A psychophysical model to control the brightness and key-to-fill ratio in CG cartoon character lighting.

Lighting is a commonly used tool to manipulate the appearance of virtual characters in a range of applications. However, there are few studies which systematically examine the effect of lighting changes on complex dynamic stimuli. Our study presents several perceptual experiments, designed to investigate the ability of participants to discriminate lighting levels and the ratio of light intensity projected on the two sides of a cartoon character’s face (key-to-fill ratio) in portrait lighting design. We used a standard psychophysical method for measuring discrimination, typical in low-level perceptual studies but not frequently considered for evaluating complex stimuli. We found that people can easily differentiate lighting intensities, and distinguish between shadow strength and scene brightness under bright conditions but not under dark conditions. We provide a model of the results, and empirically validate the predictions of the model. We discuss the practical implications of our results and how they can be exploited to make the process of portrait lighting for CG cartoon characters more consistent, such as a tool for manipulating shadow while maintaining the level of perceived brightness.