--- title: "LOD工具调研" author: "Perrin Yong" author_profile: https://www.pystone.net/profile/ published_by: "Perrin Yong" canonical: https://www.pystone.net/notes/lod-tools-survey/ type: note content_role: unspecified visibility: public id_stability: rename-stable source_path: "10-计算机、信息技术与工程/05-游戏图形与运行时/渲染基础/LOD工具调研.md" content_hash: f393f02ce08ab129bfbbff2938110700a0de52b5304d6fa67e54dbbcabf6e65e knowledge_version: 224c990773de.5fa8af6e39fa site_commit: 224c990773de166d23a886306577dd90379529ce notes_commit: 5fa8af6e39fa3891d1b9b4832bfa6c4e0ecaaf0a --- # LOD工具调研 > 创建时间：2021/1/28 14:13 * LOD工具调研 * LOD Introduction * generalized * Methods * Discrete Levels of Detail (DLOD) * Continuous Levels of Detail (CLOD) * HLOD (Hierarchical LOD) * Applications * LOD in Unity * LOD for shaders * LOD for meshes * workflow * 带有LOD的模型 * 不带LOD的模型 * Project-wide LOD settings * Smooth transitions between LOD levels * LOD Tools * overview * Unity AutoLOD * 发现的问题 * Ref ## LOD Introduction 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**. ### generalized Although most of the time LOD is applied to **geometry detail** , 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’d” when the object is simplified by the underlying LOD-ing algorithm. ### Methods #### Discrete Levels of Detail (DLOD) **Provide various models to represent the same object.** Creating multiple, discrete versions of the original geometry. At runtime, the full-detail models are substituted for the models with reduced detail as necessary. **Using less detailed versions of your models that are viewed when they’re further away from the camera**. Due to the discrete nature of the levels, there may be visual **popping** (sometimes called switchover) when one model is exchanged for another. This may be mitigated by **alpha blending** or **morphing** between states during the transition. #### 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. #### HLOD (Hierarchical LOD) grouping different objects together ### Applications > * **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. > > * GIS(geographic information system) and 3D city models > > * Rendering and modeling software > > ## LOD in Unity ### LOD for shaders You can assign a LOD (level of detail) value to a SubShader. This value indicates how computationally demanding its shader programs are. At runtime, you can tell Unity to exclude SubShaders above a given LOD value, so that Unity falls back to using SubShaders that have a lower LOD value. Unity does not calculate SubShader LOD automatically, and this feature does not relate to distance from the Camera. You must set the maximum LOD manually. Shader.maximumLOD Shader.globalMaximumLOD ### LOD for meshes The farther a GameObject is from the Camera, the lower-detail LOD level Unity renders. This technique reduces the load on the hardware for these distant GameObjects, and can therefore improve rendering performance. > For the very lowest level of detail, you can use a Billboard Asset, which Unity displays instead of a 3D mesh. #### workflow ##### 带有LOD的模型 * 制作模型时，LOD命名规范，Unity会自动识别LOD，添加LOD Component ![Alt text](/media/e5f80c13a0e55415dc05.png) ![Alt text](/media/b522ce3f2648de370bea.png) ![Alt text](/media/15a126ce1dba7f1296bb.png) ![Alt text](/media/588990a823461e71e453.png) > 如果制作时，多个LOD的坐标相同，在Unity当中直接就可以使用。如果多个LOD坐标不同，需要调整LOD的坐标 * 命名不规范，无法识别LOD - manually create a GameObject with a LOD Group component, and configure the LOD levels manually. ##### 不带LOD的模型 * AutoLOD，勾选了Generate on import后，在导入模型时，会将fbx中的Mesh都视为LOD0，自动生成LOD1和LOD2 的Mesh，生成lod资源，为FBX模型添加LOD Group组件，放入场景中就是带有LOD的模型。 #### Project-wide LOD settings Maximum LOD Level: Exclude meshes above a specified LOD level from your build. LOD Bias: Determine whether to favor higher or lower LOD levels at threshold distances. #### Smooth transitions between LOD levels Smooth transitions take place inside transition zones, where Unity renders both the current and next LOD levels separately, and then cross-fades them together. **Cross-fading** is the technique of rendering two levels at the same time, with a weighting of 1 to 0 for the current LOD level and 0 to 1 for the next LOD level: ![Alt text](/media/795f2746fc0b3706276e.png) ## LOD Tools ### overview * 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. ### Unity AutoLOD ### 发现的问题 AutoLOD会把生成的Mesh的顶点的格式自动变为UInt32，该格式的Mesh无法进行动态合批，会增加DrawCall。 ## Ref