How to Do a Chroma Key

by ugop98 on March 21, 2017 - 4:06pm

      During the last years, there have been a massive use of green screen technology. Every day, spectators see green screens. On TV, for example, on the weather channel, series, in movies; every movie uses green screen. It allows production to cost less. Everyone can use those technics, as a Chroma key. It is a special effect technique for compositing two images based on color hues. It is a really easy process, with six major steps divide in two moments. During the production, thinking about your green screen, lighting and your digital camera will be necessary. Then there will be post-production, with the transfer of your images on the computer, the use of the ultra key effect and then the placement of the digital background.

PRODUCTION

      To begin, on the step of production, different parameters will need to be set to have the best results. At first, you will need a green screen to do your Chroma key. It is use in motion pictures, video games, and every day in the industry of visual effects. The semi-neon-green that is used in green screens is a color that is not common in nature and it will simplify all the process.

Then, think about your lighting. It is an important parameter for a shooting on a green screen. The lighting of the green background must be uniform. Green is also easier to illuminate than other colors, so not much light will be need. In fact, there must be no shadow, this is a work of precision.

Finally a good quality digital camera is needed to shoot those images. The more details there will be on the image, the more real your Chroma key will be. In this example, moving the camera is prohibited. If you move it, matchmoving (creating a 3D space, with a virtual camera corresponding to the movement of the digital camera) will be needed in post-production: a lot of work! The object or the person filmed, should be placed in front of the green screen.

POST-PRODUCTION

      After a well-done production, let’s go to post-production. Put your images on the computer. This, must of course, contain editing or special effects software as After Effect, Nuke x or Flame. In our Adobe Premiere Pro will be used. In order, to create our Chroma key, import the images in the software. Then, create a new sequence (right click- create a new sequence) and drag and drop it on your timeline. Double click on your sequence will open it. Once this is done, in the effect tab of the software, choose “Ultra Key Effect”.

The principle of this effect is to select a color in the image to be removed, here the green. First, by using the eye dropper, we select on the image a sample of our green background, just by clicking on it. The result could not be good, due to some noise. Image noise is the presence of parasite information. It is visible in low light areas and also in uniform areas such as a blue or a green background. It results in loss of sharpness in detail and edge.

The matte generation is here to give solutions to those problems. There is no miracle recipe! Settings are adjusted according to the situation. Noise, will be the main parameter to change to create a nice Chroma key.

After modifying those settings, it is important to use the matte generation to perform a second cleaning in the image which will be based mainly on the details and edge. Finally, when all is done, place your digital background behind your sequence on your timeline. A color correction and a check that the whole effect work will be necessary. It’s that simple!

 

      To conclude, Chroma key is not a difficult process. Good production is needed, with great images there will be a faster process and better results. Light, has a first role in your production. Then, during the post-production, a sense of the detail will be a key to your success. Also, as always in post-production, patience is required! Today, green screens are changed for new technology and virtual reality.

Comments

Really nice job! Clear and well organised, we can easily follow the steps! A concept not easy not understand at first, but now clear with your explanations!

A great process description that I would for sure recommend to people who are beginners in post production. Although I would have love to see images to support your technicals terms. However, the definitions you gave for them are clear and simple. You did a really nice job at showing that Chroma Key is simple if well understood.

About the author

Currently a student at the Cégep de Jonquière in production and post-production techniques, I continue my training in post-production and VFX. I previously obtained a vocational training courses in editing and post-production in Villefontaine in France.