Croma Key is a method for trimming a film, where the background is blue or green, they are used from the beginning of television in simple applications to sophisticated multi-layer systems, basically consisting of inserting letters or images in a video, replacing a color by the video to insert. The teams in charge of realizing these effects in real time (live or online) are Video Switchers or Mixers , in post-production (off line) video editing programs such as Adobe Premiere, AVID, etc. are also used.
The term “key” refers to the operation of switching a base video with one that contains the letters or another image, so that they are superimposed on the main image. The key effects are classified into two groups:
- Luminance key (Luma key).
- Color key (Chroma key).
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- 1 Chroma Key Effect
- 2 Luminance and Chrominance
- 3 Video Features for Key
- 4 Some characteristics of the key font
- 5 Scene lighting for chroma key
- 6 Easy or difficult
- 7 Source
Chroma Key effect
The image that occupies the plane closest to the viewer is known as Foreground, the one that occupies the last position (the one furthest from the viewer) is called Background, the intermediate planes are called Mediumgroung.
Chroma Key effect
Luminance and Chrominance
As already known, the Luminance (Y) is the part of the electrical video signal that contains the scene’s black, white, and gray information, and is used by black-and-white televisions to display images.
On the other hand, the Chroma (Cr) is the electrical signal that contains the color information of the scene and that together with the luminance are used by color televisions to display the images in color. The composite video signal (SVc) contains luminance, chroma, horizontal sync, vertical, and color information.
Luminance should not be confused with Chrominance, Luminance is what you see when you put a television in black and white and contains information on the different lighting levels of the scene, Chrominance is the color of each part of the scene.
Key video features
The video that is used as “base” does not require higher specifications and can have any artistic or technical characteristics, this is because it is not used for trimming processes. On the other hand, the video used as a “key source” must comply with certain technical characteristics so that the electronic trimming is carried out as naturally as possible.
Some characteristics of the key font
- Opaque areas should be as large as possible.
- There must be a marked difference or “detachment” between the opaque part and what is wanted as transparent.
- The color to be trimmed should be high to medium saturation. Preferably blue or green.
- Good resolution of the camera that takes the shot and adequate lighting.
- If possible, use colors in the opaque area that differ greatly from the color selected for trimming.
- The area that will finally be transparent must be a color that has the most uniform hue and saturation possible within the clipping area.
- The black and white balance of the image must be very well adjusted to achieve precise cropping.
Scene lighting for chroma key
The blue color as the Scene background for Chroma key is used in most cases and allows to achieve a good edge trimming mainly when people appear in the scene, this is because the background of the scenery reflects light towards the back of the subject and because blue is complementary to yellow-orange (skin tone) at the edges of the figure, the sum results in white or in the worst case, blue loses saturation, substantially improving the quality of the cut in the limits of the subject’s face and the blue background. This characteristic is not achieved when using the color green. However, depending on the type of work, other colors may be used.
Easy or difficult
The Chroma key produces mixed feelings in the filmmakers. For some, getting a good cut is trivial. Others tremble at the thought of “lighting a chroma.” But there is something we all have in common: some horror story related to a troublesome chroma key – long hours of tweaking, testing, and despair – trying to achieve decent cropping, wiping out an awkward shadow, or regaining the natural color of a presenter with electronic jaundice. Why is it so difficult sometimes to achieve one?
Let’s start by establishing an important premise: obviously, there are some teams better than others … but any device capable of chroma key effects is going to give a decent result if fed with good quality material. And good quality is supported by three factors: good camera, good lighting and good background. And when it comes to achieving post-production color clipping, the recording format counts, too.
There are many ways to achieve a chroma key effect. The traditional way is to do it live in a studio, in the best “climate men” style. This is usually accomplished by mounting a small corner with a perfectly lit background and using a high-quality effects generator, which ensures excellent results.