The spectral shape, irradiance, direction, and diffuseness of daylight vary regularly throughout the day. The variations in illumination and their effect on the light reflected from objects may in turn provide visual information as to the time of day. We suggest that artists' color choices for paintings of outdoor scenes might convey this information and that therefore the time of day might be decoded from the colors of paintings. Here we investigate whether human viewers' estimates of the depicted time of day in paintings correlate with their image statistics, specifically chromaticity and luminance variations. We tested time-of-day perception in 17th- to 20th-century Western European paintings via two online rating experiments. In Experiment 1, viewers' ratings from seven time choices varied significantly and largely consistently across paintings but with some ambiguity between morning and evening depictions. Analysis of the relationship between image statistics and ratings revealed correlations with the perceived time of day: higher "morningness" ratings associated with higher brightness, contrast, and saturation and darker yellow/brighter blue hues; "eveningness" with lower brightness, contrast, and saturation and darker blue/brighter yellow hues. Multiple linear regressions of extracted principal components yielded a predictive model that explained 76% of the variance in time-of-day perception. In Experiment 2, viewers rated paintings as morning or evening only; rating distributions differed significantly across paintings, and image statistics predicted people's perceptions. These results suggest that artists used different color palettes and patterns to depict different times of day, and the human visual system holds consistent assumptions about the variation of natural light depicted in paintings.

Visual material perception is often studied with physically well-defined stimuli that lack ecological variety. Yet, even the visual variety found in our natural environment is limited when compared to artistic depiction. A similar object can be depicted in numerous different ways that all make visual sense. We studied the perception of translucency using 38 paintings of sea waves as experimental stimuli. It has previously been shown that translucency depends on the shape of the translucent object and on the light conditions. Both shape and light appear in many variations in depictions of seas. In the first experiment we explored the use of Thurstonian scaling and introduce the concept of Number of Distinguishable Levels (NDL). We found that the NDL ranged between 1.5 in a set with small waves to 4 in a set with large waves. While Experiment 1 took place in the lab, Experiment 2 was performed online and replicated the data from Experiment 1 qualitatively, although the NDL was lower in the online experiment. Furthermore, in this experiment we conducted Thurstonian scaling on a number of other attributes that possibly contribute to translucency perception, such as wavetip shading, surface reflections and realism. We found that many of these correlated significantly with translucency. In sum, this study advocates and demonstrates the use of uncontrolled stimuli, in our case paintings, to explore the wide variety of input the human visual system can process.