Shooting Green/Blue Screen

Green screen is a process by which a single color is selected in the frame and digitally replaced with another image or video source. This process is called keying and an image can be keyed any number of ways, including keying out a specific luminance (brightness) level, or in this case, keying out a particular color (chromakey). The green screen process is a popular method for extracting actors or objects and compositing them into another environment. 

Green Screen vs. Blue Screen

Although any color can technically be keyed, blue and green are the primary choices of directors of photography.  First, green and blue are both the color opposites of flesh tones, minimizing the chance of keying out part of an actor’s face.  In general, the choice of whether to use blue or green depends largely the actor’s wardrobe or the color of the objects in the shot.  If the actor is wearing blue jeans, consider using a green screen. If the actor is wearing a green shirt, consider using blue screen.  Nothing in the foreground should have the same color as the background screen or you will end up keying through your subject.

• Green screens are more reflective than blue screens and require less light to illuminate.  Choose a green screen when working with limited lighting resources, or when a lot of light is required for high-speed cinematography.  Often, green screens will be chosen when shooting indoors reducing the amount of light needed for exposure, while blue screens are used outdoors in direct sunlight. 
• Before computer generated imagery (CGI) was adopted as the primary visual effects tool in the industry, models were shot against a red or orange screen illuminated with infrared light, which greatly reduced the risk of bounce back from the background onto the model.
• When traveling, neon green poster board purchased from an office supply store is a great inexpensive solution.  It’s cheap and can be taped together to make a screen of virtually any size and its highly reflective color is easily lit and picked up by digital cameras.

Digital vs. Film

There are two types of blue screens and two types of green screens.  The first set of blue and green screens, called chromakey blue and chromakey green are designed for use when shooting film and aren’t as saturated in color as their digital counterparts. Film has a much wider tolerance for color and brightness, making it better suited for these muted colors.

When shooting digitally, a special type of green and blue screen was created called digital blue and digital green. The colors are much brighter and feature much stronger, saturated color, making them easier for CCDs to capture.

COMPRESSION

Once light enters the lens, digital cameras separate it into the red, green and blue components before they are compressed and saved either to tape or to solid state media.   Unlike the red and blue channels, which are heavily compressed, the green channel isn’t and retains much of the original data.  For this reason, using a digital green screen will make for a much easier key in post-production because of the amount of data in the green channel.  The edges around the subject are sharper and more defined, enabling the software to isolate and cleanly remove the green screen. The overall quality of the key is much better than if a blue background were used

Shooting green screen also depends heavily on the type of digital format used. 

• Number of chips - When shooting green screen plates, choose a 3-CCD or a CMOS camera.  Single CCD, or chip, cameras use one CCD to convert all the light into the recorded electrical signal, whereas a three-CCD camera will break the incoming light into red, green and blue channels.  Of these three channels, the green channel contains the color information for the green screen, so by keeping it separate from the red and blue channels, the resulting image is sharper with no color bleeding. The final key will be clean, and edges around the keyed subject will be sharp.
• Format -  Once the camera converts light into an electrical signal, it is then compressed for storage on either a tape or onto another digital medium like DVD or solid state card. Different formats will compress the footage differently, mostly by throwing away color information; information that is critical to creating a clean chromakey. When shooting green screen, avoid formats like consumer analog formats, miniDV and HDV as the compression is so severe, attempting to pull a key will result in stair-stepping along the edges of the subject.  Choose a professional broadcast format like DVCProHD, Digital BetaCam, HDCAM, or any uncompressed formats for the optimal key.  Although it may be more expensive to shoot, the resulting images will yield a clean, flawless key.

Lighting a ChromaKey Screen

Lighting a green or blue screen can be tricky, and the final quality of the key is dependent on your ability to properly shoot the screen on set.  Follow these tips for shooting a good green or blue screen.  For ease of discussion, I’ll only refer to the background screen as a green screen, although this applies to any color you may choose.

