The surface appearance of glossy objects is constrained by generative processes, which determine the sharpness (b) of specular highlights. Specifically, image structure is constrained by the way that surface reflectance properties interact with 3D surface geometry and the light field 31 . Microroughness: whereas matte surfaces scatter light in all directions, glossy surfaces reflect light directionally, preserving the structure of the illumination field. Highly smooth surfaces produce narrow specular lobes 68 that increase the contrast and clarity (sharpness) of the reflected image relative to microscopically rougher surfaces, which produce broader specular lobes that blur this structure. Surfaces with higher microroughness also cause more spread-out specular highlights (higher coverage) owing to the larger range of surface normal orientations reflecting bright light sources towards the observer. Specular strength: whereas dielectric materials like plastic reflect only a proportion of light specularly, metals reflect all light specularly, increasing the contrast of the reflected image. Illumination sources: because of the structure-preserving properties of glossy objects, the contrast, sharpness and coverage of specular highlights depend on the incident light that is being reflected; that is, the intensity and structure of the illumination field. Three-dimensional shape: furthermore, 3D shape distorts this illumination structure. Specular reflections cling to points of high surface curvature and are elongated along (but slide rapidly across) directions of minimal surface curvature 23 , 24 , 69 , 70 . This means that 3D shape and observer viewpoint affect the location and distortion of specular reflections and thus the proximal stimulus properties of coverage, sharpness and contrast of the highlights.
