Perception of material properties from dynamic line drawings

Recent studies have shown that people are able to recognize material qualities based on motion information alone (Schmid et al, 2018, Bi et al 2018). In these experiments, stimuli were deforming nonrigid objects (e.g. liquids, jelly, cloth) with dots ‘stuck on’ at random places in and on the object. The dots ‘inherited’ the kinematic material properties and global shape deformations of the object and the overall motion pattern yielded vivid nonrigid material qualities, such as wobbliness. Dynamic dot materials contain both, interior-, and boundary motion. However, how much does each of these types of motion contribute to the percept of a given material quality? To answer this question we contrast ratings of material qualities of dynamic dot stimuli with those for dynamic (out)line drawings of the same object-material deformations, as well as with ratings for corresponding full-texture (i.e. color & reflectance) renderings (for sample stimuli, see https://jlubox.uni-giessen.de/getlink/fi9xJ5W9kN1drXPBodkz1HCY/). Animations of five material categories (fabrics, hard breakables, jelly, liquids, smoke) were rated each six times on eight material attributes (dense, flexible, wobbly, fluid, airy motion, motion coherence, oscillatory motion, and motion dynamics), blocked by rendering style (dots, lines, full). Comparing dissimilarity matrices and cluster analysis of attribute ratings between the three rendering conditions suggest that 1) also animated line drawings vividly convey mechanical material properties, 2) similarity in material judgements between line drawings and fully textured animations was larger than that between dynamic dots and fully textured stimuli. We conclude that boundary motion might play a critical role in the perception of mechanical material qualities.