Blurred Areas in Very Short Range Photogrammetry: A Flexible Alternative to Focus-Stacking. Case Study of a Complex-Geometry Ornament of Strasbourg’s Cathedral, France

Abstract. Cultural Heritage documentation of small artifacts with complex geometry processed in photogrammetry has to deal with narrow depth-of-field inherent to optics. The focus-stacking method is traditionally employed to overcome this issue, but has to be deployed on a stable environment in terms of movement and light. It requires a very large number of photographs which are modified in colors and geometries, depending on focus-stacking algorithms, to be able to generate the focus-stacked images needed by Structure from Motion (SfM) processes. This article proposes a more flexible method, suitable for unmovable artifacts or acquisitions in the field where tripods for lights and cameras are excluded. The proposed method uses multiple single-shot photographs, as in traditional photogrammetry of larger items, improved with paired masks on areas suffering from optical blur. These masks are automatically generated using Laplacian Pyramid after an operator reviews the settings to fit the full set of the acquisition pictures. The case study is a Strasbourg’s Cathedral ornament carved in red sandstone in the Flamboyant Gothic style. This delicate sculpture presents a complex geometry with recesses and protrusions. Three processes leading to photogrammetric 3D dense clouds are described and evaluated: the proposed method with masks, the same method but without masks and a focus-stacking approach. These models are compared with a reference acquired using a metrology laser scanner, demonstrating the benefits of masking blur for accuracy and noise. When assessed relative to the focus-stacking approach, the proposed method with masks achieves similar accuracy while offering greater speed and adaptability.