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Tuesday, March 27 • 2:00pm - 6:15pm
1 - 3D Representations from Reflectance Transformation Image Datasets: A case study from early Egypt

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We present a visualisation technique which uses the set of digital images captured in an illumination dome for the construction of 3D representations of material surfaces. In conventional practice (Mudge et al., 2005) the images are processed by the reflectance transformation imaging (RTI) technique to generate a file that represents the variation in intensity when the surface is illuminated from any direction. This enables interactive display of the object as if illuminated by a 'virtual torch' under control of the observer, and provides a useful means of enhancing the contrast of surface relief. We have developed an alternative means of processing the image sets, using the photometric stereo technique (MacDonald, 2011) to extract surface normals. The method achieves a higher angular resolution than in the normals derived from the RTI representation (Malzbender et al., 2001) by employing many triplet combinations of the lamps in the dome. From these the height of each point may be accurately reconstructed, producing a digital terrain model of the surface. The method will be illustrated through a case study on the early Egyptian (c. 3100 BCE) Hunters Palette, a shield-shaped slab of mudstone elaborated with relief-carved figures of humans and animals (Petrie, 1953). Smaller less-elaborate mudstone palettes were used to grind minerals such a malachite and galena, perhaps for cosmetic purposes. The Hunters Palette lacks evidence for mineral grinding, but its surface bears a range of other marks which can aid our understanding of its 'life history'. Such surface details are difficult to discern in conventional photographs and are often ignored in line drawings, but with the aid of RTI images (acquired as part of the AHRC-funded RTISAD project, see Earl et al. 2011), the finer details of surface topography can be visualised in striking detail. RTI analysis is already providing new insight into the manufacture process and the embodied actions and habits of the artisan(s) who created the scenes, from the direction, sequence, and depth of carving to possible tool types and techniques (see Piquette forthcoming). One area of the Hunters Palette in particular shows intriguing evidence for recarving, where it seems that the position of the rope may have been altered. Making sense of these surface transformations provides challenges which can be overcome by deploying RTI capture data in new ways. We extracted surface normals using 36 of the 76 images in the set taken in the dome, corresponding to all LED lights of zenith angle greater than 22° to avoid self-shadowing of the object surface. From the normals the surface gradients (partial derivatives in X and Y directions) were calculated and the height reconstructed. Cross-sectional profiles through the digital terrain map show clearly the depth of the carving with an overall maximum peak-to-trough amplitude of approximately 9 mm, and fine details of 1 mm in depth, as in the pleats of the hunter's kilt. The reduction of height in the stone above the hunter's back is clear in the vertical section, suggesting reworking.



Tuesday March 27, 2012 2:00pm - 6:15pm
Building 65, 1177 Streamed into room 1095

Attendees (12)