Extrinsic MANOVA for Oriented Projective Shapes

Abstract

For configurations of landmarks reconstructed from camera images, classical projective shape analysis identifies antipodal spherical representatives of projective coordinates, while oriented projective shape (OPS) retains their distinction. In 3D, a fixed oriented-projective coordinate representation of a OPS is point on product of 3D spheres. We develop nonparametric multisample inference for equality of population extrinsic means on this product. Quaternion multiplication helps create a global orthonormal tangent frame field, in which we obtain explicit extrinsic covariance blocks that preserve dependence between registered landmarks. A covariance-weighted MANOVA statistic has a chi-square null limit under nonfocality of distributions and positive-definite extrinsic covariance assumptions. An inverse-free discrepancy has a weighted chi-square limit when covariance matrices may be singular, and centered, groupwise nonparametric bootstrap procedures are justified asymptotically. We apply a fixed-observed-covariance nonparametric bootstrap version of the weighted statistic to 31 seven-landmark facial reconstructions from five subjects. Separate tests at the right inner brow and chin reject equality of the five OPS mean directions after Bonferroni adjustment. The analysis concerns a common mixed-sign frame domain and marginal landmarks; it does not establish pairwise subject differences or a joint six-dimensional facial test.