The majority of filmgoers are familiar with the use of blue- or green screens in filming effect shots in big budget films such as Star Wars or the Matrix. But with the advent of digital compositing in post production, a wide variety of colored screens can be used depending on the color of the foreground objects that are shot in front of it. So there are blue screens, green screens, black screens and even red screens in some cases.
The most famous of these is the blue screen. Here we will explore the process involved in replacing those “blue” with an image.
In the old, “optical” days of Alien and Aliens, blue screen was used for all optical effect shots. For example, the shot of the Sulaco approaching the planet LV-426 towards the beginning of the film. This kind of shot uses a “traveling matte” background. That means the background is not a still image, but is a moving film shot.
To make a composite of the foreground and background, each element must first be shot on separate negative films. The miniature model of the Sulaco was shot before a blue screen. Regardless of how dark the model needs to appear in the final shot, the blue background was lit very brightly so a clear separation was possible.
The matte painting of the planet was filmed in a more traditional manner. The camera moved across the large painting to give the illusion that the camera had panned in space.
Now, this is where the fun begins.
Before digital compositing emerged in the post-production industry in the early 1990’s, it was very complicated to composite these kinds of shots. Everything had to be done using an optical printer.
First, the Sulaco shot needed to be copied onto a high contrast panchromatic black and white film. This type of film is sensitive to all colors, unlike some black and white films that don’t capture red, which was why red lights have traditionally been used in dark rooms.
The image from the color negative was then copied through a blue filter which absorbs every color except blue. Then you are left with an image where the blue colors are extremely bright, and the red and green colors are very dark. Since the black and white film has such high contrast, you will basically get a black silhouette of the Sulaco surrounded by white.
Imagine using a photograph and increasing its contrast on a computer to the maximum. In the end, you would get sharp-edged surfaces that resemble the shapes that were in the photo. This is how this black and white film turns the smooth image into an image where there are two surfaces, the black ship area and the white blue screen area.
But this black and white film is also a negative film, so the situation will be reversed – the blue screen will be black and the ship would be white.
This kind of image will be used later as a “mask” in the optical printer. But we’ll get to that in a moment.
Next, the image of the ship was copied onto a new piece of film. And since you only want the ship from this shot, and not the blue screen around it, you need to place a mask around the ship to block the blue screen. This mask is set in front of the Sulaco footage to precisely block the blue area with its black surface. As a result, only the image of the ship was captured, since the ship area was white (transparent) on the black and white film mask.
The same process needs to be repeated in order to place the matte painting of the planet into the background. But you can’t use the same mask. You need a new mask that will allow the image of the matte painting to be captured around the ship.
So for that the black and white mask film was just copied onto another black and white negative film and the colors were reversed. The ship area was black and the blue screen area was white. Piece of cake!
This new mask was then placed in front of the planet footage and copied onto the same color negative film as the ship. The black area covered the image of the ship, allowing only the matte painting to be recorded.
Lastly, the film was processed and you got a shot of the Sulaco flying towards the planet.
In my experience, people always enjoyed these shots but generally took them for granted. They were simply not aware how much work went into creating these images that only represent, at times, a few seconds of screen time. This technique is very exact. If one mask is off by a millimeter, you would get a fuzzy, half meter thick double exposed area on the screen. Perfection is important.
With today’s technology, the blue screen technique has become very easy. It can even be done in “real time”, like with weather forecasts. And if you look at modern SF films such as Star Wars: Episode II – Attack of the Clones, you will notice that there are very few shots that don’t feature blue screen.
While this technique has become commonplace in today’s filmmaking, it’s important to note that at the time Aliens was filmed, it was still an expensive and time-consuming process. Perhaps in understanding this, you will better appreciate the hard work and dedication it took to make those brief images look as good as they did.