Bidirectional Reflectance Measurement and Reflection Model Fitting of Complex Materials Using an Image-Based Measurement Setup

Abstract

Materials with a complex visual appearance, like goniochromatic or non-diffuse, are widely used for the packaging industry. Measuring optical properties of such materials requires a bidirectional approach, and therefore, it is difficult and time consuming to characterize such a material. We investigate the suitability of using an image-based measurement setup to measure materials with a complex visual appearance and model them using two well-established reflection models, Cook–Torrance and isotropic Ward. It was learned that the complex materials typically used in the print and packaging industry, similar to the ones used in this paper, can be measured bidirectionally using our measurement setup, but with a noticeable error. Furthermore, the performance of the reflection models used in this paper shows big errors colorimetrically, especially for the goniochromatic material measured.

Measurement and rendering of complex non-diffuse and goniochromatic packaging materials

“…Ngan et al [35] presented an image-based measurements to measure anisotropic materials like velvet by wrapping it around a cylinder in different orientations. A conceptually similar setup was used in [44] to investigate the suitability of image-based measurements to estimate the reflectance of isotropic packaging materials, represented using two wellknown reflectance models (Cook-Torrance [7] and Ward [50]). Three flexible packaging materials, with different optical properties ranging from fairly diffuse to goniochromatic, were measured using both their setup and a goniospectrophotometer; both their setup and the goniospectrophotometer cannot measure retro-reflectance.…”

“…Their results show a large relative error, in particular, for the goniochromatic print sample, due to the optimisation algorithm often converging to local minima. In [44], both BRDF models used have 3 free parameters (enforced in the Cook-Torrance model by replacing the Fresnel term with a constant). Phenomenological and physically based models with a higher, yet reasonable amount of free parameters, could provide better generalisation properties for packaging materials.…”

“…2. The gold sample is a metallic gold thin cardboard commonly used for decorative purposes in print and packaging industry, while the blue-green sample is a packaging paper printed using effect Colour shift obtained from the spectral radiance factor measurements of the blue-green sample surface when measured at different viewing directions [44] pigments and varnish coatings. Both samples are non-diffuse with the blue-green sample also being goniochromatic.…”

“…Print samples, such as the ones used in [44], show non-diffuse and goniochromatic properties which are challenging to visualise. Due to these complex optical properties, we test two different fitting metrics.…”

On the Acquisition and Reproduction of Material Appearance

“…Further, we have investigated the suitability of using such a measurement setup to measure materials with a complex visual appearance. These complex materials showed non-diffuse and gonio-chromatic properties which were difficult to model using well established reflection models [35].…”