Chroma key is set colours. You can key almost any colour. The reason you use a specific Green/Blue is that those tones are not found skin shades and hues.
We use it alot . But you can come unstuck with crap imatations and naff software. There are many softwares out but most are only ok fot Event work. If you want a really high quality software to key Chroma you are looking £400 upwards. we need it to also batch process 600+ portraits.
The principal subject is filmed or photographed against a background consisting of a single color or a relatively narrow range of colors, usually blue or green because these colors are considered to be the furthest away from skin tone.[2] The portions of the video which match the preselected color are replaced by the alternate background video. This process is commonly known as "keying", "keying out" or simply a "key".
Green is currently used as a backdrop more than any other color because image sensors in digital video cameras are most sensitive to green, due to the Bayer pattern allocating more pixels to the green channel, this mimicks the human increased sensitivity to green light.[5] Therefore the green camera channel contains the least "noise" and can produce the cleanest key/matte/mask. Additionally, less light is needed to illuminate green, again because of the higher sensitivity to green in image sensors.[6] Bright green has also become favored as a blue background may match a subject's eye color or common items of clothing, such as jeans, or a dark-navy suit.
Blue was used before digital keying became commonplace because it was necessary for the optical process, but it needed more illumination than green. However, it is also further in the visual spectrum from red, the predominant color in human skin.
The most important factor for a key is the color separation of the foreground (the subject) and background (the screen) – a bluescreen will be used if the subject is predominately green (for example plants), despite the camera being more sensitive to green light.
In analog color TV, color is represented by the phase of the chroma subcarrier relative to a reference oscillator. Chroma key is achieved by comparing the phase of the video to the phase corresponding to the preselected color. In-phase portions of the video are replaced by the alternate background video.
In digital color TV, color is represented by three numbers (red, green, blue). Chroma key is achieved by a simple numerical comparison between the video and the preselected color. If the color at a particular point on the screen matches (either exactly, or in a range), then the video at that point is replaced by the alternate background video.
The biggest challenge when setting up a bluescreen or greenscreen is even lighting and the avoidance of shadow, because it is best to have as narrow a color range as possible being replaced. A shadow would present itself as a darker color to the camera and might not register for replacement. This can sometimes be seen in low-budget or live broadcasts where the errors cannot be manually repaired. The material being used affects the quality and ease of having it evenly lit. Materials which are shiny will be far less successful than those that are not. A shiny surface will have areas that reflect the lights making them appear pale, while other areas may be darkened. A matte surface will diffuse the reflected light and have a more even color range. In order to get the cleanest key from shooting greenscreen it is necessary to create a value difference between the subject and the greenscreen. In order to differentiate the subject and screen a two-stop difference can be used, either by making the greenscreen two stops higher than the subject or vice versa.
Sometimes a shadow can be used to create a special effect. Areas of the bluescreen or greenscreen with a shadow on them can be replaced with a darker version of the desired background video image, making it look like the person casting the shadow is actually casting a shadow on the background image instead.
Another common, related, problem is lighting the subjects in a manner that is complementary to the scene being keyed in behind. Attention has to be paid to matching the direction of lighting and the colour balance in the background. Any spill of the chromakey colour will make the result look unnatural. Even a difference in the focal length of the lenses used can affect the success of chromakey.
I've seen so many struggle with lighting. It must be even lit and the bg must not have marks and creases in as that will come out in the keying process.
