Myoung Kook Seo

Acquisition and representation of pearlescent paints using an image-based goniospectrophotometer

The use of pearlescent paints has grown significantly for many industrial products, due to their special visual effect, which originates from optical interference between many small pearlescent platelets. This makes the visual appearance of pearlescent paints vary with the light and view direction. Since pearlescent paints are very sensitive to specific wavelength bands, multispectrum-based representation methods that use spectral distributions give us more accurate image synthesis of them than does the use of RGB-based ones. In this paper, we present a novel image-based goniospectrophotometer system and its characterization method to acquire the spectral bidirectional reflectance distribution functions (BRDFs) for realistic image synthesis of pearlescent paints by combining two promising technologies, namely, high-dynamic-range images and multispectral images. The capability of our system is demonstrated by generating rendering results from four different material samples and comparing them with the RGB-based results and fully measured BRDF data.

Efficient representation of bidirectional reflectance distribution functions for metallic paints considering manufacturing parameters

Metallic paint has been widely used to provide for special vi- sual appearance to various products so that photorealistic rendering of this material has been an important issue in the development of new products. We introduce a new approach that predicts the reflectance of metallic paint while considering manufacturing parameters. Our main idea is to simulate the appearance of various metallic paints having different composition of constituent materials by combination of measured bidi- rectional reflectance distribution functions and thereby to find a paint that provides optimal appearance of a product with appropriate composition of constituent materials. We mainly focused on two paint parameters, aver- age size and density of aluminum flakes, because they significantly affect the appearance of metallic paint. We also present a compact represen- tation to approximate a large size of measured data using a few curve functions. We demonstrate the efficiency and usability of our reflectance estimation method using some examples. C 2011 Society of Photo-Optical Instru-

High-dynamic-range camera-based bidirectional reflectance distribution function measurement system for isotropic materials

We present a novel high-dynamic-range (HDR) camera-based bidirectional reflectance distribution function (BRDF) measurement system that can measure the reflectance property of isotropic materials. Our developed system can measure the BRDF of highly specular materials much faster than previous systems. It measures highly dense BRDF samples for a wider reflection angle with less noise so that it provides accuracy that is necessary for computer graphics application. To estimate the reflectance of a given material, we perform an absolute photometric calibration for the HDR camera. Our system is verified by checking the Helmholtz reciprocity and comparing the performance with that of previous image-based systems. The capability and efficiency of the developed system is demonstrated by comparing the images generated by measure-and-fit and direct-rendering methods using the measured data of four different isotropic materials.

Spatial augmented reality for product appearance design evaluation

Abstract Augmented reality based on projection, called “Spatial Augmented Reality (SAR)”, is a new technology that can produce immersive contents by overlapping virtuality and real-world environment. It has been paid attention as the next generation digital contents in media art and human–computer interaction (HCI). In this paper, we present a new methodology to evaluate the product appearance design more intuitively by means of SAR technique. The proposed method first projects the high-quality rendered image considering the optical property of materials onto the mock-up of a product. We also conduct a projector-camera calibration to compensate a color distortion according to a projector, a projection surface and environment lighting. The design evaluation methodology we propose offers more flexible and intuitive evaluation environment to a designer and user (evaluator) than previous methods that are performed via a digital display. At the end of this research, we have conducted a case study for designing and evaluating appearance design of an automobile.

Representation of homogeneous translucent materials using the P3 approximation and an image-based bidirectional subsurface scattering reflectance distribution function measurement system

In computer graphics applications, the standard diffusion model (SDA) is used to represent translucent materials. However, the SDA is not suitable for the representation of the translucent materials with weakly scattering properties. We represent the translucent materials with the weakly scattering properties by using the P3 approximation, since the P3 approximation can more accurately represent such translucent materials than the SDA. We also present an efficient and stable image-based bidirectional subsurface scattering reflectance distribution function measurement system that can measure the reflectance property of homogeneous translucent materials. From the high-dynamic-range image of a translucent material acquired from the proposed image-based measurement systems, the reduced scattering and the absorption coefficients of the material are estimated to represent its translucent property in computer graphics applications. We demonstrate the strength of the P3 approximation for representing the translucent materials using various examples with realistic illumination based on an environmental map.

