Perceiving Gloss in Surfaces and Images

Color Appearance Models are successfully used to model the color perception differences seen when the same stimuli are presented on different media, e.g. hard copy or a self-luminous display. It is currently unknown if the similar effects are present in gloss perception and if there is need for Gloss Appearance Models. Gloss communication, and the higher level material appearance communication is becoming more important everyday with the increase in customized manufacturing and the need for the costumer to preview a final product while short-runs, time and cost constraints prohibit the use of hard-copy proofs. Three experiments are proposed in order to analyze this phenomenon. The Gloss matching performance of observers on real objects is first going to be studied. Then, the same experiment will be repeated with synthetic images. Finally, a cross-media matching experiment will be performed, where the observers will have to match a real material with synthetic representations. The same trend was observed in the experiment using only real objects and in the cross-media situation, where a high matching accuracy was obtained for low gloss samples, and the gloss of mid and high gloss samples was underestimated. The same accuracy for low gloss samples was obtained for the experiment with only synthetic images, but mid and high gloss samples were overestimated. The sensitivity of the observers was higher when only real samples were used, it decreased when the display was used due the lack of visual disparity and multiple viewing conditions, and it was lowest on the last experiment, influenced by the multiple media and the above limitations. Introduction Gloss communication, and the higher level material appearance communication is becoming more important every day with the increase in customized manufacturing and the need for the costumer to preview a final product while short-runs and cost constraints do not allow the use of hard-copy proofs. Color Appearance Models were developed to account for the viewing conditions and its effects on the perception of color. The same color stimuli seen on a hard copy and on a self-luminous display produces different color perceptions. Color Appearance Models are successfully used and have been widely evaluated [10] to model those changes in appearance and enable to create the same color perception on different media. It is currently unknown if the same effect is present in gloss perception, or if there is any need for Gloss Appearance Models. This project is designed to study if there exists a fundamental difference in cross-media gloss perception. Gloss communication could be improved with a transformation that accounted for the difference between the representation of a material seen on a display and the real material. Gloss also varies in other dimensions than color. Vangorp et al. [13] studied the gloss perception dependence on an object’s shape, and found that the material appearance perceived varied depending on the shape of the object. By using the uniform gloss space defined in Pellacini et al. [8] the authors modeled the shape dependence and were able to correct for it, being able to match the gloss appearance of two objects with different shapes. In this project, in order to understand the gloss perception difference between real objects and synthetic objects seen on a display, three different matching experiments will be conducted. In the first experiment, the observers will have to match real objects in a custom-built light booth. This will enable an understanding of the accuracy of the observers and their variability when performing the task with real objects. In the second experiment, the observers will repeat the same task but they will perform it on a display with synthetic images representing the real objects. As with the previous experiment, this will enable an understanding of the accuracy and variability of the observers performing this task on another media. More interestingly, it will allow us to compare how the accuracy and variability varies from the real objects to the simulations. In the third experiment, a cross-media matching experiment will be performed in which the observers will have to select the simulation of an object that matches a real object. This will allow us to understand the influence of the media used in the matching task. Related work Gloss is a perceptual attribute related to the physical phenomenon of the Bidirectional Reflectance Distribution Function (BRDF). The BRDF is a 4-dimensional function that describes how light is scattered by a surface and it is defined by the following equation: f (ωi,ωo) = L(ωo) E(ωi) (1) where E defines the irradiance due to the light source in the incoming direction defined by ωi, and L defines the radiance of a surface in the outgoing direction ωo, where the directions are defined in spherical coordinates. In their classic publication, Hunter and Harold [5] described six features that relate to the perception of Gloss: Specular gloss This property models the specular reflection at different angles, commonly 20◦, 30◦, 45◦, 60◦, and 75◦. The integration of the reflected light at a given aperture for a material in respect to a black glass defines the specular gloss. Figure 1. Setup used for the experiment. From left to right, 30-inch HP ZR30w display, custom-built light booth, and lazy susan used to provide easy access to the samples to the users. The color difference seen between the different media is due to the camera response. Lower angles are used to compare high-specular materials and higher angles are used to compare low-specular materials. Sheen This property models the specular reflection at grazing angles and it is defined at 85◦. Contrast gloss or luster Defines the difference between the highlight areas and its surrounding. This effect can be clearly seen in velvet cloth, which has distinct highlights and dark areas. Absence-of-bloom gloss Also known as absence of haze, which is defined as the spread of the specular component of the reflected light from a glossy surface. Distinctness-of-image gloss This property defines how well a material allows to distinguish the reflected background on the surface of the material. For example, a mirror will have a higher distinctness-of-image than a brushed metal as the mirror is going to sharply reflect the background, while the brushed metal will introduce some amount of blur to the reflected image of the background. Surface-uniformity gloss This property defines how smooth a surface is, being able to perceive a non-uniform texture when the surface is rough. The importance of gloss in the finishing of commercial products drove the creation of international standards concerning the measurements of some of those perceptual gloss attributes: Specular Gloss is defined in ISO 2813, ISO7668, ASTM D523, ASTM D2457, DIN 67530, and JIS 8741, Distictness-of-image gloss is defined in ASTM D5767, and Haze is defined in ASTM E430, and ISO 13803.