Image Analysis and Synthesis Using Physics-Based Modeling for Pearl Quality Evaluation System

Analysis by image synthesis using CG has attracted wide attention in machine vision. This paper proposes a method of modeling and synthesizing pearls that will be the central technique of a pearl quality evaluation system. Pearls manifest a specific optical phenomenon that is not dependent on the direction of the light source. To investigate this feature, we propose a physical model for multilayer film interference called an "illuminant model." The synthesis algorithm has been configured from such representations of physical characteristics as interference, mirroring and texture which correspond to the main evaluation factors obtained from human experts. Further, portions of photos of real pearls and the synthesized images were analyzed based on a scale of psychological evaluation of"pearl-like quality" demonstrating thereby that the generated images can present such a pearl-like quality. 1 I n t r o d u c t i o n Image synthesis using computer graphics has recently come to be used in machine vision to enhance inspection systems. This approach is an analysis by synthesis method which is employed to find the optimum inspection conditions or inspection criteria through the simulation of the item for inspection. It is considered to be an important technology which will meet the needs to upgrade inspection systems and improving their accuracy. In developing the pearl quality evaluation system, the authors have so far made various analytical approaches [ 1,2], and succeeded in deriving a relationship between the physical information regarding pearls and their evaluation by human experts. We have this time studied a synthetic approach which can form a virtual pearl sample in order to verify and correct the analytical results shown in Fig. 1. Pearls are widely known and remarkably popular as jewelry items. They also have the specific optical and structural features given below [3]. • A pearl has a lustrous iridescence with its multilayer, thin-film structure, due to its diverse optical behavior such as interference and multiple reflection. Above all, the phenomenon of the hue distribution of the interference of light is characteristic. ° A pearl is a natural substance whose film thickness and surface roughness are nonuniform indicating natural irregularities and fluctuations. The modeling and synthesis of a pearl, a substance interesting from the optical point of view as was seen above, is, therefore, a worthwhile topic for investigation. Several studies have so far been made concerning the modeling of light behavior [4]. However little attention has been paid to multilayer, thin-film interference. Moreover, an investigation of the optical phenomenon of pearl mica paint has been reported [5], however, no such report has ever been made on the pearl.