Shape-from-shading is a well-known, although ill-posed, 3D-reconstruction technique. Photometric stereo is an extension of shape-from-shading, where several light sources are used to illuminate the scene from different points of view. This problem is well-posed when the light sources are known. However, when there are unknown ("uncalibrated photometric stereo"), an ambiguity still occurs, which is called the "generalized bas-relief ambiguity". In my talk, I will detail this ambiguity and how to remove it.