Yasuki Yamauchi

Brightness, not luminance, determines transition from the surface-color to the aperture-color mode for colored lights

Whether a color stimulus appears in the surface-color or in the aperture-color mode depends on the luminance relationship between the center color stimulus and its surround. We investigated how chromaticity of a color stimulus affected the luminance level at which the appearance of the stimulus changed from the surface-color to the aperture-color mode. Mode estimation points were obtained for 10-cd/m2 color stimuli with different chromaticities presented in the center of a white surround of variable luminance. The color stimuli tended to appear in the aperture-color mode as purity increased, similarly to the increase of the brightness-to-luminance ratio for equal-luminance colors. It was also found that the mode-transition sensitivity function was similar in shape to the brightness sensitivity function for 440–660-m monochromatic light. Our results indicate that brightness is a determining factor for mode transition between the surface-color and the aperture-color modes. We discuss a possible assumption for relationships between brightness and lightness limits of a surface color.

Depth information affects judgment of the surface-color mode appearance.

The mode of color appearance is determined not solely by physical properties of the stimulus but also by the conditions of surrounding stimuli. Coplanar ratio hypothesis suggests that the information provided in the same plane plays an important role in the judgment of lightness. We measured the upper-limit luminances of the test stimulus for the surface-color mode in a three-dimensionally represented environment to study the effects of depth and luminance conditions on the mode perception. The test stimulus and two array-type surrounding stimuli composed of 10 different colors were presented at different depths. The test stimulus was presented at three different depths. Subjects set the luminance of the test color to the point where it just ceased to appear in the complete surface-color mode. The upper-limit luminances of the test colors varied as the luminances of the surrounding stimulus displayed in the same depth changed. Our results indicate that the perception of the surface-color mode is mainly determined by the stimulus displayed in the same depth. These results support that belongingness--to which group in the environment the stimulus belongs--is important, and that the mode of color appearance is determined coplanar in a three-dimensional environment.

Upper-limit luminance for the surface-color mode appearance

A series of experiments were carried out to reveal determinants for the mode of color appearance by measuring the upper-limit luminance of a color chip for the surface-color mode. We used a CRT color monitor to present test and surround stimuli in the surface-color mode. The stimuli were composed of a three-by-three array of color chips on a gray background with a white frame. The observer increased the luminance of a center test color until it just ceased to appear in the surface-color mode. Our results show that this upper-limit luminance was different among test colors, but their brightnesses, calculated from the luminance and brightness/ luminance values, were almost the same and were slightly below the brightness of the white frame. The existence of the surrounding color chips affected the results, but their sizes and spatial arrangements did not. When all of the luminances of the surrounds changed equally, the upper-limit luminances of the test colors for the surface-color mode appearance changed by the same ratio. This result indicates that the brightness of a target was a determinant for selecting the mode of color appearance and that the brightest surround stimulus acted as a cue for determining the judgment.

Contrast adaptation to luminance and brightness modulations.

Perceptual brightness and color contrast decrease after seeing a light temporally modulating along a certain direction in a color space, a phenomenon known as contrast adaptation. We investigated whether contrast adaptation along the luminance direction arises from modulation of luminance signals or apparent brightness signals. The stimulus consisted of two circles on a gray background presented on a CRT monitor. In the adaptation phase, the luminance and chromaticity of one circle were temporally modulated, while the other circle was kept at a constant luminance and color metameric with an equal-energy white. We employed two types of temporal modulations, namely, in luminance and brightness. Chromaticity was sinusoidally modulated along the L-M axis, leading to dissociation between luminance and brightness (the Helmholtz-Kohlrausch effect). In addition, luminance modulation was minimized in the brightness modulation, while brightness modulation was minimized in the luminance modulation. In the test phase, an asymmetric matching method was used to measure the magnitude of contrast adaptation for both modulations. Our results showed that, although contrast adaptation along the luminance direction occurred for both modulations, contrast adaptation for luminance modulation was significantly stronger than that for the brightness modulation regardless of the temporal frequency of the adaptation modulation. These results suggest that luminance modulation is more influential in contrast adaptation than brightness modulation.

Subjective evaluation of specular appearance for multiple observations using projector-based appearance reproduction

This paper presents the subjective evaluations of multiple observers as part of an investigation into the relationship between CG parameters of digital mockups and visual sensibility. In our experiments, the specular appearance of CG imagery is reproduced on an actual mockup using a projector-camera system. The specular intensity and position are evaluated in terms of magnitude and inauthenticity. For the specular intensity evaluation, it was found that changes to specular intensity in the CG parameters were equal to the changes in the brightness sensibility of the object viewed. Furthermore, the results of inauthenticity evaluations clarified the limitations of the viewpoint range. The specular appearance from the 60° viewpoint gave observers the impression that the form and position of the specular reflection were inauthentic. Therefore, it was determined that the control of appearance in our digital mockup was only suitable for observations within the range from -45° to 45°.