(Excerpt taken from the visual effects chapter in my new film book, "Making Your First Blockbuster", due for release early 2019 by Michael Wiese Productions.)
Using a green screen for film work is very common now, and has a wide range of applications. Green screens come in all shapes and sizes, from a full curtain and floor studio setup to a 3-foot-by-5-foot foldaway-material version small enough to carry. It all depends on what your shot is and what you’re trying to achieve.
When shooting any visual effects involving green screen and keying footage, you need to make sure the color information in the image you’re recording is the best it can be. The minimum requirement for high quality green screen work is using a codec with a color sampling of 4:2:2 in order to execute a good, clean key of the image. Keying requires quality color information in the image, so work with a camera that can provide this.
It’s important to know what the background image will be so the director of photography can light the subject in the same way as the background, and the two elements will match perfectly once composited. If the background plate has a low setting sun in the bottom right corner (like the image above), then that orange glow should appear on the left shoulder of your actor if they're facing the camera. It is these small touches that help sell the shot to the audience. You also need to position your actor far enough away from the green screen to avoid any spill from the screen itself. If too close to the screen, the green color can reflect or spill on to the actor and when you remove the green screen, you’ll also be removing part of them. Being too close to the screen also means that when you light the actor, they might cast shadows back onto the green screen. These issues alone sometimes mean you’ll need a bigger screen than what you think you do. In addition to this, the actor’s movements need to be kept within the scope of the green screen. If they wave their arms about even for a second and their fingertips go outside of the green screen, but still in shot, you’ll lose them. (You would mask or rotoscope around their hands to salvage the shot if this happened.)
The green screen needs to be lit separately from the actor and also evenly lit across the breadth of the screen if the actor is moving in front of these areas. Having it darker at one end will result in varying strengths of green, and this might make keying the shot harder for the visual effects artist. It is also important that the screen around the actor is even, with no creases or folds. The software will remove the green color, but any obvious texture in the material will carry across to your background image. Imagine if you painted a wall of bricks green and used this as your green screen. Once keyed, the green would disappear, but the brick texture would then immediately be applied to your background plate.
The angle at which you film your actor, and your camera height, tilt angle, and lens size, must all match the background plate you are using. It must look like the composited shot was all filmed at the same time. Having a different lens size or height will destroy this illusion.
Some DPs like to expose the green screen one f-stop over the one used for shooting the actor. Having that extra brightness and solid green means the edges of the actor can be easily defined. Others light it to the same stop, and some a stop under. If the green screen is too bright, you can lose saturation, and there is a little danger of more spill on the actor. Everyone has their own preferred method. Personally, I think lighting a stop over is good. The problems arise when you go too far in either direction — too bright or too dark. I’ve sat in a visual effects suite where the screen was too dark to pull a good key. The other thing to make sure you do is to talk with whomever is in charge of costume so that the actor doesn’t have anything green on them — although this could result in that part of the actor also disappearing from the shot! With software these days being so sophisticated, keying programs can differentiate between different shades and color saturations of green, but those are best avoided if you can. Failing that, try a blue screen instead. However, if you are filming a scene with a floating head, or maybe a war film with a character missing an arm, wrapping their arm or body in blue or green material could give you the means of achieving this shot. If the camera is locked off, then no tracking markers will be required. However, if you pan, tilt, or do any type of camera movement during the shot, then you’ll need tracking markers to track and record the camera’s new position as seen in the image below. The compositing software can then apply those new movement coordinates to the background image.
As mentioned, keying software is very intuitive now and can make allowances for poor green screen work. Sometimes the shoot is rushed, or the resources just aren’t there to do the best job you can. You might well have an uneven green screen containing the odd fold, but software can now compensate for this (a little). Within the keying application there are various parameters and variables that can be adjusted to pull a solid key. Of course, this shouldn’t result in a carefree, “Oh, it will be fine . . .” attitude on set, but if you are down to the wire, then there are ways to fix things in the application. Having an even green screen only applies to the areas around the subject, though. If the sides of the green screen are uneven, with the light levels falling off but the actor is not moving near that area, this isn’t an issue. A “garbage matte” to mask off the uneven area can easily be produced. If the actor is perfectly lit and so is the green screen, but the shot is wider than the green screen, also capturing some of the surrounding set, ceiling, or area around it, this is all okay. When the artist pulls a key from the shot, they can mask around the unwanted elements and create the garbage matte. Then the software can be told to pull the key and remove the green from what is within the garbage-matte mask, as in the picture below:
The secret to any visual effects work is in the planning before you get to the studio or location. Ask questions like: What is the action? How wide are we? I’ve seen directors ask for walking shots when they have brought green screens big enough to shoot a medium shot from the waist up. They think that because they have a green screen they can shoot anything, which unfortunately isn’t the case. Planning and preparation are again crucial here.
"Making Your First Blockbuster" is due to be released early 2019 from Michael Wiese Productions.