May 1, 2008
By Chew Tze Chuan
The term “chromakey,” often referred to as “greenscreen,” is a bit misleading. Primarte will work with just about any colour backdrop. Since subjects normally do not wear green clothing, it is the one used the most. I prefer to use blue in most instances – light-colored hair and skin tone separate easier. The use of fabric chromakey backdrops from Filmtools help reduce colour spill. There seems to be a myth that you can’t photograph blue on blue backdrops, or green on green backdrops. When Primarte is processing an image, it’s creating an action in Photoshop. If blue jeans are removed, just use Photoshop’s History brush to recover the lost blue color. If you use an existing background that you’ve purchased or created, study the background light direction and contrast closely. Then light your subject, using the same light angle and contrast range as the background. Chromakey will work in almost any photography market; however, high-volume events present a problem because of the time it takes to process individual images. When Digital Anarchy introduced Primarte 3.0, it was a definite upgrade. The new version has better colour-spill correction features. The Automask feature has a button that will mask 80 per cent of the images in one click, allowing me to skip the standard three-step process that Primarte 2.0 was built around. Many companies offer high-resolution backgrounds. When you offer theme videography, there may be times when a client or event may request a design be created for them. Use Photoshop, Illustrator, and Autodesk’s 3ds Max (or a combination of them) to create backgrounds; most times, though, its easier to purchase backgrounds. Automask analyses an image and automatically removes its background, eliminating or reducing the need for manual keying. The Primarte control panel, called Light Wrap, provides much better spill suppression and provides a better foreground to background blend – great for creating sophisticated composites. Greenscreen usage is becoming increasingly popular because it is less expensive than taking actors on location, and everything from the weather to the lighting can be controlled. However it is best to make sure there is a visual effects specialist on set. It is not wise to ask for a greensreen shot and let the DP shoot what they think [is right]. They might shoot something that is technically correct but a VX specialist may be able to make suggestions on-set that would make it easier, more flexible, cheaper in post. The greenscreen is one element, and one challenge in a greenscreen shoot, but there are other elements of hot-spots, uneven lighting, shadows and, even worse, when there’s equipment in the view of the camera. Even if they see it, a gaffer is not going to move a light unless someone tells them to. Greenscreen with HD Visual effects specialists will check camera format, lighting and stage set-up to avoid all sorts of problems that will cost time and money in post. One of the issues of shooting HD with greenscreen is some of the lesser formats are basically unusable. It’s better if the shot is done on Digi Beta, then in a larger format like HD because there’s so much noise. An essential element is to check the actual greenscreen fabric quality. Another concern is skin tone. Sometimes a darker skin tone reflects the green a little more – there is a little more green saturation. It is an extra step you have to take but it’s one other level of keying and noise reduction, which is really useful. It makes good keys look better and it makes unusable keys barely usable. One of the problems with compressed format is that it is not the greatest for greenscreen keying. It’s doable, but it can be problematic. You have to make sure the greenscreen is lit very well. Proper lighting helps solve the problem of green spill. Consistent lighting and colour temperature are very important. Recently, Mechanism Digital worked on 900 greenscreen shots for the History Channel special. The Last Stand of the 300, the real story of the Spartans, a 90-minute special released in conjunction with the feature film 300. Mechanism suggested shooting it entirely on greenscreen to mimic the approach Hollywood took. On-set the entire time, they made several time and cost-saving suggestions: shooting things with a locked-off camera and then moving it in post, shooting separate passes for more flexibility with depth of field and blurring or even replicating people in post. That was a big one since the 48 live actors needed to become 3,000 in the special. Greenscreen shots were used for the backgrounds or little greenscreen people were put in the distance to add life to the shot. Since Mechanism artists composited, they relied on their usual prepro tests. In this instance, they use a greenscreen paper and tabletop with toy action figures and mocked up some concepts to see if they would work. Then, at the backend in post, they used Eyeon’s Fusion to composite the more complicated shots and Adobe After Effects for the more straightforward ones. Hardware was custom-built PCs and Mac G5s. On greenscreen it is not always necessary to move the camera, flip the actors for the turnaround and change the lighting a little bit on them. Typically, when you shoot a movie scene, you shoot the master, then you go in for coverage and then you have to turn the camera around and relight. Inevitably, visual effects studios do have to work with both good and bad greenscreens. Their techniques and tools are their weapons. 10 Green Screen Tips 1. Don’t over expose the screen. Under exposed screen is better than over exposed. Too bright increases spill and decreases saturation. One stop under is okay. 2. Light screen evenly with softboxes/diffusers. Check with light metre within 1/4 F-stop variation and all same temperature. 3. Avoid using dimmers on screen light (talent is okay); they create flicker, which may not key every other frame. If dimmers must be used, test in keyer on-set before shooting. 4. Turn off any in-camera colour gain or filters, sharpening or detail. Don’t forget to set white balance! 5. Place talent as far from screen as possible to avoid spill. Light talent separately to match planned backplate, avoid unwanted light spill and shadows on screen. Don’t use coloured backlights to reduce spill. Turn off the screen lights after everything is lit to see actual talent lighting. 6. Use blocking tape with colour close to screen so they both key in one pass. Use tracking marks with similar, not exact, colour when crossed by talent or a contrasting colour for better tracking results if track marks will not be crossed by talent. They can be garbage matted out later. 7. Avoid interlaced footage if possible. It’s just one less hassle in post. 8. Keep talent in focus, especially if using compressed footage (focus/depth can be simulated in post). Screen out of focus is okay. 9. Keep screen as clean as possible. Dirt won’t key and must be removed by hand. Wear booties, or roll out brown paper when working. 10. All problems can be fixed in post if a sufficient amount of money is available.