3D модельдердің көрсетілу деңгейін реттеу

Adjusting the complexity of a 3D model representation to save storage and computation
In computer graphics, level of detail (LOD) refers to the complexity of a 3D model representation. LOD can be decreased as the model moves away from the viewer or according to other metrics such as object importance, viewpoint relative speed or position. LOD techniques increase the efficiency of rendering by decreasing the workload on graphics pipeline stages, usually vertex transformations. The reduced visual quality of the model is often unnoticed because of the small effect on object appearance when distant or moving fast. Although most of the time LOD is applied to geometry detail only, the basic concept can be generalized. Recently, LOD techniques also included shader management to keep control of pixel complexity. A form of level of detail management has been applied to texture maps for years, under the name of mipmapping, also providing higher rendering quality. It is commonplace to say that "an object has been LOD ed" when the object is simplified by the underlying LOD ing algorithm as well as a 3D modeler manually creating LOD models.

Although the algorithm introduced above covers a whole range of level of detail management techniques, real world applications usually employ specialized methods tailored to the information being rendered. Depending on the requirements of the situation, two main methods are used:

The first method, Discrete Levels of Detail (DLOD), involves creating multiple, discrete versions of the original geometry with decreased levels of geometric detail. At runtime, the full detail models are substituted for the models with reduced detail as necessary. Due to the discrete nature of the levels, there may be visual popping when one model is exchanged for another. This may be mitigated by alpha blending or morphing between states during the transition. The second method, Continuous Levels of Detail (CLOD), uses a structure which contains a continuously variable spectrum of geometric detail. The structure can then be probed to smoothly choose the appropriate level of detail required for the situation. A significant advantage of this technique is the ability to locally vary the detail; for instance, the side of a large object nearer to the view may be presented in high detail, while simultaneously reducing the detail on its distant side. In both cases, LODs are chosen based on some heuristic which is used to judge how much detail is being lost by the reduction in detail, such as by evaluation of the LOD's geometric error relative to the full detail model. Objects are then displayed with the minimum amount of detail required to satisfy the heuristic, which is designed to minimize geometric detail as much as possible to maximize performance while maintaining an acceptable level of visual quality.

The basic concept of discrete LOD (DLOD) is to provide various models to represent the same object. Obtaining those models requires an external algorithm which is often non trivial and subject of many polygon reduction techniques. Successive LOD ing algorithms will simply assume those models are available. DLOD algorithms are often used in performance intensive applications with small data sets which can easily fit in memory. Although out of core algorithms could be used, the information granularity is not well suited to this kind of application. This kind of algorithm is usually easier to get working, providing both faster performance and lower CPU usage because of the few operations involved. DLOD methods are often used for "stand alone" moving objects, possibly including complex animation methods. A different approach is used for geomipmapping, a popular terrain rendering algorithm because this applies to terrain meshes which are both graphically and topologically different from "object" meshes. Instead of computing an error and simplify the mesh according to this, geomipmapping takes a fixed reduction method, evaluates the error introduced and computes a distance at which the error is acceptable. Although straightforward, the algorithm provides decent performance.

Because hardware is geared towards large amounts of detail, rendering low polygon objects may score sub optimal performances. HLOD avoids the problem by grouping different objects together. This allows for higher efficiency as well as taking advantage of proximity considerations.

LOD is especially useful in 3D video games. Video game developers want to provide players with large worlds but are always constrained by hardware, frame rate and the real time nature of video game graphics. With the advent of 3D games in the 1990s, a lot of video games simply did not render distant structures or objects. Only nearby objects would be rendered and more distant parts would gradually fade, essentially implementing distance fog. Video games using LOD rendering avoid this fog effect and can render larger areas. Some notable early examples of LOD rendering in 3D video games include The Killing Cloud, Spyro the Dragon, Crash Bandicoot: Warped, Unreal Tournament and the Serious Sam engine. Most modern 3D games use a combination of LOD rendering techniques, using different models for large structures and distance culling for environment details like grass and trees. The effect is sometimes still noticeable, for example when the player character flies over the virtual terrain or uses a sniper scope for long distance viewing. Especially grass and foliage will seem to pop up when getting closer, also known as foliage culling. LOD can also be used to render fractal terrain in real time. Unreal Engine 5's Nanite system essentially implements level of detail within meshes instead of just objects as a whole.

LOD is found in GIS and 3D city models as a similar concept. It indicates how thoroughly real world features have been mapped and how much the model adheres to its real world counterpart. Besides the geometric complexity, other metrics such as spatio semantic coherence, resolution of the texture and attributes can be considered in the LOD of a model. The standard CityGML contains one of the most prominent LOD categorizations. The analogy of "LOD ing" in GIS is referred to as generalization.

MeshLab an open source mesh processing tool that is able to accurately simplify 3D polygonal meshes. Polygon Cruncher a commercial software from Mootools that reduces the number of polygons of objects without changing their appearance. Simplygon a commercial mesh processing package for remeshing general input meshes into real time renderable meshes.