• Choose the appropriate color for your background.  Make sure the color of the background is unique and isn’t present in the actors’ wardrobe or in the subject. When shooting digitally, consider using a digital green screen.
• Light the screen evenly.  Even lighting is the single most important aspect of obtaining a good key, as the compositor will be able to select a narrower range of green for a cleaner key. As a general rule, a well-lit green screen should not vary in brightness by more than half a stop at most. There are several ways to evenly light a green screen:
  • Cyc Lights - Most green screen plates are shot at professional studios against a smooth, curved wall called a cyc.  The cyc is painted green, and broad overhead lights are rigged from the grid in the ceiling to create an even light, especially when lightly diffused. Cyc lights are inexpensive, easy to hang and are ideally suited for lighting large areas and are commonly used on standard 18 ft. high lighting grids.
  • Space Lights - Space lights are large soft lights hung from a studio grid. Unlike cyc lights, which can be focused onto the green screen with minimal spill onto the subject, space lights are placed in the center of the stage, and flood the area with light. Although the green screen is evenly lit, the subject is also flooded with light, making it extremely difficult to control.
  • Kino-Flos - Kino-Flos are an outstanding lighting option for smaller green screens. When lighting interview subjects, products or other small contained objects, 4’x4 bank Kino - Flos are the perfect choice.  Studios with smaller cycs can benefit from 4’x8 bank Kino-Flos, called Image 80s.  Since Kino-Flos can be outfit with different color bulbs, the best choice for lighting green screen is either the green, 5000k bulb, or the 5600k daylight globe.  Although tungsten bulbs will work, the blue hue of the daylight bulb brings out the green color frequency, making the green screen more saturated on screen. 
  • Reflected Light -  If you don’t have access to a lighting grid but still need to light a large green screen, buy four 4’x8’ sheets of foam core, black on one side and white on the other.  Tape two of them together along the long edge and open them up like a book sitting on the short edge with the white side on the inside. Build two of these and place each one just off both the left and right sides of the frame.  Inside these reflectors, place a 1.2k or 4k HMI focused into the foam core. You have just made a large 8’x8’ soft source that illuminates the green screen.  Position the foam core so it blocks any spill off your actors.
  • Construction Lights – If traditional lighting equipment isn’t available, consider purchasing construction work lights from your local hardware store. Add a light diffusion in front of each light, and place the lights 3-4 feet away from the green screen to ensure an even light.

• Always light the green screen first.  Lighting a subject in front of a green screen can be tricky, because the DP needs to light both the green screen and the subject without the light from one affecting the other. When lighting the green screen, avoid any unnecessary light spill and try to keep the subject in darkness.  The DP will then create a separate lighting setup specifically for the subject.
• Properly expose the green screen. A good rule of thumb is to expose the green screen roughly one f- stop below the brightness of the subject’s key light.  If using a waveform monitor, the luminance of the green screen should be around 60 IRE, or 60% brightness value.
• Avoid spill - If the actors are positioned too close to the green screen, green light will be reflected back onto the actors, making it extremely difficult to key them out later.  Avoid green spill by placing the actors at least 8-10 feet away from the background. When working on a green screen floor, place a furniture pad or duvatyne on the floor whenever it’s not in the shot to eliminate upspill.
• Ensure good separation of the subject.  Make sure your subject is properly lit so there is a distinct difference between the subject and green screen.  When shooting actors with dark hair, add a rim light to increase separation. When shooting blond haired actors, reduce the rim light and consider rigging a flag to reduce the light on the subject’s head. 
• Avoid reflections. Be extremely careful when shooting reflective objects to ensure they do not reflect the green screen.  Reflective surfaces like marble, granite, glasses and jewelry can cause compositing problems. 
• Avoid spill.  Make sure your subject is far enough away from the green screen to avoid any green spill on your subject.  It’s nearly impossible to cleanly key a subject  that has too much green spill. If you’re shooting against a green cyc with a green floor, consider laying down furniture pads on the ground to avoid green reflection bouncing off the floor.
• Know what the background will be.  In narrative projects where the subject is being keyed into a real environment, make sure you light the subject properly so that the key light is the same as the background plate in color, intensity and texture.  It’s a good idea to have the background images available on set as reference.

SHOOTING FULL BODY

I’ve shot over a thousand green screen shots in my career, and one issue that arises every time is whether to frame the actor’s feet or not.  The set-up required to properly shoot an actor head-to-toe is significantly more expensive and time consuming than framing a loose medium shot where the bottom frame line falls mid-thigh.

• You will need a full green cyc with floor – Since the actor is seen full body, a smooth, even transition is needed in the green screen, curving from the wall behind the actor to the floor underneath them. It may be necessary to rent a professional studio with the proper facilities, costing upwards of a thousand dollars a day. If you shoot a medium shot, the DP can set-up a flat green screen behind the actor in practically any location.
• Lighting is more complicated. – It’s time consuming, requires additional lighting equipment and more crew members to evenly light the cyc from wall to floor without any variation in the brightness. This can take several hours, whereas lighting a flat greenscreen behind the actor can take less than half and hour and a couple Kino-Flos.
• The lens settings and placement of the camera when shooting the green screen subject must match the lens setting and placement of the camera shooting the background plate. It is very difficult to get the perspectives, lighting angles and optics correct to seamlessly key an actor head-to-toe into an environment.  Pushing into to a tighter medium shot provides much more leeway when compositing the background image.