Visual acuity in day for night

In lm production, it is sometimes not convenient or directly impossible to shoot some night scenes at night. The lm budget, schedule or location may not allow it. In these cases, the scenes are shot at daytime, and the 'night look ' is achieved by placing a blue lter in front of the lens and under-exposing the lm. This tech-nique, that the American lm industry has used for many decades, is called 'Day for Night ' (or 'American Night' in Europe.) But the images thus obtained don't usually look realistic: they tend to be too bluish, and the ob-jects ' brightness seems unnatural for night-light. These problems stem from the fact that this Day for Night pro-cess is exclusively optical (lters, lm exposure), and hence it can't provide the modications that account for how we see the world at night: different spectrum for the night illuminant, desaturation of colors, brightness modication according to wavelength, contrast modi-cation according to luminance adaptation levels, and non-uniform and non-linear loss of resolution. In this article we introduce a digital Day for Night algorithm that achieves very realistic results. We use a set of very simple equations, based on real physical data and visual perception experimental data. To simulate the loss of visual acuity we introduce a novel diffusion Partial Dif-ferential Equation (PDE) which takes both luminance and contrast into account, produces no ringing, is stable, very easy to implement and fast. The user only provides the original day image and the desired level of darkness of the result. The whole process from original day image to nal night image is implemented in a few seconds, computations being mostly local.