Abstract
Abstract. As a rule, image-based documentation of cultural heritage relies today on ordinary digital cameras and commercial software. As such projects often involve researchers not familiar with photogrammetry, the question of camera calibration is important. Freely available open-source user-friendly software for automatic camera calibration, often based on simple 2D chess-board patterns, are an answer to the demand for simplicity and automation. However, such tools cannot respond to all requirements met in cultural heritage conservation regarding possible imaging distances and focal lengths. Here we investigate the practical possibility of camera calibration from unknown planar objects, i.e. any planar surface with adequate texture; we have focused on the example of urban walls covered with graffiti. Images are connected pair-wise with inter-image homographies, which are estimated automatically through a RANSAC-based approach after extracting and matching interest points with the SIFT operator. All valid points are identified on all images on which they appear. Provided that the image set includes a "fronto-parallel" view, inter-image homographies with this image are regarded as emulations of image-to-world homographies and allow computing initial estimates for the interior and exterior orientation elements. Following this initialization step, the estimates are introduced into a final self-calibrating bundle adjustment. Measures are taken to discard unsuitable images and verify object planarity. Results from practical experimentation indicate that this method may produce satisfactory results. The authors intend to incorporate the described approach into their freely available user-friendly software tool, which relies on chess-boards, to assist non-experts in their projects with image-based approaches.
Highlights
Recent CIPA symposia are witness to the importance of imagebased digital documentation, including both reconstruction and visualization, of cultural heritage. All such projects rely today on un-calibrated digital cameras. Their images are, usually, processed with some commercial software, handled more than often by experts in other fields – architects, engineers, archaeologists or conservationists – who may not be familiar with photogrammetry
It is not surprising that the issue of camera calibration has been gaining in significance
Perhaps the simplest among them is self-calibration based on recording unknown textured planar objects in several views
Summary
Recent CIPA symposia are witness to the importance of imagebased digital documentation, including both reconstruction and visualization, of cultural heritage. A response to this demand is represented by freely available user-friendly tools for automatic camera calibration, which are usually based on structured 2D patterns (typically of the chess-board type); besides low cost and ease in construction, their contrast and pattern are suitable for automatic feature extraction and, establishment of point correspondences Such tools, by exploiting different views of chess-board patterns to determine interior and exterior camera parameters, have been originally inspired by the “plane-based calibration” approach (Sturm & Maybank, 1999; Zhang, 1999), a process relying on the homographies between a world plane with known metric structure and its views. The authors’ intention is to incorporate this approach into their freely available user-friendly software tool to assist non-experts in photogrammetry in projects in the field of cultural heritage documentation
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