## 2 Background and Related Work  
As it has been explained earlier, one of the main elements of a virtual TV set is the monochrome cyclorama that surrounds the stage to allow the use of the chroma-keying technique. Any color can be used as the key, being its main requirement the existence of a difference between the key color and the hue of any element that must not be erased in the final composition. The most common colors used for chroma-keying are the saturated green and blue. Blue was more used traditionally due to the use of analogical cameras, as it created less reflections and its presence in nature and human skin is less prevalent. Nowadays, green is the prevalent color, as the sensors of the digital video cameras are more sensitive to this hue [8]. This better sensitivity for the green channel makes the captured image less noisy, reducing the amount of light needed for the cyclorama [9], thus avoiding undesired reflections in the elements on the stage that may be stained with green light. The size of the green areas on the stage may vary from a simple screen, to a plane background or a complete cyclorama surrounding the stage (Figure 2 top). However, the most usual configuration in a professional virtual TV set is based in a green room, including three walls (both sides and the background) and the ground. One of the main characteristics of this kind of cycloramas is the absence of sharp edges in the union of the walls and the ground in order to avoid shadows that would lead to the appearance of artifacts in the final composition. Besides the physical construction of the stage, the chroma-keyer itself must be considered in order to improve the chroma-keying process. It receives three video signals: the first one recorded from the cameras, the second one with the computer-generated elements and the third one with a mask that indicates when a virtual element is in front or behind the real scene. Then it uses these three signals to create a composite image combining real and synthetic elements using the color information captured from the cameras and the position information in the mask. The chroma-keyer can be implemented by software (as a computer program) or by hardware (using specific devices). Regardless of its implementation, the process can be configured in order to optimize its behavior for each specific TV set. Mainly both the base hue to be erased and the threshold of the accepted color variability can be configured. The increment of the threshold implies a bigger tolerance to color variations, admitting a worse illumination of the background but being less precise in the hue that is being erased (Figure 2a). As it has been remarked before, in order to reduce the threshold, thus improving the chroma-keying process performance (Figure 2b), the cyclorama illumination must be as diffuse and uniform over all its surface as possible. Shadows and shines that may cause tone variations should be avoided as they may produce noise and artifacts in the final composite image, forcing a threshold increase and thus reducing the precision of the process. In this way, in order to separate the background from the elements to preserve in the final composition adequately, the light of the cyclorama must be uniform and diffuse, avoiding shines and shadows. As a general rule, there must be a difference in the light that helps increasing the color difference between the actors and objects and the monochrome background [10]. To achieve this goal, the most common solution in commercial virtual tv sets consists of two different illuminations:  
Figure 2: Chroma-keying process. Original images (**top**), chroma-keying mask (**middle**) and final composite image (**bottom**). Incorrect keying threshold configuration result (**a**). Proper chroma-keying configuration result (**b**). one for the actors, atrezzo and scenography (usually based on the traditional configuration with frontal, side and back lighting) and another one for the cyclorama.