Realistic Visualisation of Cultural Heritage Objects
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Realistic Visualisation of Cultural Heritage Objects Lindsay William MacDonald 3DIMPact Research Group Department of Civil, Environmental and Geomatic Engineering University College London Supervisor: Professor Stuart Robson Thesis for the Degree of Doctor of Philosophy (Ph.D.) October 2014 Revised April 2015 Declaration of Authorship I, Lindsay William MacDonald, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Signed ………………..…………………………. Date ……October 2014.…..…………..…….. Acknowledgements This has been a long journey, after eight years of study (part-time). The research was informed by my two careers, the first for 20 years in industrial R&D and the second for 15 years in academia. I acknowledge the benign and liberal spirit of Jeremy Bentham, who permeates the whole institution of UCL and enlivens inter-disciplinary collaboration. I am deeply indebted to my supervisor, Professor Stuart Robson, first for taking me on as a mature student and then for his infinite patience, encouragement and friendly guidance throughout. I have also benefitted greatly from discussions, interactions and practical assistance from other students in my cohort, notably Mona Hess, John Hindmarch and Ali Hosseininaveh Ahmadabadian. Most important of all has been the unwavering support from my wife Sandra, without whose positive attitude this extensive project would probably never have been completed. Set in 11-point Gill Sans MT PhD Thesis, October 2014 – Lindsay W. MacDonald, Faculty of Engineering, UCL Realistic Visualisation of Cultural Heritage Objects Abstract Digital imaging finds its perfect application in the domain of cultural heritage, enabling objects to be captured, rendered and displayed in many ways. It facilitates public access to objects that would otherwise be too rare, valuable or fragile to be widely seen, and it enables conservators and palæographers literally to see the objects in a new light. A key issue for reproductions, both digital and physical, of cultural heritage objects is how realistic they should be. There are broadly two modes of reproduction – accurate and pleasing – and there are deep implications of this dichotomy for the digitising and rendering of objects in museum collections. For documentation and recording purposes, metric accuracy is important, and ideally the archival digital dataset should contain all the information of every aspect of the actual object. For presenting the object through digital media, such as displays in computers and mobile devices, however, the user experience depends on ease of access, interactivity, quality of annotation, as well as an attractive appearance. What matters for realistic visualisation is to convey the illusion of looking at a real 3D object, rather than the accuracy of the underlying representation. This research programme was motivated by a desire to understand the limits of quality that could be achieved in the digitisation of 3D objects and their visual rendering on displays. Attributes of interest included colour, tonal range, surface finish, spatial resolution and dimensionality. Much of the investigation stemmed from photography in a hemispherical dome, enabling a set of 64 photographic images of an object to be captured in perfect pixel register, with each image illuminated from a different direction. This representation turns out to be much richer than a single 2D image, because it contains information at each point about both the 3D shape of the surface (gradient and local curvature) and the directionality of reflectance (gloss and specularity). So it enables not only interactive visualisation through viewer software, giving the illusion of 3D, but also the reconstruction of an actual 3D surface and highly realistic rendering of a wide range of materials. The following seven outcomes of the research are claimed as novel and therefore as representing contributions to knowledge in the field: A method for determining the geometry of an illumination dome; An adaptive method for finding surface normals by bounded regression; Generating 3D surfaces from photometric stereo; Relationship between surface normals and specular angles; Modelling surface specularity by a modified Lorentzian function; Determining the optimal wavelengths of colour laser scanners; Characterising colour devices by synthetic reflectance spectra. Twenty-one papers relevant to the topic have been written and presented at international conferences (all published in the conference proceedings) since 2009, during the course of the research leading to this thesis. PhD Thesis, October 2014 – Lindsay W. MacDonald, Faculty of Engineering, UCL Ithaca When you start on your journey to Ithaca, then pray that the road is long, full of adventure, full of knowledge. Do not fear the Lestrygonians and the Cyclopes and the angry Poseidon. You will never meet such as these on your path, if your thoughts remain lofty, if a fine emotion touches your body and your spirit. You