Im2Flow: Motion Hallucination from Static Images for Action Recognition

Existing methods to recognize actions in static images take the images at their face value, learning the appearances---objects, scenes, and body poses---that distinguish each action class. However, such models are deprived of the rich dynamic structure and motions that also define human activity. We propose an approach that hallucinates the unobserved future motion implied by a single snapshot to help static-image action recognition. The key idea is to learn a prior over short-term dynamics from thousands of unlabeled videos, infer the anticipated optical flow on novel static images, and then train discriminative models that exploit both streams of information. Our main contributions are twofold. First, we devise an encoder-decoder convolutional neural network and a novel optical flow encoding that can translate a static image into an accurate flow map. Second, we show the power of hallucinated flow for recognition, successfully transferring the learned motion into a standard two-stream network for activity recognition. On seven datasets, we demonstrate the power of the approach. It not only achieves state-of-the-art accuracy for dense optical flow prediction, but also consistently enhances recognition of actions and dynamic scenes.Comment: Published in CVPR 2018, project page: http://vision.cs.utexas.edu/projects/im2flow

Achieving full diversity in multi-antenna two-way relay networks via symbol-based physical-layer network coding

This paper considers physical-layer network coding (PNC) with M-ary phase-shift keying (MPSK) modulation in two-way relay channel (TWRC). A low complexity detection technique, termed symbol-based PNC (SPNC), is proposed for the relay. In particular, attributing to the outer product operation imposed on the superposed MPSK signals at the relay, SPNC obtains the network-coded symbol (NCS) straightforwardly without having to detect individual symbols separately. Unlike the optimal multi-user detector (MUD) which searches over the combinations of all users’ modulation constellations, SPNC searches over only one modulation constellation, thus simplifies the NCS detection. Despite the reduced complexity, SPNC achieves full diversity in multi-antenna relay as the optimal MUD does. Specifically, antenna selection based SPNC (AS-SPNC) scheme and signal combining based SPNC (SC-SPNC) scheme are proposed. Our analysis of these two schemes not only confirms their full diversity performance, but also implies when SPNC is applied in multi-antenna relay, TWRC can be viewed as an effective single-input multiple-output (SIMO) system, in which AS-PNC and SC-PNC are equivalent to the general AS scheme and the maximal-ratio combining (MRC) scheme. Moreover, an asymptotic analysis of symbol error rate (SER) is provided for SC-PNC considering the case that the number of relay antennas is sufficiently large

Detecting Byzantine Attacks Without Clean Reference

We consider an amplify-and-forward relay network composed of a source, two relays, and a destination. In this network, the two relays are untrusted in the sense that they may perform Byzantine attacks by forwarding altered symbols to the destination. Note that every symbol received by the destination may be altered, and hence no clean reference observation is available to the destination. For this network, we identify a large family of Byzantine attacks that can be detected in the physical layer. We further investigate how the channel conditions impact the detection against this family of attacks. In particular, we prove that all Byzantine attacks in this family can be detected with asymptotically small miss detection and false alarm probabilities by using a sufficiently large number of channel observations \emph{if and only if} the network satisfies a non-manipulability condition. No pre-shared secret or secret transmission is needed for the detection of these attacks, demonstrating the value of this physical-layer security technique for counteracting Byzantine attacks.Comment: 16 pages, 7 figures, accepted to appear on IEEE Transactions on Information Forensics and Security, July 201

Storage, fractionation and melt-crust interaction of basaltic magmas at oceanic and continental settings

This study uses phenocrysts and xenoliths to examine storage, fractionation and melt-crust interaction of basaltic magmas. Gabbroic xenoliths from Hualalai Volcano, Hawaii include fragments of lower oceanic crust (LOC) cumulates. Oxygen and Sr isotope compositions of these gabbros indicate minimal hydrothermal alteration. Magmas from fast ridges fractionate on average at shallower and less variable depths and undergo more homogenization than those from fast ridges. These features suggest a long-lived shallow magma lens exists at fast ridges, which limits the penetration of hydrothermal circulation into the LOC. Anorthitic plagioclases in these LOC gabbros therefore unlikely derive from hydrous melting or hydrothermal replacement. The strongly positive correlation between plagioclase anorthite content and whole rock Re concentration of Hualalai LOC gabbros may place further constraints on the origin of anorthitic plagioclase at mid-ocean ridges. Most Hualalai xenoliths represent Hualalai melt-derived cumulates. MELTS modeling and equilibration temperatures suggest Hualalai shield-stage-related gabbros crystallized within local LOC. Therefore, a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt–crust interaction between Hawaiian melts and Pacific crust partially overprinted Sr, Nd, and Pb isotope compositions of LOC-derived gabbros. The modified isotope compositions of Pacific LOC (and likely lithospheric mantle) are similar to Hawaiian rejuvenated-stage lavas. Although minor assimilation of Pacific crust by Hawaiian melts cannot be excluded, the range of oxygen isotope compositions recorded in Hawaiian magmas cannot be generated by assimilation of the in situ LOC. The Papoose Canyon (PC) monogenetic eruption sequence in the Big Pine volcanic field, California displays temporal-compositional variations indicating mixing of two distinct melts. PC phenocrysts and xenoliths derive from melt that is more fractionated and enriched than PC lavas. Pressure constraints suggest these phenocrysts and xenoliths crystallized at mid-crust depths. PC lavas also show evidence of crustal contamination. Therefore, PC phenocrysts and xenoliths likely derive from early PC melts that ponded, fractionated and assimilated continental crust in mid-crustal sills, which were mixed with more primitive melts as the eruption began. The temporal-compositional trends thus reflect gradual exhaustion of these sills over time.Geological Science