The experimental procedures were very similar to those for reflection-image recognizability. Seven male and two female observers, including an author (HK) of the present study, participated in the experiment. Only the reference stimuli differed from the reflection-image recognizability experiment. The objects of the reference stimuli were illuminated by a square area lamp instead of the illumination maps; this lamp was used to control the highlight-related features. The experiment included three tasks to rate the contrast, sharpness, and coverage of specular highlights. In the contrast and sharpness rating tasks, reference stimuli with five levels of the
c and
d parameters suggested by Ferwerda et al. (
2001) were used. These parameters roughly represent different psychophysical dimensions of perceived glossiness and can be controlled using surface roughness and contrast between specular and diffuse reflectance. The parameters were set independently for the two tasks: For the contrast rating task, (
c,
d) = (0.1, 0.9), (0.2, 0.9), (0.3, 0.9), (0.4, 0.9), and (0.5, 0.9), and for the sharpness rating task, (
c,
d) = (0.48, 0.0), (0.48, 0.58), (0.48, 0.83), (0.48, 0.93), and (0.48, 0.97). In the coverage rating task, reference stimuli with five levels of highlight area, controlled by the size of the square area lamp, were used. Four of the reference stimuli were created with lamps of differing side lengths (1, 5, 10, or 15 m) in the Blender modeling space. The other reference stimulus was an image of an object with a specular reflectance of 0 illuminated by a square area lamp with a side of 10 m, and thus had no specular highlights. All reference stimulus images were captured by a camera with an obliquely downward position.