“Continuous tone representation of three-dimensional objects illuminated by sky light” by Nishita and Nakamae

Natural lighting models to date have been limited to calculation of direct sunlight. However, this paper proposes an improved model for natural lighting calculations that adequately considers both direct sunlight and scattered light caused by clouds and other forms of water vapor in the air. Such indirect natural light is termed skylight and can be an important factor when attempting to render realistic looking images as they might appear under overcast skies.In the proposed natural lighting model, the sky is considered to be a hemisphere with a large radius (called the sky dome) that acts as a source of diffuse light with nonuniform intensity. In order to adequately take into account the nonuniform intensity of such skylight, the sky dome is subdivided into bands. The light intensity within individual bands can be assumed to be transversely uniform and longitudinally nonuniform and therefore the total luminance emanating from each band can be calculated more accurately.The proposed method significantly improves the realism of natural lighting effects. Its advantages are particularly apparent when simulating lighting under an overcast sky or when rendering surfaces that fall within a shadow cast by an obstruction lit by direct sunlight.

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