Scaling and discriminability of perceived gloss.

While much attention has been given to understanding biases in gloss perception (e.g., changes in perceived reflectance as a function of lighting, shape, viewpoint, and other factors), here we investigated sensitivity to changes in surface reflectance. We tested how visual sensitivity to differences in specular reflectance varies as a function of the magnitude of specular reflectance. Stimuli consisted of renderings of glossy objects under natural illumination. Using maximum likelihood difference scaling (MLDS), we created a perceptual scaling of the specular reflectance parameter of the Ward reflectance model. Then, using the method of constant stimuli and a standard 2AFC procedure, we obtained psychometric functions for gloss discrimination across a range of reflectance values derived from the perceptual scale. Both methods demonstrate that discriminability is significantly diminished at high levels of specular reflectance, thus indicating that gloss sensitivity depends on the magnitude of change in the image produced by different reflectance values. Taken together, these experiments also suggest that internal sensory noise remains constant for suprathreshold and near-threshold intervals of specular reflectance, which supports the use of MLDS as a highly efficient method for evaluating gloss sensitivity.