Bidirectional perspective ME - Similarly to the translational SI creation solutions, bidirectional perspective motion estimation is performed with the perspective transforms selected in the previous step to refine the perspective deformations already obtained.
Regarding the several motion models available in the literature, the eight-parameter perspective motion model was selected in this paper, due to its popularity [20 23] and also its ability to model complex motion, like zooms, rotations, and perspective deformations.
Backward perspective ME - Then, the process performed in the previous step is repeated for the opposite direction (from X f to X b ), thus obtaining the perspective deformations (defined by the associated PTVs) from the future reference frame, X f, to the backward reference frame, X b.
The perspective motion model used for SI creation (in a distributed video decoder) enables a more accurate characterization of complex motion that might occur in the video sequence, such as zooms, rotations, and other affine and perspective deformations, without transmitting any parameters or vectors from the encoder to the decoder as in predictive video coding.
Quarter-pel up-sampling - To provide a more accurate estimation for the possible perspective deformations, the backward and forward reference frames are first up-sampled with the H.264/AVC quarter-pel up-sampling filter [1]; this is performed so that the next steps can benefit from increased precision reference frames.
image data acquired with different spatial and/or temporal sampling rates, image structures of different spatial and/or temporal extent, objects at different distances from the camera, the linear component of relative motions between objects in the world and the observer, and the linear component of perspective deformations.
Velocity adaptation was applied to optic flow estimation by Nagel and Gehrke (1998) and was shown to improve the accuracy in optic flow estimates in a similar manner as affine shape adaptation improves the accuracy of image descriptors under perspective image deformations outside the similarity group.
Specifically, a mechanism of affine shape adaptation was proposed for reaching affine covariant interest points in affine scale space, and it was shown that the computation of such affine-adapted image measurements improved the accuracy of later-stage processes in situations when there are significant perspective image deformations outside the similarity group.
