Practice based comparison of imaging methods for visualization of toolmarks on an Egyptian Scarab

Transcription

1 Department of Civil, Environmental & Geomatic Engineering (CEGE) 3D Imaging, Metrology, Photogrammetry Applied Coordinate Technologies (3DIMPact) ICISP 2014, Cherbourg, France Practice based comparison of imaging methods for visualization of toolmarks on an Egyptian Scarab Lindsay MacDonald 1, Maria Filomena Guerra 2, Ruven Pillay 3, Mona Hess 1, Stephen Quirke 4, Stuart Robson 1, Ali Hosseininaveh Ahmadabadian 1 1 Dept. of Civil, Environmental and Geomatic Engineering, UCL, UK 2 UMR 8096 CNRS, France 3 C2RMF, Musée du Louvre, Paris 4 Institute of Archaeology, University College London

2 Acknowledgements Maria Filomena Guerra CNRS Ruven Pillay C2RMF Mona Hess UCL

3 Analytical study of Bronze Age Egyptian gold jewellery Origin of gold and composition of the alloys for forming, soldering and decoration Main partners CNRS first UMR 8220 now UMR 8096 (Paris) Institute of Archaeology, UCL (London) Partners Petrie Museum (UCL, London) National Museums Scotland (Edinburgh) British Museum (London) Associates Garstang Museum of Archaeology (Liverpool) Louvre Museum (Paris) PICS 5995 CNRS France-UK ( )

4 Egyptian sites Main excavated sites where studied objects were found. Several objects from other origins selected from museum collections for typology and chronology. 120 objects studied by sciencebased techniques: Optical microscopy SEM EDS X Radiography XRF Ion Beam analysis Synchrotron XRF

5 Selected object scarab Scarab of steatite with gold band, underside inscribed in hieroglyphs within scroll border with personal name and title: Estate manager of the granary Iufseneb. Place: Lahun (Faiyum governorate) Middle Egypt Period: Late Middle Kingdom c.1700 BC Materials: steatite, gold Dimensions: 26.7 x 18.6 x 11.4 mm The UCL Petrie Museum of Egyptian Archaeology UC11365 Catalogue Image

6 Photographic views Top Left Nose Bottom Right Tail

7 X ray fluorescence analysis Composition of scarab s gold band was analysed at Petrie Museum with the mobile XRF system of Institute of Archaeology UCL.

8 XRF results on gold band Cu wt% Ag wt% Au wt% Standard 1 reference measured Standard 2 reference measured Standard 3 reference measured Scarab average σ

9 Objectives of 3D imaging study To produce a 3D representation with resolution high enough to visualize tool marks, ~50 µm To display detailed surface geometry in correct colour To evaluate whether transport of imaging equipment to museums abroad is feasible To compare imaging outcomes with established but costly and time intensive analytical methodologies in laboratories

10 Imaging methods studied Arius 3D scanner Photogrammetry SfM #1 Photogrammetry SfM #2 Reflectance transform imaging (RTI) Photometric stereo Specular modelling

11 Arius 3D colour laser scanner Laser wavelengths: nm Laser spot diameter: 80 µm Resolution: point spacing 100 x 100 µm with geometric and colour information (XYZ RGB IJK) Accuracy: single point better than 25 µm

12 3D colour laser scanning 26 scans, with scarab turned to many different positions Point clouds processed in Pointstream software Unified 3D representation with ~402,000 points. Good geometric accuracy but insufficient resolution Colour metameric because of R,G,B laser wavelengths

13 Photogrammetry / VisualSFM PMVS 80 images from Nikon D3200 camera with 105mm macro lens Structure from motion (SfM) image processing pipeline Point clouds cleaned and fused together during post processing Realistic colour and detail, with resolution ~20 pixels/mm Compromised by holes due to specular reflections from gold.

14 Photometric stereo Set of 64 images captured in the UCL illumination dome Nikon D200 camera with 200mm macro lens Image resolution ~65 pixels/mm (15 micron) Normals calculated by bounded regression method Surface reconstructed and exported as 3D point cloud Rendition in 3D did not provide a completely metric image Successful for detail, especially hieroglyphs on underside.

15 Photometric stereo Comparison of point cloud from photometric stereo with point cloud from laser scanner Mean fitting error < 0.25 mm

16 Details in comparison Ability of methods to represent surface detail on object Photogrammetry #2 3D colour laser scanning Photometric stereo

17 Reflectance transform imaging Base image components: Normal vector Albedo Calculate specular quotient at each pixel Ratio of actual intensity/diffuse ~1 for matte areas >>1 for shiny areas Specular vector Specular colour

18 Specular quotient vs radial angle High values near specular peak. Falling to asymptote of 1 with increasing radial angle from peak. Plotted here for 3x3 pixel cell, giving 9x64 = 576 values.

19 Angular distribution functions Various functions for BRDF models in computer graphics. Lorentz function chosen for its broad flanks: 1 1

20 Fitting angular distribution First fit linear flank. Then fit Lorentz function for values above flank. Four parameters in combined model: 1

21 Simulation of original image Diffuse base image plus specular highlights (Lorentz function) Combination gives excellent approximation to original image Illuminated from same hemisphere coordinates as original lamp. Original photograph Simulated image

22 Comparison of imaging methods Technology Colour Specularity Geometry Resolution (detail) Cost Portability Photogrammetry via SfM Medium Poor Good Medium Low Good 3D colour laser scanning Poor Poor Excellent Low High Impossible Photometric stereo + specular model Good Excellent Poor High Medium Poor

23 Conclusions The ideal 3D image acquisition system would have: Geometric accuracy of the laser scanner Convenience and portability of photogrammetry Fine surface detail of photometric stereo Realistic appearance of specular rendering model