Level of detail (LoD) simplifies 3D objects viewed from a distance, increasing rendering speed. Discrete LoD uses pre-made models, while continuous LoD dynamically adjusts detail. Other variations include reducing image quality and using geometric primitives.
Level of detail (LoD) in computer graphics refers to the process of simplifying the various graphical aspects of a three-dimensional (3D) object that is viewed from a distance. The purpose of implementing level of detail in a program is that the rendering speed can be increased when there are more objects in the virtual scene and some of them are so far away from the observer that the extraneous details would not be visible anyway. Depending on how exactly the level of detail is integrated into a program, it can be of two types: discrete or continuous. Discreet LoD replaces 3D models or textures with separate, usually pre-made models that have less detail. Continuous LoD uses algorithms to dynamically increase or decrease detail on a model as needed.
Almost all 3D graphics programs use some form of level of detail, especially video games. When implemented correctly, LoD is almost unnoticeable to the viewer. When the calculated distance between an object in the virtual scene and the viewer’s position is greater than a certain number, the level of detail can be reduced. This reduction usually results in fewer polygons making up the 3D object, reduced quality texture images, or a combination of both. The rendering speed of a scene using level of detail is considerably faster than the rendering speed of a scene where objects are constantly at full detail.
Discrete level of detail is a method that uses a certain number of distances to determine which model of an object will be rendered. The different quality levels of the 3D model can be created in a modeling program or calculated when starting the program and then archived for later use. This method works well, because it’s quick and easy. It is also not favored by some programmers and artists because, at times, jumping from one LoD model to another might appear too obvious and suddenly make a larger and more detailed object appear before the viewer.
Continuous level of detail actually uses algorithms to take the polygons that make up an object and subdivide them for more detail, or consolidate faces for less detail. This version of LoD allows for a smooth transition from a single pixel to full detail as the distance is reduced. Algorithms, however, can sometimes be very processor intensive and also have the potential to produce unwanted results, such as missing polygon faces or changes to 3D models that distort the initial geometry.
Other variations of LoD include reducing the image quality of texture mapped images, instead using small images of very low quality, or even simply using a solid color with no texture mapping to approximate the look from a distance. In another method, instead of actually loading a low-quality 3D model for LoD purposes, the remote model is replaced by a geometric primitive such as a sphere or rectangle, which renders much faster than arbitrary polygons. There are other algorithms that can be used for more specific types of LoD calculations, such as those needed to modify terrain meshes and those that can be used to approximate objects moving rapidly through the scene.
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