The Chroma Key Effect

chroma key filter

Media for this tutorial

Did you know that the first blue screen effect in cinema was employed in the production of The Old Man and the Sea (based on the book by Ernest Hemingway, starring Spencer Tracy)? Commonly known as the "traveling matte" technique, utilizing separate "male" and "female" film mattes and a complex projection setup to achieve the final composite, the process was only available to high budget productions. Today, a better-quality effect can be achieved in digital post-production with the help of a specialized software filter.

In this tutorial we will use BCC Chroma Key filter included as a standard feature with BORIS RED. However, the same BCC Chroma Key is available in the lower-priced BORIS FX package. Both RED and FX plug into a wide range of NLE systems from Avid, Apple, Adobe, Sony (Vegas), Harris (DPS Velocity), Canopus (Edius), Media 100, and others. In addition, BCC Chroma Key is part of the BORIS Continuum Complete filter set available as native plug-ins Adobe, Apple, Avid, and other NLEs.

How does the chroma key work? The foreground object has to be separately filmed in front of an evenly colored and lit backing. The footage is brought in the compositing or editing system to be combined with the new background video or still. We have to consider three different color hues: the foreground object such as a person (its skin and clothing), the backing color that is used as a temporary backdrop behind the person, and the new background video to be used as a replacement of the backing color.

chroma keyingA few words about producing the foreground video: first, you need to decide on the color of the backing. You have a choice of three possible colors: blue, green or orange. The choice will depend on both the foreground and new background types. The backing color must be opposite to the colors of the foreground object and similar to colors of the new background. Example: if the foreground is a person then blue or green backing color is recommended as these colors are not present in human flesh pigments. In fact, human skin color is 70% red for all people regardless of race. However, you may want to choose the backing color to match the new background. Example: if the new background is mostly sea or sky then blue backing will work better but not if it is grass or trees in which case green is preferable. The reason for that is in the color spill cast on the foreground object from the backing material during the shooting. Things to consider when filming people: blue eyes (use green screen), ties and other accented clothing (never allow blue or green ties). Also note that some digital formats favor green over blue as green channels get higher sampling rates.

The second factor is the lighting of the backing during the shoot. The backing can be made of cotton cloth or a wall painted with flat green color. Avoid shiny materials that reflect light. The most recent technique that employs retro-reflective fabric and a ring of green LED lights mounted on the camera lens is very effective in green screen production.

The task of isolating the foreground object is not simple even in the perfect shots. The system has to handle transparent objects such as smoke or glass, fuzzy edges such as hair or clothes, cast shadows, green or blue color spill, etc. In this tutorial we will examine these challenges and provide solutions using the BCC Chroma Key filter. The foreground plate that we will use is a semi-transparent glass bowl shot in front of a green backing. The scene has a cast shadow from the leaves on the left. Let's see how well the BCC Chroma Key filter handles the first two challenges.

green screen

The first step is to choose the BCC Chroma Key filter from the Keys and Mattes category in the Filters menu. This step may vary if you are using the BCC package directly in the editing program. However, the Filter controls are consistent in every host.

Note that the default color of the key is a shade of blue. This is the most commonly used standard blue color but we need to change it to match our green backing. The best way to pick the key color is by sampling directly from the source image. Using the eyedropper tool let's pick the color from an area close to the foreground object. This step is very important for achieving the best result as no amount of further tweaking will compensate for poorly sampled backdrop color. The reason for picking near the edge of the object is that this is the area most difficult to key. So the choice of color must favor the weakest spot in the image.

filter controls

Note that as soon as we sampled the green color the preview showed the seemingly perfect key. The checkerboard background helps to examine a single frame plate in the preview. Observe the left side of the bowl where the checkerboard is clearly visible through the transparent glass.

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