supervisor before making the call).
I could write a whole article just on tracking markers but someone else did. But the short version is here: The greenscreen is featureless and if the camera moves the software will need some high contrast points to latch onto so it can replicate the move in the background. These markers need to be:
- Simple dots or circles (that way the centre is still obvious when they are out of focus). Forget funky shapes, letters or crap like that.
- Not in the way of hair or other tricky detail – obviously you are breaking the nice even greenscreen by sticking markers on it so try to keep them where peoples hair and such won’t be.
- Identifiable – make the pattern irregular so you can tell one marker from another
- No bigger than they need to be – depends on their size in shot but huge markers aren’t helpful. They need to be just big enough to remain high contrast in the final image.
- Not flapping in the wind – if the wind is flapping the greenscreen then any tracking markers on it will not be useful (since they will be measuring the flapping wind motion and not the camera motion).
- Not reflective – if they are reflective you will be tracking the reflections on the marker, not the marker itself
A big thanks to Chris Poulay, Matchmove Lead at One of Us for his advice regarding markers.
Again, making greenscreen shots look convincing is not easy and should never be underestimated – but follow these guidelines on set and it will at least be feasible.