A Low-Cost Shape-from-Focus Workflow for 3D Geometric Morphometrics: Proof of Concept Using Trypoxylus dichotomus (Linnaeus, 1771) (Coleoptera: Scarabaeidae) Genitalia

Abstract

Two-dimensional (2D) photography is widely used in geometric morphometrics (GM), but loss of information and orientation errors remain concerns in volumetric structures. Three-dimensional (3D) imaging provides more accurate shape representations for such structures, but it typically requires expensive equipment. We developed a low-cost automated workflow based on a shape-from-focus (SFF) algorithm to reconstruct partial 3D surfaces from uniformly stepped focus stacks. As a proof of concept, we applied this method to the male genitalia of Trypoxylus dichotomus (Linnaeus, 1771) (Coleoptera: Scarabaeidae), comparing 2D and 3D landmark analyses. The reconstructed 3D models yielded reliable regions for landmark and semilandmark placement. Both 2D and 3D analyses revealed consistent hypoallometric scaling, but the choice of size proxy (3D centroid size versus linear length) had a greater impact on allometric inference than the dimensionality of landmarks. The SFF workflow provides a rapid and inexpensive alternative for generating 3D models suitable for GM, and highlights the importance of validating 2D measures against centroid-based proxies.