Chroma Key

Imagine what you can do if you film your subject separate from the background and put the two together in post production? This is Chroma Key, also known as greenscreen or bluescreen, a technique for mixing two images together.

The development of chroma key began as early as 1940 when Larry Butler won the Academy Award for Special Effects for Thief of Baghdad. He invented the bluescreen technique in order to achieve the visual effects in this film.

In order to shoot chroma key you simply need one image of the foreground (the subject) with an evenly colored and lit backing, then a separate new background on another image. The evenly coloured background in the one image is replaced by the other new background image. This is done by removing (or making transparent) one colour (often green or blue) in the image.

What camera to use
In order to achieve a nice result you should have a 3CCD camera, which resolves at least 500 lines, it should preferably have manual exposure controls and zebras.

3CCD means that the camera has three separate charge-coupled devices (CCDs), taking in one separate measurement of red, green and blue light. This makes it easier to key out one certain colour in post production.


The high resolution is important because this gives more detail and contrast in the image, which means smoother edges and hence a better result.

The manual exposure controls and the zebra stripes (a setting which will put stripes on the image areas exposed at the same level) will make it very easy to see, adjust and make sure that the background is evenly lit and within the luma range you want.

You simply adjust the iris and gain manually until the entire background is evenly covered with zebras, and as this most often means overexposing you must remember to find the right exposure again before recording.

How to place the camera and the subject

Try to place the subject as far away from the background as possible in order to avoid reflected colour from the background onto the subject and to prevent the subject from casting shadows on the background.

There should be at least the same distance between the subject and the background as between the subject and the camera, and the distance between camera and subject could be even further.

Use the zoom if possible, as this will give a lower depth of field, meaning that the background will be out of focus and with less detail. This is an advantage as you need it to be even and without contrast and detail.

Blue, green or orange?

The background can be made of either cotton cloth or a wall painted with flat green colour, but not shiny materials as that will reflect light.

The colour of the background depends on both the foreground and new background. The background colour needs to be the opposite of the colours in the foreground subject and similar to the colours in the new background. The background is usually green or blue, but can also be orange.





The reason green or blue are most common is because most often the foreground subject is a person, and blue and green are the colours furthest away from human flesh pigments. It is also important to keep in mind what the new background will be - if it is mostly sea or sky, then blue backing will work better but if it is grass or trees green is the best. This is in case of colour spill on the foreground subject from the background during the shooting. Don´t forget details like the eye colour of the subject - if it is blue, then use a green screen.

The lighting

The key to get a good result is to give the chroma key background an even lighting across the entire surface. Large softlights are good as they will hide shadows and imperfections in the surface.

The foreground subject needs to be lit so it fits with the new, replaced background, so if the new background is outdoors and the sun shines from one direction, you need to consider this when lighting your subject.

You also need to be careful so that the light on the subject doesn´t spill or cast shadows or hot spots on the background. The best is to angle these lights from the sides.


New development

The most recent technique for shooting chroma key is a ring of green or blue LEDs (Light Emitting Diodes) placed on the camera lens, providing all the light required to illuminate a fabric designed specifically for use as backdrops when using these LED rings.

These fabrics differ from the more traditional blue or greenscreens, as they look grey to the human eye. However, the fabric contains millions of tiny glass beads that act as reflectors.

When the light from the LED ring hits the fabric, the camera actually sees the grey fabric as a perfectly even blue or green background.

Keying - the Postproduction

Keying is the process of removing specified colours in a frame. You can either use luma or chroma keying. Chroma means that the editing program finds in each frame every pixel containing the colour you have chosen for a background key colour and makes that pixel transparent.

Then whatever you have on the video layer underneath will show through. Luma keying works the same way, but instead of colour it will select all the pixels in the frame that have a higher (or lower, depending on what you choose) brightness value than the specified value. Find a greenscreen (or blue or orange), get creative and experiment!

Links:

Thief of Bagdad in Wikipedia

Chroma Key in Wikipedia
Reflecmedia Chroma Key Video at YouTube

More about the LED ring at Reflecmedia.com
Videomaker - Green Screen Post Production at YouTube
Lighting for Chromakey at Videomaker.com


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