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MillimeterSpeaking About Compositing: Six Things Producers Should Know
May 1, 1999 12:00 PM, Michael Goldman
Millimeter asked 15 prominent visual effects artists to discuss the compositing issues they would most like their clients to understand. All 15 artists repeatedly used the terms "communication" and "buzzwords" in discussing their craft. All emphasized that detailed, early communication is central to executing a composite that meets competing creative, technical, financial, and deadline goals.
When asked to explain exactly what clients and visual effects providers should be communicating about during the creation of multi-layered effects shots, artists turned the discussion to the dangers of "buzzwords." Artists point out that the digital explosion has left heads spinning all around the production universe, making it virtually impossible for even the most technically savvy producer or advertising agency executive to keep pace with the limitless nuances of compositing. They say that clients therefore routinely toss around terms like "rotoscoping" and "blue screen" without really understanding the impact such concepts will have on their project.
"Compositing is like marriage," says Brian Buongiorno, VP and visual effects artist at 525 Studios, Santa Monica. "It is totally based on communication between the parties involved. If you talk it out early in the process, the shot will probably work out. If you don't, you are likely to have problems."
To communicate effectively, producers should understand as much about compositing as possible, artists say. Such knowledge can save money, says Kristin Johnson, an Inferno artist at Post Logic Studios, Santa Monica.
"We still see elements come in for a composite that weren't shot or lit properly, or elements are missing altogether," says Johnson. "The end result of work that consists mainly of fixing flawed elements will never be as good as work that consists of making good elements great. That's why I think clients should really try to understand compositing."
In an effort to serve that noble goal, the artists offer Millimeter readers these six major issues that smart producers should discuss with their visual effects partners.
1. What's the deal with motion control? New topics make producers cringe like motion control. They associate the use of computer-controlled, moving camera rigs with swelling budgets. Nevertheless, artists say that knowing when to shoot motion control can save a project time and money.
"It is true that motion control can be expensive," says Geraint Owen, senior producer at Pinnacle Studios, Seattle. "But using it can also cut down on post work. If you try to get by with a locked-down camera and then try to match a camera move in post, if things don't line up, you are looking at tons of rotoscoping work. Whereas, if you shoot motion control, you are guaranteed different shots will match up, and that can save time in post. The other good thing about motion control is that you don't have to shoot as much film, which also saves money. You can rehearse the shot using a video tap on the film camera, working on it until you get it just right. Then, you can shoot it onto film."
There are, of course, alternatives to using expensive motion-control set-ups. According to Kevin Mullican, senior compositor at Look! Effects, Los Angeles, there are "in-between" solutions, such as the use of camera rigs that incorporate motion-control technology but "don't go the whole way."
"There are many methods to get away without having to spend money on a full motion-control rig," says Mullican. "Motion-control, pan-and-tilt camera heads, rather than the full rig, for instance, can often do the job."
The other hot alternative these days involves tracking motion or camera moves inside a computer. Various types of tracking software on the market allow computers to replicate live camera moves using data captured with tracking markers placed on actors during the live-action shoot. (See last month's Millimeter for more on tracking software.) This kind of software is popular, particularly for shots involving the marriage of live-action elements to computer-generated elements. Artists say, however, that "tracking" is one of many compositing "buzzwords" that producers often talk about without fully understanding.
"What preparation you need to track a shot well depends on its complexity," says Todd Vaziri, lead digital artist at Banned From the Ranch, Santa Monica. "One extreme, when you are tracking a giant CG creature, for instance, involves allowing the effects crew to go on set right before or after shooting to collect detailed data through the use of tracking markers or taking actual surveying measurements on location. In any case, even with simple wire removal, it's important that production people provide us with clean background plates to match up with. It might sound surprising, but some producers never think about getting us clean plates, and that's especially frustrating because you can shoot a clean plate in minutes."
2. What's the difference between blue screen and green screen? Harri Paakkonen, co-founder of Mixin Pixls, Santa Monica, says: "Blue and green screen is such a standard technique that people take it for granted. But it is incredibly important, and there is lots of confusion about what constitutes good blue screen or green screen."
Paakkonen points out what many compositing artists might think is obvious: that the color of the foreground element should determine whether blue or green screen is used in the background. Yet, he and many other artists say producers need to discuss the details of such work in advance with the visual effects company or risk providing inferior shots for compositing.
"Obviously, the background should be as different from the foreground as possible," Paakkonen says. "If the actor is wearing blue, you shouldn't use blue screen. But you see shots all the time where they picked blue screen in such cases. You also see green screen that is poorly lit, which is a big problem because green screen tends to have more spill than blue screen."
Jonathan Keeton, co-founder and visual effects director at Radium, San Francisco, points out that many producers also neglect to shoot the background plate before they conduct the blue or green screen shoot. Many also neglect to use a video assist to visualize the composite live on set when shooting blue or green screen. Failure to take such measures can wreck a composite because there is no other good way to match light and shadows other than to try and "fix things in post," a term most compositors despise.
