In this paper we show that typical nonrigid structure can often be approximated well as locally rigid sub-structures in time and space. Specifically, we assume that: I) the structure can be approximated as rigid in a short local time window and 2) some point-pairs stay relatively rigid in space, maintaining a fixed distance between them during the sequence. First, we use the triangulation constraints in rigid SjM over a sliding time window to get an initial estimate of the nonrigid 3D structure. Then we automat ically identify relatively rigid point-pairs in this structure, and use their length-constancy simultaneously with trian gulation constraints to refine the structure estimate. Lo cal factorization inherently handles small camera motion, short sequences and significant natural occlusions grace fully, performing better than nonrigid factorization meth ods. We show more stable and accurate results as compared to the state-of-the art on even short sequences starting from 15 frames only, containing camera rotations as small as 2° and up to 50% contiguous missing data.

The self-organization of actin filaments is a topic that links cell biology with condensed matter physics. In vitro assays allow precise manipulation of component mechanical and chemical properties, needed for rigorous tests of theoretical models. We review recent work on in vitro motility assays that documented emergence of ordered actin filament microdomains powered by myosin motor proteins at high filament densities. Motor and filament surface density and mechanochemical cycle kinetics are additional parameters under current investigation. Individual filament collisions have been studied in order to elucidate the emergent population behavior. Apolar, weak  interactions evidenced by local filament deformations during crossover events are attenuated at high motor densities. Theoretical analysis requires refinement of rigid rod filament models. In intact cells, accessory proteins modulate actin filament length, bundling or sliding and this gives rise to complex emergent structures and behaviors such as cell motility and chemotaxis. The development of generic, mechanical and biochemical frameworks with predictive power that link molecular properties with micro- and macroscopic phenomena seen in living cells requires dialogue between theoreticians and experimentalists.