Sensory representation of surface reflectances: assessments with hyperspectral images.

Specifying surface reflectances in a simple and perceptually informative way would be beneficial for many areas of research and application. We assessed whether a 3×3 matrix may be used to approximate how a surface reflectance modulates the sensory color signal across illuminants. We tested whether observers could discriminate between the model's approximate and accurate spectral renderings of hyperspectral images under narrowband and naturalistic, broadband illuminants for eight hue directions. Discriminating the approximate from the spectral rendering was possible with narrowband, but almost never with broadband illuminants. These results suggest that our model specifies the sensory information of reflectances across naturalistic illuminants with high fidelity, and with lower computational cost than spectral rendering.

Colour expectations across illumination changes.

This study investigates human expectations towards naturalistic colour changes under varying illuminations. Understanding colour expectations is key to both scientific research on colour constancy and applications of colour and lighting in art and industry. We reanalysed data from asymmetric colour matches of a previous study and found that colour adjustments tended to align with illuminant-induced colour shifts predicted by naturalistic, rather than artificial, illuminants and reflectances. We conducted three experiments using hyperspectral images of naturalistic scenes to test if participants judged colour changes based on naturalistic illuminant and reflectance spectra as more plausible than artificial ones, which contradicted their expectations. When we consistently manipulated the illuminant (Experiment 1) and reflectance (Experiment 2) spectra across the whole scene, observers chose the naturalistic renderings significantly above the chance level (>25 %) but barely more often than any of the three artificial ones, collectively (>50 %). However, when we manipulated only one object/area's reflectance (Experiment 3), observers more reliably identified the version in which the object had a naturalistic reflectance like the rest of the scene. Results from Experiments 2-3 and additional analyses suggested that relational colour constancy strongly contributed to observer expectations, and stable cone-excitation ratios are not limited to naturalistic illuminants and reflectances but also occur for our artificial renderings. Our findings indicate that relational colour constancy and prior knowledge about surface colour shifts help to disambiguate surface colour identity under illumination changes, enabling human observers to recognise surface colours reliably in naturalistic conditions. Additionally, relational colour constancy may even be effective in many artificial conditions.