It's nothing new.

We've all seen it. From TV shows as old as Monty Python's Flying Circus to films like Sin City, Sky Captain and the World of Tomorrow and 300 - epic blockbuster films with monumental cinematography - all shot without ever leaving the studio.

Yes folks, it's the good old blue screen - the age old technique of shooting a subject in one place, and making them appear to be in another place by replacing the blue portion of the pictures with alternate footage.

Yet even though it's been done a squillion times, we're still coming up with new ways to do it, and we still struggle to make it real. So what can be so difficult? How have things changed since the technique first appeared so many years ago? And why is getting it right such an art form? In this article, we explore the many techniques, issues, challenges and other factors involved in creating an effective blue-screen composition.

History digest

The history of chroma-key compositing extends well beyond the scope of this article. There has, however, been one single giant leap the technique has taken over the years: it went digital. In fact, digital application has changed it so much that it's no longer known as chroma-key compositing (and in fact, the primary manufacturer of chroma key products has even opted to change its name to "digicomp", to reflect the changes made in standard screen colours).

Traditional analogue chroma key techniques used thresholds on the chrominance signal of a video feed (or video split) to determine alpha information, and that was pretty much that. Whilst thresholds were adjustable, there was little that could be done about spill, hair detail, artifacts, matte edge lines or any other common keying problems.

Once the whole affair went digital, anything became possible. Computer software has been able to make it virtually flawless, with such invaluable tools as spill suppression, shadow keying, matte thinning, artifact reduction and translucence calculations. It has also taken keying beyond simple chroma thresholds and into the RGB world.

Digital keying techniques

Many important factors in screen compositing are the keying techniques made possible by digital imaging technology. They make the difference between a successful marriage of two images, and the appearance of a magazine clipping crudely cut out and pasted onto a photo print.

The most basic digital keying technique after the initial key is pulled is matte choking and hole filling. Because of the finite detail in digital images, the pixels representing the edges of the subject often combine a mixture of both foreground and background - especially with soft focuses or motion blurs. Because it isn't always possible to perfectly balance the keying thresholds to show only foreground and no background, it's quite common to have a little bit of background left over on the subject's edges. This is commonly known as a matte line.

Matte lines are typically eliminated by shrinking (or 'choking') the matte. With still photography, it's also possible to manually 'paint out' matte lines, although this is typically unnecessary and overly tedious. With video, such extreme matte adjustments would be considered painstaking, although not unrealistic for some key shots.

Similarly, blue buttons on clothing or reflected blue highlights on jewelry can put 'holes' in the matte. Given a simple threshold, imaging software can scan a matte for these holes and fill them in. Likewise, the the screen has dirty patches or a bug flies past the camera, software can detect these small objects in the key and eliminate them.

Even with a matte line eliminated, subjects frequently contain noticeable amounts of 'spill', where the intense colour of the screen has illuminated the subject, causing a white shirt or skin tone to appear blue. This can have the nasty effect of giving the subject an unnatural appearance, or worse still, if the thresholds are broad enough, confusing the software into thinking the spill portions of the subject are actually part of the screen itself, and consequently making them invisible altogether! Fortunately, if a screen is lit evenly enough, the latter can be easily avoided, leaving the relatively solvable problem of discolouration.

There are an abundance of spill suppression tools available, all using relatively similar techniques of sampling the base hue of the screen, and desaturating or re-balancing any similar colours to neutralise the spill effect. In portrait photography, it's generally more effective to apply different colour balances to different parts of the image, rather than using a single filter for the whole frame. With video, the latter is the more realistic option.

Once in a while, an object will need to be photographed that has translucent or transparent portions. This is an area where digital keying reigns supreme, with its infinite mathematical capability to detect such portions and replicate their semi-transparency. For this, we tend to have to rely more on specialty software rather than fundamental Photoshopping, although with enough fiddling, it's still possible. It's basically a case of calculating how much of the screen is visible through the subject material, balancing out the screen's colour, and using the aforesaid calculation to reduce the subject's opacity.

Putting a subject into a background means we also have to reproduce any effect that subject should have on that background; the most obvious being a shadow. Especially when the subject is seen to make contact with the background, having a realistic shadow added in is what sells that contact. Digitally reproducing shadows is an art form in itself, and can be as simple as using the shadow cast be the subject onto the screen from the original foreground plate, or as complex as using 3D modeling to digitally simulate the shadow. With portraits, and especially with soft lighting, it's usually a relatively simple matter of burning (darkening) the portion of the background, but it's important to do so with a solid understanding of the way light behaves. It doesn't just hit a surface, bounce of and project an image - it gets inside things, shakes around, soaks up colours, throws them every which way and spills all over the place. Predicting what light should be doing is the best way to reproduce a convincing shadow.

It's rare that a subject will colour-match its background. Especially with outdoor backgrounds and studio foregrounds, there will always be differences in colour temperature, contrast and even specularity and ambient light. Even the most basic of digital imaging programs come with an arsenal of tools for colour adjustment, so achieving a decent match between foreground and background will never really be beyond any serious photographer's grasp. I would like to point out, however, that it's a good idea to be familiar with the technical differences in lighting before trying to match them, as many colour adjustment techniques can appear similar, but with enough difference to throw the results a little far off the mark. For example, an overly yellow colour temperature would require a very different neutralisation technique than an image shot on a camera with a particularly high yellow response. Similarly, low lighting or under-exposure should be handled differently to low contrast or flat lighting. It is often necessary to break an image up into several different sections to match colours properly, as different colours will respond to adjustments in varying fashions.

Just as a screen can spill its colour onto a subject (as mentioned earlier in this article), so too can a regular background. One of the finer points in colour matching is recreating the spill that should be on a subject if it were placed in the intended background. One good way to do this is to use a screen with a similar colour to your background. For example, if your subject is going to be composited into a leafy or grassy scene, use a green screen! This way, any green spill on your subject may actually be beneficial to the final composition, and may only need mild reduction, rather than total elimination. In cases where using matching screens isn't possible (which are sadly a majority), it is often necessary to fabricate spill to completely sell a composition. In still photography, it's fairly simple to colourise a portion of an image to match its surroundings. In video it's a lot harder, although with enough motion we can afford to cheat the eye a little more.

Other keying techniques

Foreground and background lighting should always match. This may sound like a no-brainer at first mention, but it can be a particularly involved and detailed task to achieve, depending on a scene's complexity. Essentially, light-matching revolves around matching either the foreground light to the background light, or vice versa. Aspects of lighting including angle and elevation, brightness ratios, softness and colour balance/tint should be noted in one scene and replicated in the other. The preferred approach is to note lighting on the background at the time of photography, and systematically reproduce it on a bluescreen stage. When this is not possible, the background can be analyzed using shadows and colour tones as clues as to what the original light sources should have been.

When lighting the screen itself, care should be taken to ensure it is evenly lit. Whilst it is easy enough for a computer to remove a distinct blue colour from a picture, removing very light or very dark shades of blue starts to get a little tricky. Dark hair, shadows and other foreground elements get lost in the mix. By lighting a blue screen evenly, you minimise the percentage of the total colour spectrum you need to remove.

Finally, while digital spill suppression is often useful, prevention always beats cure; the more distance you have between your subject and screen, the less the screen will spill onto your subject. for a typical human subject, 3 metres (15') is a good rule of thumb.

Chroma key compositing is a cornerstone of modern digital artistry. Whilst simple by nature, it bares enough complexity to be considered an art form in and of itself. Mastering this art form takes care and patience, but is well worth the effort for any serious photographer.