Efficient Gonio-Spectral Imaging for Diffuse Objects Based on Optical Reflectance Properties

Precise color and texture reproduction based on goniospectral reflectance properties of object is required in many fields such as E-commerce, digital archives, criminal investigation and so on. However, spectral imaging at all geometries requires much measurement tasks and huge data storage. In this paper, efficient gonio-spectral imaging was performed for diffuse objects of Lambertian-like reflection by taking account of its optical reflectance properties. The proposed method assumes relative spectral distribution of reflected light could be regarded being constant, and only the power changes. This is preserved for wide range of geometries, except for neighbor of specular direction. Therefore, gonio-spectral images were created from only one basic spectral image at certain geometry. It was achieved by modulating power of the basic spectral image at each pixel with relative power change between illumination geometries. The relative power change was calculated from grayscale data of image reproduction geometry and that of basic spectral imaging. And also, spatial resolution was enhanced by recording grayscale images at higher resolution than basic spectral image. Experiments were performed to Japanese washi paper and cloth. Basic spectral data was obtained at geometry 45/0, and grayscale images were taken at geometry 81/0 and 87/0. From created spectral images, color images were reproduced under illuminant A and D65. Light-and-shade induced by surface textures were precisely represented together with color information based on spectral data. Good correspondence between appearance of real sample and reproduced images was achieved with smaller number of sampling points and less data amount than the previous method. Introduction Diffuse objects such as papers or cloths are very common materials, and they have deep relation to our daily life. In the fields such as E-commerce and digital archives of electronic museums, it is very important to record the color and material feelings of diffuse objects precisely with digital data, and display them faithfully on the monitor. For the matter of color, a number of studies have been performed extensively, and precise color reproduction have achieved by recording spectral reflectance properties, which is color information peculiar to objects. Additionally, gonio-spectral reflection properties, which are spectral reflection properties concerning illumination and observation geometries, were recorded for faithful reproduction of material feelings of objects together with precise color information. These techniques are expected to be one of the new phases in imaging science of next generation. Generally, enormous memory capacity and high computational costs are required for recording and processing spectral images because of the large data amounts. Problems on explosion of data amount will be critical in case recording gonio reflectance properties also. Therefore efficient methods for gonio-spectral imaging are indispensably required. A lot of studies have been performed on the encoding of the spectral information, most of them based on the expansion of spectral function into a system of orthogonal basis functions that are estimated from a set of typical color spectra by using such as principal component analysis. However concerning the spectral reflectance including gonio reflection properties, few methods were reported yet. In this paper, we propose an efficient method for recording and reproducing gonio-spectral images of diffuse objects. The proposed method is based on optical reflectance properties of diffuse objects and visual properties of human eyes. In the following chapter, IS&T's 2003 PICS Conference