Material properties and image cues for convincing grapes: The know-how of the 17th-century pictorial recipe by Willem Beurs

Painters mastered replicating the regularities of the visual patterns that we use to infer different materials and their properties, via meticulous observation of the way light reveals the world's textures. The convincing depiction of bunches of grapes is particularly interesting. A convincing portrayal of grapes requires a balanced combination of different material properties, such as glossiness, translucency and bloom, as we learn from the 17th-century pictorial recipe by Willem Beurs. These material properties, together with three-dimensionality and convincingness, were rated in experiment 1 on 17th-century paintings, and in experiment 2 on optical mixtures of layers derived from a reconstruction of one of the 17th-century paintings, made following Beurs's recipe. In experiment 3 only convincingness was rated, using again the 17th-century paintings. With a multiple linear regression, we found glossiness, translucency and bloom not to be good predictors of convincingness of the 17thcentury paintings, but they were for the reconstruction. Overall, convincingness was judged consistently, showing that people agreed on its meaning. However, the agreement was higher when the material properties indicated by Beurs were also rated (experiment 1) than if not (experiment 3), suggesting that these properties are associated with what makes grapes look convincing. The 17thcentury workshop practices showed more variability than standardization of grapes, as different combinations of the material properties could lead to a highly convincing representation. Beurs's recipe provides a list of all the possible optical interactions of grapes, and the economic yet effective image cues to render them.