Proposal of Radiometric Correction for the In-Situ Modelling of Polychrome Religious Carvings

Abstract. Digitalizing cultural heritage demands accurate 3D models for documentation, conservation, and restoration, focusing on both geometry and texture. For medium-sized objects, 3D scanning commonly provides accurate geometry, while photogrammetry excels at capturing texture. Therefore, a hybrid workflow is often used. However, on-site acquisition of complex objects, such as the polychrome 'Saint Catherine of Alexandria' sculpture in Jaén, Spain, presents significant radiometric challenges due to variable lighting and shiny surfaces. This study details a two-phase methodology to create a photorealistic 3D model of this carving. The first phase involved capturing geometry with a structured light scanner and acquiring photogrammetric images using a conventional camera. These datasets were then fused, combining the 3D scan and oriented photogrammetric block to obtain the initial 3D model. Additionally, a colorimeter simultaneously measured true colour values of seven distinct chromatic segments (e.g., gold tunic, face, shoes) to address the carving's challenging reflectivity. The second phase focused on radiometric correction. Images were segmented using a divided 3D mesh and depth maps generated for each segment. Each segment's RGB values were then adjusted to match the colorimeter's average reference value for that specific segment. This zonal correction strategy ensured colour homogeneity. The resulting 3D model, textured with these corrected images, showed a significant improvement in colour realism. The average RGB distance between colorimeter measurements and the model's texture was substantially reduced using this approach. This preliminary study demonstrates the potential and robustness of this method for achieving accurate colour fidelity in 3D models, even under challenging on-site conditions.