Measurement and rendering of complex non-diffuse and goniochromatic packaging materials
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ORIGINAL ARTICLE
Measurement and rendering of complex non-diffuse and goniochromatic packaging materials Aditya Sole1
· Giuseppe Claudio Guarnera1,2
· Ivar Farup1
· Peter Nussbaum1
Accepted: 11 September 2020 © The Author(s) 2020
Abstract Realistic renderings of materials with complex optical properties, such as goniochromatism and non-diffuse reflection, are difficult to achieve. In the context of the print and packaging industries, accurate visualisation of the complex appearance of such materials is a challenge, both for communication and quality control. In this paper, we characterise the bidirectional reflectance of two homogeneous print samples displaying complex optical properties. We demonstrate that in-plane retroreflective measurements from a single input photograph, along with genetic algorithm-based BRDF fitting, allow to estimate an optimal set of parameters for reflectance models, to use for rendering. While such a minimal set of measurements enables visually satisfactory renderings of the measured materials, we show that a few additional photographs lead to more accurate results, in particular, for samples with goniochromatic appearance. Keywords Visual appearance · Goniochromatism · Genetic algorithm-based optimisation · BRDF measurement · Retroreflection
1 Introduction Materials like non-diffuse metallic paints, varnish coatings, and effect paints have complex optical properties that produce fascinating appearance in manufactured products. Metallic paints contain metal flakes, causing the incident light to be specularly reflected. Effect paints are made using thin metal oxide layers on transparent mica platelets [28] and contain pearlescent pigments. The multi-layered structure of pearlescent pigments helps increasing changes in visual appearance of a material, with respect to the incident and viewing directions [28], including angle-dependent spectral reflectance [22]. These materials, often referred as “goniochromatic” [32], are commonly used in the print and packaging industry. Such materials are produced using different printing techniques (e.g. offset, gravure, screen printing [22]) and contribute to some of the main challenges in the printing line, including:
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Aditya Sole [email protected]
1
Norwegian University of Science and Technology, Gjøvik, Norway
2
University of York, York, UK
1. performing fast and easy process control measurements; 2. synthetically reproduce and match visual properties of packaging materials for customer approval and quality control; 3. communicate material appearance across production and quality control departments in a print production line.
To characterise and communicate the visual appearance properties of goniochromatic print and packaging materials during print production, typically bidirectional reflectance distribution function (BRDF) measurements are required [10,32]. Commercially available devices, such as multi-angle spectrophotometers and goniospectrophotometers, could be used to perform such bidirectional measurements [22]. However, su
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