Extension of the WPS Model with Hierarchical Scene Representation and Analytic Ray–Primitive Intersection for Native Support of RAMI Actual Scene Descriptions in Radiative Transfer Simulation

Radiative transfer (RT) models of vegetation canopy serve as powerful tools for the quantitative interpretation of remotely sensed signals. Based on the Monte Carlo ray-tracing (MCRT) model, WPS (Weighted Photon Spread), we performed extensions with new functionalities to better adapt the three-dimensional (3D) scenarios in the Radiation Transfer Model Intercomparison (RAMI) exercise. In terms of 3D canopy representation, the two most recent phases of the RAMI exercise (RAMI-IV/V) addressed two features: (1) extensive use of the instancing technique, where an individual component serves as a prototype, and its copies are transformed to generate an entire tree’s foliage; and (2) new scene definitions with more geometric primitives alongside flat objects like disks and triangles, e.g., spheres, cylinders, and cone frusta, and the utilization of transformations of these geometric primitives to describe actual scenes. New extensions were implemented in WPS to represent the 3D canopies with the instancing technique at hierarchical levels and to handle ray intersection for different geometric primitives and their transformed forms. Two new modes were therefore developed: WPS3INS, based on a two-layer axis-aligned bounding-box structure, and WPS3BVH, based on a bounding volume hierarchy that stores each transformation only once. Because spheres, cylinders, and cone frusta are intersected analytically and are never tessellated into triangles, every scene component of the RAMI-V actual scenarios is ray-traced at its native per-primitive fidelity, meaning that no geometric uncertainty of tessellation origin is introduced into the RT simulation. The extensions also deliver large memory savings: for the birch stand winter scene (HET15_JBS_WIN), which contains 138,251,607 primitives, WPS3BVH reduces the runtime memory by a factor of about 57 compared with WPS3. Evaluations through ROMC (Radiation transfer model intercomparison Online Model Checker) show that the accuracy of the new modes for 3D structure representation is within ±1% of the reference solution. Comparisons with the LESS model for selected RAMI-V actual canopy scenarios also show close agreement, with the lowest R2 of 0.999 and highest Relative Root Mean Square Error of ~1.553%. Future work based on the extended WPS model is suggested for the exploration of high divergences of RT simulations from the RAMI-V actual scenarios among the participating MCRT models.