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Creating an Event-Driven Cinematic Camera, Part One
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Creating an Event-Driven Cinematic Camera, Part One


January 7, 2003 Article Start Page 1 of 3 Next
 

In the beginning, Auguste and Louis Lumiere made their first short movies without changing the position of the camera. It would take others, such as the magician Georges Melies, to stop and move the camera to create more dynamic scenes than could be done with a stationary, continuously running camera. Over the next century, cinematographers and editors have learned the best ways to film, cut, and transition between different shots to make the movie experience larger than life.

When it comes to camera setup, the dynamic nature of games puts us back to the days of Lumiere. With minimal or no control of the placement of the actors in a game, camera shots are often set as stationary or dragged behind one of the actors. Here and there, we see games that try innovative camera techniques — some that work and others that do not. One way to improve the chances of success is to take the film industry’s century of experience and adapt it to our industry.

One method at our disposal in games is to create a system that automates camera placement and scene transitions (similar to the jobs of the cinematographer, director, and film editor on a major film production). In this, the first of a two-part series of articles on this subject, I’ll look at automating the work the cinematographer does in setting up cameras to capture the best view of a scene. This will lay the groundwork for choosing among available scenes and camera shots required by the director and editor, which I will cover largely in part two.

Describing the Shot

Since the position and orientation of the actors is not known ahead of time, a method of describing the shot without exact positioning information is necessary. In order to arrive at a suitable set of parameters, let’s look at the basic rules and descriptions that a cinematographer follows when setting up a camera shot. We are not searching for a complete description of how the shot is set up, but just for enough information from which to establish a position and orientation for the camera. For example, while we may eventually wish to implement different camera filters for interesting effects, what we care about now is mainly the field-of-view for the camera.

The most important rule of cinematography is called the 180-degree rule: The camera should not cross the line of action. The line of action is an imaginary line that partitions a scene into two distinct areas, usually going through the main actors or in the direction of movement. By not crossing the line between shots, a scene’s screen direction and space is preserved. This idea can be extended to account for the three-dimensional nature of certain games by using a plane to partition space rather than just a line. This information is constant across all or several of the shots in a scene.


Figure 1. Common triangular camera arrangements: (a) standard, (b) over-the-shoulder, (c) point of view, and (d) profile.

Once the line of action has been established, camera placement within the valid area follows what is known as the triangle system. Figure 1 shows the four basic camera positions for two actors: standard, over-the-shoulder, point of view, and profile. Each of these layouts has the same camera at the top point of the triangle, which is used for the establishing (or master) shot. The remaining cameras are meant to favor one actor over another. Figure 1 shows the approximate locations of the cameras on a two-dimensional plane, leaving the need for another parameter to describe the height or vertical angle of the camera.


Figure 2. Visible portion of human actor for various shot sizes.

Two remaining parameters involve the framing of the actor or actors on the screen. The first of these parameters is the shot size, which will determine the distance of the camera from the actor. Figure 2 shows the visible portions for a human actor in the common shot sizes. In addition to these shots, the long shot frames the actor with plenty of space between the edge of the screen and the actor. The other parameter determines the emphasis given to one actor or another, or the approximate screen space taken up by each actor. This information should be sufficient for us to move on to laying out the actual data structure and working out the equations for determining the final position and orientation. From here I’ll assume a basic knowledge of vector math, otherwise, you may wish to refer to Eric Lengyel’s Mathematics for 3D Game Programming and Computer Graphics (Charles River Media, 2002) or other similar graphics text.


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