Chroma Key is used so much as an effect today that we no longer notice this kind of digital trickery. Provided, of course, that the use of blue or green screen is professional and thus according to all rules of the art. Here we show you what you have to consider.
Whether feature film, television or image video: digital life also makes life easier in the production of films and videos. When it comes to combining picture elements or replacing them with completely digitally animated objects, greenscreen / bluescreen, and thus Chroma Key, plays a decisive role.
This article not only explains how to use Chroma Keying professionally, but also offers tips and tricks for the best possible quality and results in videos with Bluescreen or Greenscreen
In a nutshell
- Chroma Keying allows you to separate subject and background, making two different image sources in a video appear as one image.
- This combination is only possible if the foreground image is clearly distinguished from a uniform background. If this is the case, the exchange is done automatically using software.
- Most commonly, a standardized blue or green background color is chosen, known as blue screen or green screen.
- The setup and illumination of the background is crucial for the quality of the effect.
- 3 exclusive professional tips are waiting for you at the end of this article.
What is the difference between green screen and blue screen or chroma key?
All three terms, Greenscreen, Bluescreen and Chroma Key, stand for the same digital process. A person or object is always “punched out” of the picture. The process is called Chroma Keying.
The two most common application data of Chroma Keying are blue screen or green screen. They stand for the most important basic mechanism that makes this effect possible.
An example: a video testimonial for a management video must be produced. The CEO to be shot is only available for a very limited period of time. At the same time, however, the interview should take place in front of a branch of the company abroad.
If it were an (animated) photo, Photoshop would cut out the head of the manager (photographers call this “cropping out”) and insert it in front of the desired background. In theory, this would also be possible with video. In practice, this would mean elaborately editing 25 individual images per second.
Ever since the advent of analogue film, there has been a method of combining foreground and background in a simple and efficient way: chroma keying.
How does Chroma Keying work?
The mode of operation of Bluescreen / Greenscreen can be explained easily. You command a computer software to automatically remove a certain colour in the video recording. This colour is blue. Or green. Wherever it is present in the image, a new background or video image can be added.
That’s how you walk on water with Chroma Key!
Probably the best known example of chroma keying is the weather forecasts on television. The announcer stands in front of a monochrome background during the recording. But the TV viewers see the person in front of the animated weather map. The exchange of the background image for a new video image, an animation or a freeze frame takes place in real time.
This process can also be used to change or replace objects in an image. Of course, this is only possible if the object to be replaced is monochrome and the colour to be replaced does not occur elsewhere in the image.
This is exactly the reason why there is not only blue screen. But also green screen.
Green screen or blue screen?
Imagine the weather prophet from the example above has green eyes. Or wears a green tie. At the same time he stands in front of a green background. What happens?
Right! Even where the eyes were and the tie, the computer, stupid as it is, replaces the foreground with the background.
So whether green screen or blue screen is used always has to do with which of the two colours is already in the foreground.
Bluescreen used to be considered the measure of all things. Already in the analogue film – at that time a complex chemical process- a motive for a new background could be set in this way. There is a simple reason why the colors blue and green – and not also yellow, red or black – are used as background colors today:
Green and blue offer the strongest contrast to the color of human skin. And thus enable a clean definition of foreground and background.
Technically, this is done by the software used in video image processing live or after recording, making the background color transparent: it replaces it with an alpha channel. Even though the color green is now more frequently chosen than the former uniform blue, green has a major disadvantage that must be taken into account when illuminating the background:
Green reflects the light much more strongly (up to a factor of two) than the color blue.
If the bluish reflections fall on the person or object in the foreground, it becomes “unclean”.
As a result, the software has trouble distinguishing between foreground and background. And because it does what it was programmed to do, it then also punches out parts of the foreground and delivers planted image edges.
Application types of Chroma Keying
In addition to its daily use in TV news and video interviews, Chroma Keying has become increasingly important in feature films over the last few years.
Through 3D animation, worlds can be created that are hardly distinguishable from reality. However, mythical creatures and futuristic worlds are still created on the computer – and only after the actual shooting.
For the actors, this means that they no longer play in real worlds, but in a green screen or blue screen studio. A green or blue tennis ball on a stick made of playing partners can be used to direct the view.
What do I have to consider when illuminating for greenscreen and keying?
For professional chroma keying you don’t want (!) to paint your studio background with any blue or green color. You’re making life more difficult for the keying software.
What you want, when you’ve made the decision between greenscreen and bluescreen, is a ready-made greenscreen or bluescreen. Or a special color, either “Chroma Green” or “Chroma Blue”.
These standardised colours are admittedly more expensive than the stuff you can get at the DIY store. But they have an unsurpassable advantage. Because of their standardization they are matched to the software for Chroma Keying. Even if you or your cameraman is not a master of lighting, the computer will unerringly recognize what it has to do.
You want to illuminate the background as evenly as possible, whether as a stretched canvas or all painted studio walls. The brightness must be even. Shadows are also undesirable.
Especially make sure that the light source you use does not sabotage you! Daylight fixtures or LEDs are ideal. Unlike with warm light, you don’t run the risk of the headlights adding a shade of gray to your beautiful blue or the green in front of the camera turning brownish.
Spill is your shot whenever the foreground is …
- casts shadows or light on the background, or
- the background casts light on the edges of the object or person in the foreground.
Both of these create color stains, which are reliably but undesirably removed by Chroma Keying. Unprofessional keying is particularly noticeable when the edges of the subject in the foreground are unclean.
In practice, as a professional you have two tricks at hand to avoid spill.
- You illuminate the background where you have a reflection or shadow with an additional light source. So you fight the irregularities and “spots” with an additional, coloured background light.
- You can control the colour tone in the background not only with the light source, but also with a special foil (gel) that follows a standard magenta value. You can find this under the name “Minus Green”.
Your goal must be to have an evenly illuminated background already during the shooting. A waveform monitor will help you achieve this. Such a monitor can also be rented by the day.
Programs and software for keying
Professional software for video editing usually has the function for chroma keying already integrated. This also applies to freemium or free programs for video editing.
Recommendable and popular are greenscreen editors:
- DaVinci Resolve
- Filmora9 Video Editor
- final cut pro
- AVS Video Editor
- Sony Vegas Pro
- virtual dub
In addition to the programs mentioned above, there are various other video editing software with a built-in chroma key tool.
3 professional tips and tricks for an even background
1Use a Waveform Monitor
The Waveform Monitor shows you the color spectrum as bandwidth. The narrower this horizontal band is horizontally, the more regular your background. If you see a diagonal gradient, one of the background sides is darker than the other.
Working with a waveform monitor for chroma key | © Aputure
2Working with depth of field
Another trick is to work with shallow depth of field. If you place the sharpness level on the foreground and highlight it, the background will appear more uniform due to the slight blurring. This way, irregularities and slight shadows are “swallowed” by the lack of sharpness.
At the same time the focus on the foreground helps to get sharper edges. This also helps when chroma keying with green screen or blue screen.
3Optimize color saturation in image editing
You can also make life easier for yourself and your shots by applying a color correction to the image before keying. So you can optimize the saturation of the background color, of course separated from the foreground.
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