Many previous works on photometric stereo have shown how to recover the shape and reflectance properties of an object using multiple images taken under a fixed viewpoint and variable lighting conditions. However, most of them only dealt with a single point light source in each image. In this paper, we show how to perform photometric stereo with four images which are taken under distant but general lighting conditions. Our method is based on the representation that uses low-order spherical harmonics for Lambertian objects. Attached shadows are considered in this representation. We show that the lighting conditions can be estimated regardless of object shape and reflectance properties. The estimated illumination conditions can then help to recover the shape and reflectance properties.
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