A fast and accurate image-based measuring system for isotropic reflection materials

We present a novel image-based BRDF (Bidirectional Reflectance Distribution Function) measurement system for materials that have isotropic reflectance properties. Our proposed system is fast due to simple set up and automated operations. It also provides a wide angular coverage and noise reduction capability so that it achieves accuracy that is needed for computer graphics applications. We test the uniformity and constancy of the light source and the reciprocity of the measurement system. We perform a photometric calibration of HDR (High Dynamic Range) camera to recover an accurate radiance map from each HDR image. We verify our proposed system by comparing it with a previous imagebased BRDF measurement system. We demonstrate the efficiency and accuracy of our proposed system by generating photorealistic images of the measured BRDF data that include glossy blue, green plastics, gold coated metal and gold metallic paints.

Developing a multispectral HDR imaging module for a BRDF measurement system

Most recent bidirectional reflectance distribution function (BRDF) measurement systems are the image-based that consist of a light source, a detector, and curved samples. They are useful for measuring the reflectance properties of a material but they have two major drawbacks. They suffer from high cost of BRDF acquisition and also give inaccurate results due to the limited use of spectral bands. In this paper, we propose a novel multispectral HDR imaging system and its efficient characterization method. It combines two promising technologies: high dynamic range (HDR) imaging and multispectral imaging to measure BRDF. We perform a full spectral recovery using camera response curves for each wavelength band and its analysis. For this, we use an HDR camera to capture HDR images and a liquid crystal tunable filter (LCTF) to generate multi-spectral images. Our method can provide an accurate color reproduction of metameric objects as well as a saturated image. Our multi-spectral HDR imaging system provides a very fast data acquisition time and also gives a low system setup cost compared to previous multi-spectral imaging systems and point-based commercial spectroradiometers. We verify the color accuracy of our multi-spectral HDR imaging system in terms of human vision and metamerism using colorimetric and spectral metric.

Rendering of human skin during physical exercise

Many researchers have shown interest in the realistic rendering of a human face since it is crucial to the success of many applications in computer graphics, games and animation. However, it is difficult to represent the realistic appearance of a facial skin since it has complex physical properties and its appearance tends to vary as the situation changes. For example, the glossiness of a facial skin increases after exercise and the color of it becomes red after drink. In this paper, we introduce a method to simulate the appearance change of a human skin during physical exercise. Our key idea is that the appearance of a skin under exercise can be divided into a surface reflectance component and a subsurface scattering component so that they are approximated by a BRDF model and a BSSRDF model, respectively. The Torrance-Sparrow BRDF model describes the glossiness and the roughness that are affected by sweat and oil on the skin surface. The Multipole model [Donner and Jensen 2005] with absorption σa and scattering σs parameters represents subsurface scattering effects that occur between cells and pigments under the surface. In this paper, the parameters of each reflectance model are acquired by measurement.

Non-parametric BRDFs for pearlescent coatings

Pearlescent coatings have been widely used in various industrial products such as automotive paints and plastics coatings since it generates special visual appearance called the angle-dependent color shift effect. Curved objects painted with these coatings would be viewed and illuminated from many different directions, and this feature makes the visual appearance change according to different view and light directions. Pearlescent coatings are composed of four layers including clearcoat, platelet coat, black base coat and substrate. Among four layers, the color shift occurs by the platelet coat layer.

Effective decomposition of pearlescent paints

This paper deals with a method to effectively compress the measured reflectance data of pearlescent paints. In order to simulate the coated surface realistically, it is requested to measure the reflectance of the pearlescent paints by using multiple wavelengths. The wavelength-based reflectance data requests a large amount of storage. However, we can reduce the size of the measured BRDF and retain the accuracy the data by using several factorization algorithms. In this paper, we analyze the decomposition of the measured BRDF of pearlescent paint and find the number of lobes or basis functions to retain the visual accuracy of the measured reflectance.