will never meet the Lestrygonians, the Cyclopes and the fierce Poseidon, if you do not carry them within your soul, if your soul does not raise them up before you. Then pray that the road is long. That the summer mornings are many, that you will enter ports seen for the first time with such pleasure, with such joy! Stop at Phoenician markets, and purchase fine merchandise, mother-of-pearl and corals, amber and ebony, and pleasurable perfumes of all kinds, buy as many pleasurable perfumes as you can; visit hosts of Egyptian cities, to learn and learn from those who have knowledge. Always keep Ithaca fixed in your mind. To arrive there is your ultimate goal. But do not hurry the voyage at all. It is better to let it last for long years; and even to anchor at the isle when you are old, rich with all that you have gained on the way, not expecting that Ithaca will offer you riches. Ithaca has given you the beautiful voyage. Without her you would never have taken the road. But she has nothing more to give you. And if you find her poor, Ithaca has not defrauded you. With the great wisdom you have gained, with so much experience, you must surely have understood by then what Ithacas mean. – C.P. Cavafy (1911) Collected Poems, tr. Rae Dalven PhD Thesis, October 2014 – Lindsay W. MacDonald, Faculty of Engineering, UCL Realistic Visualisation of Cultural Heritage Objects Contents Abstract Contents Chapter 1 – Introduction 1 1.1 Objectives of Research 2 1.2 Context of Research Programme 2 1.3 Contributions to Knowledge 3 1.4 Organisation of this Thesis 4 1.5 Related Publications 4 Chapter 2 – Literature Review 7 2.1 Photogrammetry 7 2.1.1 Principles 7 2.1.2 Software Utilities 9 2.2 Reflectance Transform Imaging 11 2.2.1 Polynomial Texture Mapping (PTM) 11 2.2.2 Implementation of PTM 12 2.2.3 Hemispherical Harmonics 15 2.2.4 Highlight RTI 17 2.2.5 Related Studies 20 2.3 Photometric Stereo 22 2.3.1 Surface Normals 22 2.3.2 Shape from Shading 23 2.3.3 Surface Height Reconstruction 26 2.4 Specular Reflectance 31 2.4.1 Reflectance Models 31 2.4.2 Gloss 35 2.4.3 Bidirectional Reflectance Distribution Function (BRDF) 37 2.4.4 Specular Distribution Models 38 2.5 Colour Specification 40 2.5.1 The CIE System 40 2.5.2 Colour Measurement 45 2.5.3 Measurement of Angular Reflectance 47 2.5.4 Imaging Techniques for Measurement of Reflectance 48 2.5.5 Metrology 50 PhD Thesis, October 2014 – Lindsay W. MacDonald, Faculty of Engineering, UCL Realistic Visualisation of Cultural Heritage Objects 2.5.6 Spectral Sampling and Principal Components 52 2.5.7 Spectral Sensitivity 54 2.5.8 Colorimetric Characterisation 57 Chapter 3 – Dome Metrology and Calibration 3.1 The Illumination Dome 61 3.2 Shadow Casting 63 3.3 Photogrammetry of Retroreflective Targets 65 3.4 Photogrammetry using Flash Lamps as Targets 69 3.5 Comparison of Lamp Coordinate Methods 71 3.5.1 Pin Shadows vs VMS Targets 71 3.5.2 VMS Targets vs Flashes 72 3.5.3 Discussion 73 3.6 Targets on Baseboard 75 3.7 Reflections off Glass Sheet 80 3.8 Reflections off Billiard Ball 83 3.8.1 Outline Detection 83 3.8.2 Highlight Detection 84 3.8.3 Horizon Effect 84 3.8.3 Locating Ball Centre 86 3.8.5 Determining Lamp Coordinates 88 3.9 Analysis of Errors 90 3.9.1 Glass Sheet 90 3.9.2 Billiard Ball 91 3.9.3 Summary 93 3.9.4 Finding Lamp Coordinates in RTI 95 3.10 Conclusion 96 Chapter 4 – Reflectance Transform Imaging 97 4.1 Image Acquisition 98 4.1.1 Image Processing Workflow 98 4.1.2 Pixel Intensity Vectors 99 4.1.3 Vector Projection 100 4.1.4 Hemispherical Projection 100 4.1.5 Polar Projection 100 4.1.6 Making an Outline Mask 101 4.2 Polynomial Texture Mapping 103 PhD Thesis, October 2014 – Lindsay W. MacDonald, Faculty of Engineering, UCL Contents 4.3 Extensions of PTM 104 4.3.1 Separating and Combining PTM Files 104 4.3.2 Extracting Normals from PTM 105 4.3.3 Image Detail Enhancement 107 4.4 Hemispherical Harmonics (HSH) 108 4.4.1 Basis Functions 108 4.4.2 Fitted HSH Components 109 4.4.3 Quality of Fit 110 4.4.4 Angular Resolution 111 4.4.5 Deriving Normals from HSH 112 4.4.6 Comparing Normals with Arius 3D Geometry 113 4.5 Conclusion 114 Chapter 5 – 3D Surface Reconstruction 5.1 Lambertian Surfaces 117 5.2 Effect of Specularity 119 5.3 Verifying Normals with Digital Test Objects 124 5.3.1 Image Noise 124 5.3.2 Square Pyramid 125 5.3.3 Hemi-ellipsoid 129 5.4 Height Reconstruction by Integration of Gradients 131 5.5 Height Reconstruction with Reference Points 135 5.6 Height Reconstruction by Fourier Transform 138 5.7 Height Reconstruction with Surface from a 3D Scanner 144 5.8 Process Flow for 3D Reconstruction 156 Chapter 6 – Specularity 6.1 Contrast of Specular Surfaces 157 6.1.1 Specular Highlight for one Light Source 158 6.1.2 Intensity Distribution in Multi-light Image Sets 159 6.2 Determining Specular Angle 162 6.3 Modelling Angular Distribution 164 6.3.1 Lorentzian Function 166 6.3.2 Specular Quotient Distributions 167 6.3.3 Cluster Lorentzian Model 170 6.3.4 Power Lorentzian Model 174 6.3.5 Comparison of Lorentzian Models 178 PhD Thesis, October 2014 – Lindsay W.