Bluescreen, what is it?
Bluescreen software offers two advantages for creating seamless image composites:
2. Believable image blends, even with very complex objects.
The combination of several images to make montages has been done since the dawn of photography and motion pictures. Huge amounts of effort went into devising ways of doing these montages easily and effectively. To this end, pioneers of the techniques in Hollywood special effects departments eventually came up with the idea of using a particular colour for a background to be cut away and replaced by another. Of the three colours in colour film blue was chosen as the most suitable for the task. Using an optical printer, the blue background would be masked, and another mask or matte would be made on another film strip. This would be used to leave an unexposed area on the film where the second image would be superimposed. This post production process was very complex, expensive and time consuming but it was worthwhile because of the enormous savings in the original shooting.
A refinement of the process captured attention when used in the epic 1959 MGM movie “Ben Hur”. After six months of experimenting, inventor and special effects master, Petros Vlahos had come up with what he called the colour difference travelling matte sequence, which worked marvellously. Some of the ship battle scenes have 17 elements! Of course this was in the analogue era when all the work was done optically on the film. With the advent of video, techniques were taken a step further. The introduction of digital computing to the field in 1992 allowed even more advances. Today’s digital technologies make it much easier and affordable while the effects can also be much more subtle and believable. Bluescreening is now routinely used to create mattes for hair, smoke and flame!
The recently released blockbuster movie Godzilla used Primatte bluescreen compositing software extensively.
Going for Green?
You might have seen TV programs about movie making where they are shooting on a green background. Green has the advantage of being, by definition, the opposite to red and is sometimes preferred when the cut-out object has a lot of red or particularly where there is a lot of skin tone in the shot. As I’ll explain later, with some software any colour of the rainbow can be used for the “bluescreen” and while theoretically you can get a better matte from a background that’s blue because the wavelength is shorter and blue light will resolve better, this is generally of little consequence and it’s convenience that counts.
Green screens were extensively used in the special effects shots for the movie “Titanic” involving people. Some of the shots of the miniature ship model were shot on red screens.
By the way, the terms matte, mask, alpha mask and alpha channel are more or less interchangeable. Matte is a movie maker term, mask is universal, while alpha channel comes from early video processing days when an extra channel was created for masking and called the alpha. In the video industry, bluescreen composition routines are known as chromakey.
How it works
Chromakey in the old analogue video version produces a matte that was a true bitmap, that is, it was either on or off which isn’t very subtle at all – so the effect may look quite fake. In the real world edges blend.
In a similar way, Photoshop’s magic wand is a crude bitmap solution. Photoshop’s Colour Select option (under the Select menu item) is a bit more sophisticated. More advanced still are Live Picture’s Silhouetting routines which have some truly elegant masking features verging on bluescreen functionality. But best results come from a combination of purpose shot bluescreen elements, together with bluescreen image compositing software, designed specifically to work with colour keyed backgrounds.
The most crucial area in a mask is the edge, where the bits you keep meet the bits you bin. So the value of any masking software is tested by the subtlety and believability of the mask edge regions. These “in-between colours” as they are known can be very tricky, and several different patented algorithms are use to process them, so that the edge feathering and decolouring solve the problems particularly in translucent objects such as glass and smoke.
Pixels at edge transition will have colour fractions from both the cut-out background and the required foreground. One old solution known as matte choking, was to partially mask the lot by increasing the density of the whole matte or just it’s edges. This can work but may lead to a matte fringe or “matte line” around the cut-out. A better solution is to evaluate both the background and the foreground object and do a colour blend at the edge, or de-tune the edge colours. This is what the latest software does.
Get it right in the shooting stage! This is the best stage to solve problems if at all possible.
The first rule of photo compositing is to match the lighting: all elements should look as if they belong together. Some recommend the use of a straw coloured back light to cancel any blue spill from the background, but in my experience this has usually been unnecessary.
It’s important that the bluescreen is evenly lit and smooth so if you are using cloth be sure to get the wrinkles out of the background. I prefer a painted background and freshly paint it each time. The television and movie industry use special blue & green paint such as that made by Rosco. If you’re using some other paint, use a vivid hue that is different from those in the foreground object.
If possible have your background image at hand so you can most easily match lighting with it. At the lighting stage of the shoot, look at how the foreground object interacts with the background, and carefully match the light levels and the direction and qualities of the light: is it hard? softly diffused? etc. This work, by the way, doesn’t have to be done in a studio. If your background image was shot outdoors consider shooting your insert image in sunlight too. It may be easier and moreover it may be more believable. As Cecil B De Mille said “ The value of special effects depends strictly on the impression of reality that they generate.”
Small bluescreen set-up, using studio flash illumination. Note the baffles on the lights to avoid stray light spill.
Keep in mind that you only need to have bluescreen behind your foreground element; no more than necessary. More is not better because it increases the chance of colour spill and colour wrap around.
Care should be taken to separate foreground lighting from background lighting to avoid spill. Use “ flags” and “cutters” where possible. These can be anything from manufacturer supplied barn doors to bits of card placed strategically. Spill on the background could degrade the hue and density of the bluescreen background. This makes calculation of a mask harder.
And a tip to cut expenses: you can use cheap blue plastic gift wrap gel instead of heat resistant Lee or Rosco filtration to colour your light . Be aware however, that unlike the expensive pro filtration gels, gift wrap is not heat resistant so use it with care! It’s fairly safe on electronic flash away from the modelling lights, but keep checking it. Wooden pegs are fine for attaching the gel but keep an eye on them too.