"It is shocking to me that many people don't realize you need to have the background shot displayed live, which you can easily do through a switcher, to ensure that what you are shooting matches the background," says Keeton. "Producers tend to worry so much about pulling a nice key that they don't always think about how they are going to marry it into the scene."
Keeton adds that he often recommends "difference matting" to a client as an alternative to blue screen. With sophisticated software now available, computers can compare two images shot on the same set and display as a white shape any portions that are different, so the compositor can tweak those areas specifically. "I particularly suggest difference matting if your lighting package is small because it can save you an entire blue screen pre-lighting day," he says.
But Kent Feeler, senior compositor at Encore Hollywood, warns that sometimes saving a few dollars early in a green screen shoot can come back to haunt a production later.
"One simple thing I tell clients is to make sure they use a physically large green screen if at all possible," says Feeler. "Lots of people use quilts of three to four screens, and that leaves compositors with all sorts of visible seams to eliminate."
Many artists say blue or green screens are not the only choices. "Keyers these days in compositing software have improved so that they can key off of most colors," says Keeton. Indeed, red screens are occasionally the best choice if foreground elements have both blue and green hues in them.
3. Why is rotoscoping such a pain? Actually, artists say, the art of rotoscoping is no longer the nightmare it once was thanks to improved tools. Rotoscoping-essentially a manual process of manipulating mattes frame-by-frame-used to require that artists paint each frame of film one at a time and usually by hand. Keeton says that situation has "improved enormously" in the last four to five years.
"There never used to be great tools to do roto," says Keeton. "If there is a subtle change in shape of an object from frame to frame, drawing them by hand over and over can drive you crazy. Now, though, you can do animated geometry on the computer, where you set points on the shape and the computer re-sets them and interpolates them for you from frame to frame. This development was essentially a revolution in rotoscoping."
But rotoscoping can still be complicated and time-consuming, so producers should consult compositors during preproduction.
4. What frame rate works best when shooting multiple elements? The issue of frame rates directly impacts rotoscoping and often makes the process more complicated even with improved tools. Artists say that some producers assume they should shoot film at 30fps, rather than 24fps, in order to match up lip synch and other elements with video material, which normally runs at 30fps (60 fields a second).
"That makes more frames to rotoscope, so that's 20-percent more work in post," says 525 Studios' Brian Buongiorno. "There are good tools that can eliminate the 3:2 pulldown, let you do the composite at the same frame rate, and then expand the pulldown back out and add the composite to those extra frames later. So shooting film at 30fps is not usually needed anymore. There are cases when it is best to shoot at 30fps, but producers should consult the visual effects people first."
Ko Muruyama, a Henry artist at Post Logic, Santa Monica, adds that the more producers know about digital video formats, the better.
"There are many new digital video formats, and some have higher compression rates than others," says Muruyama. "The more compressed you get, the lower the quality of the images you will get. Unfortunately, for economy, some companies still use formats to shoot with that have high compression rates. The problem is, when you try to rotoscope matte keys from these formats, you are not only dealing with the frame-rate problem, but you also get edges around the image that don't key very well if you shoot directly to the compressed format. It is not a problem when you transfer to digital video from film, but producers want to save money, and they sometimes shoot directly to DigiBeta or DV, and then you run into this problem."
5. Can color-correction work be performed during compositing? Artists point out that traditional color correction work is best performed during the telecine film transfer because the original negative offers a wider range of color options than the digital world. But they add that producers sometimes forget that digital compositing can only deal with elements present on the film when it is transferred to the digital universe. If certain elements or colors are wiped away or altered in ways not in the best interest of a particular composite during the telecine transfer, there is no way to return them to the shot other than complicated digital "micro-surgery." Therefore, many compositors say, certain elements are better off getting their final color pass during compositing.
"Once you lose data or images from the film, it is lost," says Noel Castley-Wright, a Henry artist at Company 3, Santa Monica. "When I'm using the Henry for compositing, I have almost as much control of color as you do anywhere else in the process. I have found that we do a lot of color correction work with the Henry. When in doubt, they are better off not altering an element's color too dramatically during the transfer and letting me deal with it later."
6. How do different film stocks impact compositing? Many projects are shot on one kind of film stock, from beginning to end, so continuity is present throughout. However, the rising use of stock footage elements purchased from outside sources can create tricky problems for compositors. Therefore, visual effects' artists recommend that they be part of the stock footage selection process from the beginning.
"There are all sorts of problems you get into when you add stock footage to a shot," says Greg Oehler, senior compositor at Composite Image Systems (CIS), Hollywood. "There can be color problems, scratches, dust, all that stuff. There are also stabilization problems, film-weave issues, and the like. Usually, our tools allow us to deal with those problems, but it can take a lot of time if it is not carefully thought out."
