In this paper we consider critical motion sequences (CMSs) of rolling-shutter(RS) SfM. Employing an RS camera model with linearized pure rotation, we showthat the RS distortion can be approximately expressed by two internalparameters of an "imaginary" camera plus one-parameter nonlinear transformationsimilar to lens distortion. We then reformulate the problem as self-calibrationof the imaginary camera, in which its skew and aspect ratio are unknown andvarying in the image sequence. In the formulation, we derive a generalrepresentation of CMSs. We also show that our method can explain the CMS thatwas recently reported in the literature, and then present a new remedy to dealwith the degeneracy. Our theoretical results agree well with experimentalresults; it explains degeneracies observed when we employ naive bundleadjustment, and how they are resolved by our method.
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