Blue colors are used for the chroma key

Instructions, tips and tricks: Green screen, blue screen and chroma key in practice

Chroma Key is used so much as an effect today that we no longer notice this type of digital trickery of color-based image cropping. Provided, of course, that the blue screen or green screen is used professionally and thus according to all the rules of the art. Here FILMPULS shows you what that means.

Whether it's a feature film, television or image video: digital life also makes life easier when it comes to film and video production. When it comes to combining picture elements or replacing them with completely digitally animated objects, green screen / blue screen, and thus chroma key, play 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 work results for videos with blue or green screens.

The most important thing on the subject

  • Chroma Key allows you to set the subject and background separate and thus to make two different image sources appear as one image in a video.
  • The combination is only possible if the foreground image clearly stands out. If this is the case, the exchange took place automatically using software.
  • Most commonly, a standardized blue or green background color, known as a blue screen / green screen, is chosen.
  • The setup and lighting of the picture 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, green screen, blue screen and chroma key, stand for the same digital process. A person or an object is always "punched out" of the picture. The procedure for this is called Chroma keying.

The two most common application dates of Chroma Key are blue screen and green screen. They represent the most important basic mechanism that makes this effect possible in the first place.

An example: a video testimonial for a management video has to be produced. The CEO to be rotated is only available for a very limited period of time. At the same time, the interview should take place in front of a branch of the company abroad.

If it was about a photo, Photoshop would cut out the manager's head (photographers call this “cropping”) and insert it in front of the desired new image. In theory, this would also be possible with video. In practice, this would mean laboriously processing 25 individual images per second.

A process has therefore existed since analogue film with which the foreground and background can be combined more easily and more efficiently: with chroma keying and green screen or blue screen.

How does a green screen / chroma key work?

The way in which the blue screen / green screen works is easy to explain. In the case of a green screen recording, a computer software command automatically removes a certain color in the video recording. This color is blue. Or green.

A new video image, graphic or photo can be inserted wherever it is present in the image. The so-called green screen works that easy.

This is how you can walk on the water thanks to the chroma key and green screen! (Video in English)

Probably the best-known example of chroma key is the weather forecast on television. During the recording, the announcer stands in front of a monochrome background. The TV viewers, however, 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 still image 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 color to be replaced does not appear in other places in the picture.

This is exactly the reason why there are not only blue screens. But also green screen.

Is Green Screen or Blue Screen Better?

Imagine the weather prophet in the example above has green eyes. Or wears a green tie. At the same time, he is standing in front of a green background. What is happening? Correct!

Even where the eyes and the tie were, the computer, stupid as it is, replaces the foreground with the new image information.

So whether green screen or blue screen is used has always also to do with which of the two colors is already present in the foreground.

In the past, blue screens were considered the measure of all things. In this way, a motif for a new background could already be set in analogue film - a complex chemical process at the time.

There is a simple reason why the colors blue and green - and not also yellow, red or black - are used today:

Green and blue offer the strongest contrast to the color of human skin. And thus enable a clear definition of the foreground.

Technically, this is done by the software used in video image processing making the background color transparent either live or after the recording: It replaces this with an alpha channel.

Even if the color green is chosen more often than the earlier uniform blue, green has a serious disadvantage that must be taken into account when illuminating:

Green reflects light much more strongly (up to a factor of two) than the color blue.

If the bluish reflections fall on the person or the object in the foreground, it becomes "unclean". As a result, the software struggles to distinguish foreground and background. And because it does what it was programmed to do, it then also cuts out parts of the foreground and uses planted picture edges.

Types of application of chroma keying

In addition to the daily use in TV news and video interviews, chroma keying and green screen technology have become increasingly important in feature film production in recent years.

The 3D animations can create worlds that can hardly be distinguished from reality. Mythical creatures and futuristic worlds are still created in the computer - and only after the actual shooting.

For the actors, this means no longer playing in real worlds, but in a green screen or blue screen studio. A green or blue tennis ball on a stick made of a play partner can then be used to direct the view.

Hollywood Without a Chroma Key: What Movies Really Look Like When Set:

What do I have to consider when illuminating for green screen and keying?

For professional chroma keying, you don't want to paint your studio background (!) With any blue or green color. Because with that you make life more difficult for the keying software.

When you have made the decision between the two options, what you want is one prefabricated Green screen or blue screen. Or a special color, either "Chroma Green" or "Chroma Blue".

These standardized colors are more expensive than the stuff you can get in the hardware store. But they have an unbeatable advantage. Due to their normalization, they are matched to the software for chroma keying. Even if you or your cameraman is not a master in the light, the computer can pinpoint what it has to do.

You want to illuminate the background as evenly as possible, whether as a stretched canvas or an all painted studio wall. The brightness must be uniform. Shadows are also undesirable.

In particular, make sure that the light source you are using does not sabotage you! Daylight lamps or LEDs are ideal. Unlike with warm light, you run no risk of the headlights giving your beautiful blue a gray shade or the green in front of the camera being tinted brown.

What is spill?

Freely translated from the English language, you can understand “Spill” as “messing up the picture”. The correct word is: spill or overflow. Your recording is always dirty when the foreground ...

  • Throws shadow or light on the background, or
  • the background casts light on the edges of the object or person in the foreground.

Both produce color spots that are reliably but undesirably removed by the chroma key. You can recognize unprofessional keying particularly by the unclean edges of the subject in the foreground.

In practice, as a professional you have two tricks at hand to avoid spill.

  1. You illuminate the background where you have a reflection or shadow with an additional light source. You fight the irregularities and "spots" with a complementary, colored light.
  2. You can control this color not only with the light source, but also with a special film (gel) that follows a standard magenta value. You can find this under the name "Minus Green".

Your goal must be to get one while shooting evenly illuminated background to have. A waveform monitor will help you with this. One can also be rented on a daily basis.

Programs and software for keying

Professional software for video editing usually has the chroma keying function already integrated. This also applies to freemium or free video editing programs.

Recommended and popular as green screen editors are:

  • DaVinci Resolve
  • Avid
  • Filmora9 Video Editor
  • Final Cut Pro
  • AVS Video Editor
  • Sony Vegas Pro
  • Virtual Dub
  • iMovie

In addition to the programs mentioned, 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 a bandwidth. The narrower this horizontal band runs horizontally, the more regular your background will be. If you see a diagonal gradient, one of the background pages is darker than the other.

Instructions: Working with a waveform monitor | © Silas F.

There are professional waveform monitors from various manufacturers, such as Canon (DP-V2420) or Telestream (WFM2200A).

2Work with depth of field

Another trick is to work with a shallow depth of field. If you put the focus layer on the foreground and lift it out, the background will appear more uniform due to the slight blurring. Irregularities and light shadows are "swallowed" by the lack of sharpness.

At the same time, the focus on the foreground helps to create sharper edges. This also helps with the chroma key with green screen or blue screen.

3Optimize the color saturation in the image processing

You can also give yourself and your recordings an easier life by later editing and a color correction beforehand for further processing running across the image. So you can optimize the saturation of the background color, of course separately from the foreground.