- freely available
ISPRS Int. J. Geo-Inf. 2019, 8(11), 504; https://doi.org/10.3390/ijgi8110504
2. Continuous LOD Data Structures for 3D Building Models
2.1. Essential Features of 3D Building Models
2.2. Representation Method for 3D Building Model
2.2.1. Representation of Triangular Mesh
2.2.2. Orientation of Triangular Mesh
2.3. Detail Transformation of 3D Building Model
2.4. The Continuous LOD Topological Data Structure of A 3D Building Model
- The coordinate value of the prehidden vertex N(vj), and the index of the prehidden vertex;
- Triangular patch pe, pf that is connected to the prehidden vertex, and the indexes of the two triangular patches N(pe) and N(pf);
- The indexes (N(pa), N(pb), N(pc)……) of the noncharacteristic surface, which need to be updated.
3. Continuous LOD Transformation Method for 3D Building Models
3.1. Pretreatment of Characteristic Surface
- Calculating normal vectors:
- Determining coplanar triangles:If cosα = 0, then the two triangles are coplanar.
- Calculating the area of each characteristic surface to determine the moving direction (the total area of the coplanar triangle):
3.2. Simplification of the 3D Building Model
- Confirm the datum level Starget where S▲max is located.
- Calculate the distance di from the center point of Ssurfacei to the datum level, and sort the planes (D1 < D2 < ……Dn) according to the distance, from small to large.
- For each simplification step, only the outermost plane Dn is simplified to the sub-outermost layer Dn−1.
- Until simplified to Starget.
- Set point stack STV and side stack STE to store newly generated points and edges, respectively.
- New vertices A′, B′, C′, and D′ have their coordinate values set to be the projected values that vertices A, B, C, and D project along normal vector to Surface Dn−1.
- New edges A′B′, A′D′, B′C′, and D′C′, and set the default state of bInUse = N.
- Update the use stage of surface Dn by replacing edges AB, AD, BC, and DC (setting edges AB, AD, BC, and DC as the nonuse state and A′B′, A′D′, B′C′, and D′C′ as the use state); then, the surface Dn is simplified to surface Dn−1.
3.3. General Transformation Method Compared with Improved Data Structure Transformation Method
4. Experiments and Analysis
Conflicts of Interest
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|The Original State||Stage I||Stage II||Stage III|
|Total number of patches stored in the list of patches||499,128||607,977||641,402||664,449|
|Time taken for the simplification process from the original model to stage i (i = 1, 2, 3) (without rendering time)||0.51||0.59||0.64|
|Time taken for the reduction process from stage i (i = 1, 2, 3) to the original model (without rendering time)||0.11||0.13||0.15|
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