Level of detail (computer graphics)
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.
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.
[citation needed] The origin[1] of all the LOD algorithms for 3D computer graphics can be traced back to an article by James H. Clark in the October 1976 issue of Communications of the ACM.
After introducing some available algorithms for geometry management, it is stated that most fruitful gains came from "...structuring the environments being rendered", allowing to exploit faster transformations and clipping operations.
However, then it might have far more detail than is needed to represent it at large distances, and in a complex environment with many such objects, there would be too many polygons (or other geometric primitives) for the visible surface algorithms to efficiently handle.The proposed algorithm envisions a tree data structure which encodes in its arcs both transformations and transitions to more detailed objects.
The significant point, however, is that in a complex environment, the amount of information presented about the various objects in the environment varies according to the fraction of the field of view occupied by those objects.The paper then introduces clipping (not to be confused with culling although often similar), various considerations on the graphical working set and its impact on performance, interactions between the proposed algorithm and others to improve rendering speed.
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.
This kind of algorithm is usually easier to get working, providing both faster performance and lower CPU usage because of the few operations involved.
OpenGL is used for rendering due to its high efficiency in managing small batches, storing each model in a display list thus avoiding communication overheads.
Because hardware is geared towards large amounts of detail, rendering low polygon objects may score sub-optimal performances.
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.
