Background and Related Work

Abstract

In computer graphics, the View Frustum determines which parts of the scene are drawn on the screen. The View Frustum is an area that is defined by 6 planes. These planes represent the borders of what is visible on the screen defined by the camera. They constrain the top, bottom, left, right, front and back of the viewing area. The most important to us the back view plane. Unlike the top, bottom, left and right frustum planes, the back plane actually removes geometry from the scene that would otherwise be visible. As the camera moves through the scene, geometry in the background will suddenly appear and disappear as it moves across the back view plane. The distance between the front and back view planes determines the depth of scene. The front view plane is usually fixed very close to the camera position. This means that the depth is determined by how far the back plane is placed away from the camera. Ideally we would like to place the back view plane infinitely far away from the camera so that no background geometry is removed from the screen. However, there are two limitations that prevent this. The first is the limitation of the Z-buffer. The Z-buffer is an area of video memory that stores the depths at which polygons are drawn for every pixel. When polygons are drawn in the scene, their depth is calculated for every pixel they will occupy. If the depth recorded in the depth buffer is closer to the camera than the current polygon at that pixel, it will not be drawn. This allows graphics application writers to not worry about the order in which they decide to draw objects in the scene. This also allows geometry to intersect while preserving the proper depth ordering. The Z-buffer can only hold a finite number of values. In any given scene, there is a range of possible distances from the camera that can map to the same Z-buffer value. This is not an issue in many graphics scenes because the depth of the scene is not very large and even very close objects are rendered properly. However, as the back plane is pushed away from the camera, the range of values mapping to the same Z-buffer value increases. The Z-buffer precision cannot change per application as it is implemented in hardware. If two differently colored objects are drawn to the same depth in the Z-buffer, the hardware may choose which object to draw per pixel in an undefined way.