Skyless polarimetric calibration and visibility enhancement

Outdoor imaging in haze is plagued by poor visibility. A major problem is spatially-varying reduction of contrast by airlight, which is scattered by the haze particles towards the camera. However, images can be compensated for haze, and even yield a depth map of the scene. A key step in such scene recovery is subtraction of the airlight. In particular, this can be achieved by analyzing polarization-filtered images. This analysis requires parameters of the airlight, particularly its degree of polarization (DOP). These parameters were estimated in past studies by measuring pixels in sky areas. However, the sky is often unseen in the field of view. This paper derives several methods for estimating these parameters, when the sky is not in view. The methods are based on minor prior knowledge about a couple of scene points. Moreover, we propose blind estimation of the DOP, based on the image data. This estimation is based on independent component analysis (ICA). The methods were demonstrated in field experiments.