Green Screen – Chroma Key

Posted by on Mar 5, 2016 in Technical | No Comments
Green Screen – Chroma Key

The idea of Chroma Keying is actually rather simple. The skill behind doing it successfully however, is not. Chroma Keying is when an actor/actress acts out a scene in front of a green (or sometimes blue) screen. The single colour background is then replaced with a background image. For example, in X Men when Archangel is flying, he of course isn’t really. He is actually held in the air by ropes and pulleys in front of a green screen. The green screen background is then replaced with a bright blue sky, giving the effect of Archangel soaring through the air at great heights!

 

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When using a green screen for visual effects, a lot of factors come into play. In order to remove the green background, all colouration of green must be removed from the recorded footage. Although all colours contain some green in them, some contain less than others. For example, a blue t-shirt does hold the slightest amount of green in it’s colouration so it will appear slightly darker once it’s been stripped of all green colouration. However, if someone in the foreground were to wear a green t-shirt and the shot was keyed (the act of chroma keying), the actor would appear to have no torso as the green would be completely removed as is the background.

Why a green screen? Why not any other colour? Well, originally it was in fact a blue screen that was used since the original process could only remove the blue colouration from video footage. However, that was in the 1930s and much has since changed! Green is now the colour most commonly used for Chroma Keying for two major reasons. The first is that it is factually a lot easier for modern day digital cameras to pick up the colour green. The difference between green and other colours is much clearer and therefore the removal of green from the footage causes less interruption to the rest of the shot as other any other colour would. The second and less technical reason for green being the colour of choice is simply because of our fashion today. A lot of our clothes are blue and therefore, it is much more common for the removal of the colour blue to cause a lot of problems when it comes to deciding what the actor/actress will wear during the shot. Although that is said, there are times when the use of a blue screen has greater benefits than the use of a green screen. For example, during the Green Goblin scenes in Spiderman, it would’ve proved extremely problematic to keep the Green Goblin’s suit green whilst removing the green screen from behind. The use of a blue screen had greater benefits, caused less colour interruptions and was therefore used.

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One of the biggest challenges faced when using a green screen is perfecting the lighting behind the actor. If the actor were to cast a shadow on the background, it would appear to the camera as a darker shade. For this reason, when removing the green pixels, the colour range would have to be wider to remove all types of green (both the lighter and the darker shades of green). However, if the background was perfectly coloured throughout, the variation of green coloured pixels that would need removing would be minimal and would therefore cause less interruption to the shot on the whole. One technique used by many keyers to achieve as few lighting problems as possible is the use of large white cards to bounce light upon the background, achieving an even amount of lighting across the whole background of the shot. However, this also comes at a price as if there is too much lighting in areas, it can appear brighter and just like a shadow, can cause colour difference in areas of the background. The lighting of a green screen is incredibly tricky and probably the most problematic of all issues keyers find themselves troubled with during the filming process.
Another challenge which can cause an issue while Chroma Keying is the spill which is caused by light bouncing off of the green screen and onto the back of the subject (most often an actor/actress). Although this may seem like no issue at all as it is barely visible from the front, when it is later processed, the computer automatically removes every tiny little trace found in the outline of the subject. This will leave jagged edges and a very unnatural and low-budget look once completed.
As well as the green screen requiring perfectly sufficient lighting, the actor in the foreground also requires lighting ñ these are called ìsubject lightsî. Again, it is extremely important that the subject is not casting any harsh or clearly visible shadows on the green screen background. A back light further helps the camera to define what is to be removed and what is not. This is a light which is found above the set and is shone directly onto the subject to help the camera pick up the clear and distinctive outline of the subject.

Perhaps you’re wondering what the green screen physically is? Well, the green screen actually isn’t anything fancy. Often, it is just a large piece of cloth or wall behind painted green. A cloth is preferred as painted walls can cause reflections which then brings issues as previously mentioned. Using a cloth also has disadvantages as you must ensure there are no creases and the cloth is perfectly flat. Otherwise, this can also cause issues with the lighting due to varying shades of green.

Chroma Keying is a simple concept but requires a lot of work and skill to perfect. In this day and age, it is regularly used and the cameras and software used to create such an effect are much more advanced than when keying was first used. This simple concept and extensive process allow us to achieve and witness great things in the film industry and many other places today.

Chroma Keying is a simple concept to understand but can require a huge amount of work and effort to perfect. A lot of films in this day and age attempt the use of a green screen for added visual effects, many of which fail as they seem so poorly created and appear very low budget. However, when done properly, it really does give the viewer a feeling of an effect that appears impossible but is seemingly happening. In today’s world, CGI and Chroma Keying is growing increasingly popular. When the concept was first found and used in the 1930s, the process was extremely limited. With the equipment and technical aid we have today, the possibilities appear limitless in comparison. With the use of computers and highly powerful digital cameras, Chroma Keying is now something that is extremely useful but requires a lot of effort, skill and time. When done properly, it looks absolutely magnificent and the rewards really are worth the required effort. It is amazing how a simple concept allows us to achieve such amazing effects.