Mitch Drain, lead compositor at Centropolis Effects, Culver City, adds that the need to match together different film grains is not limited to the use of stock footage.
"Blue screen is often filmed on different stock than first-unit photography," Drain says. "When that happens, you are faced with de-grain and re-grain issues to match the film stock together for the composite. In the case of Godzilla, we had lots of blue screen work, so we planned at the outset to create a blanket film-grain filter that would cut the blue-screen footage together seamlessly with the first unit stuff and also the CG work. Most sophisticated compositing software packages supply filters for that purpose."
Final Thoughts There are, of course, dozens of other compositing issues for producers to discuss with their visual effects partners. Artists point out, for instance, that producers tend to frequently request morphing techniques even when they are not appropriate, simply because such effects are now relatively easy to do.
They also emphasize that, in some quarters, there is still a lingering prejudice against bringing visual effects supervisors onto live-action sets due to the old perception that they will slow things down. In fact, they point out, their goal is to do just the opposite. That is why some industry professionals prefer to use a "firm bid" system when seeking jobs-they make it clear to potential clients that there are no additional fees for bringing a visual effects person onto the set.
"I'm a visual effects guy, but if I were a producer, I would want an assurance from the person I'm contracting to do the effects that they will deliver the quality of work I expect, no matter how long it takes," says Radium's Jonathan Keeton. "That's why I prefer to firm bid for a job-one price for an entire scope of work. That puts the incentive to execute at the highest levels back on the visual effects company, rather than trying to get more money for every single problem that will crop up. After all, problems will crop up."
While visual effects artists encourage clients to learn as much as they possibly can about the intricacies of compositing, many emphasize that ultimate responsibility for a multi-layered composite lies with the visual effects house.
"The visual effects people are the ones who should be asking the questions when a client brings us work," says Jimmi Simmons, director of visual effects for Western Images, San Francisco. "The producer's job is to manage the job, but we should be asking him about particular details in order to create the elements he wants."
Here is a sampling of the questions producers should be prepared to answer:
*Will there be time-code references on digital tape, so artists can easily locate elements without wasting time during expensive offline sessions?
*Can filmmakers supply clean reference plates?
*Will you need the shot produced in PAL, HD, or other formats?
*Will you need the shot reproduced for different media down the road, will clients need certain images to use for marketing purposes in various media, and will these issues require any special procedures to protect the negative or digital assets?
*Does the project incorporate stock footage? If so, what film stock is it on?
*Are creative references for the shot from film, TV, or print media available to study?
*What kind of titles or other graphics will artists apply to these images?
*Are temp audio tracks available?
*What kind of information about cameras, lighting, and lenses is available?
*At what point will the cut be locked?
Blue Screen/Green Screen: This term describes the process of filming a person or object in front of a pure blue or green background. This creates a foreground element that a computer can place in front of a separate background element by removing the blue or green background and marrying the foreground image to the background image.
Mattes: These are usually white shapes against a black background that represent isolated images or parts of images that will be manipulated into a particular composite. The matte can be a rough, blocky shape or an extremely intricate representation of the image to be manipulated. The part of the image represented by the matte can be manipulated independently from the rest of the picture and can then be composited over a new background, color corrected, or filtered without impacting other areas. Mattes are extracted in several ways, including the use of blue screen or green screen or rotoscoping.
Rotoscoping: A manual process whereby images are manipulated frame by frame. The technique is often required when there are extremely slight differences in movement in an image from frame to frame for shots captured by a locked-down camera. When artists rotoscope mattes, they draw different mattes for the object one frame at a time, which often makes it a time-consuming process.
Motion Control: Motion control is the term used to describe a computer-controlled camera rig that allows a camera to move repeatedly in an identical way. Using such rigs allows filmmakers to shoot the same element using different lighting, objects, and actors in the shot, so that the separate versions can later be precisely composited together.
Tracking: This is a manual or software-executed process in which a point in a picture is tracked as it moves. Resulting tracking data can be used in different ways. For example, a picture can be composited over the tracked point, and information from several tracking points can be used to calculate how the camera that filmed the shot moved. Tracking can be helpful in creating seamless composites, particularly when the live-action shots were captured with a locked-down camera rather than through motion control.
Clean Plate: A clean plate is a shot where only the background is present. It offers clean lighting and camera positions for compositors to actually incorporate or refer to when uniting various versions of the same shot.
Morphing: This is the process by which two images can seamlessly change from one to the other. In essence, a morph is a dissolve combined with moving areas of the first image in order to match areas of a second image. Typically, you need two shots with identical or similar camera angles and a few similar objects to create an effective morph.
Continue the discussion on “Crosstalk” the Millimeter Forum.
