  Polarization is well known for its ability to decompose diffuse and specular
reflections. However, the existing decomposition methods only focus on direct
reflection and overlook multiple reflections, especially specular
inter-reflection. In this paper, we propose a novel decomposition method for
handling specular inter-reflection of metal objects by using a unique
polarimetric feature: the rotation direction of linear polarization. This
rotation direction serves as a discriminative factor between direct and
inter-reflection on specular surfaces. To decompose the reflectance components,
we actively rotate the linear polarization of incident light and analyze the
rotation direction of the reflected light. We evaluate our method using both
synthetic and real data, demonstrating its effectiveness in decomposing
specular inter-reflections of metal objects. Furthermore, we demonstrate that
our method can be combined with other decomposition methods for a detailed
analysis of light transport. As a practical application, we show its
effectiveness in improving the accuracy of 3D measurement against strong
specular inter-reflection.
