Drishti, a volume exploration and presentation tool

Among several rendering techniques for volumetric data, direct volume rendering is a powerful visualization tool for a wide variety of applications. This paper describes the major features of hardware based volume exploration and presentation tool - Drishti. The word, Drishti, stands for vision or insight in Sanskrit, an ancient Indian language. Drishti is a cross-platform open-source volume rendering system that delivers high quality, state of the art renderings. The features in Drishti include, though not limited to, production quality rendering, volume sculpting, multi-resolution zooming, transfer function blending, profile generation, measurement tools, mesh generation, stereo/anaglyph/crosseye renderings. Ultimately, Drishti provides an intuitive and powerful interface for choreographing animations

High-Resolution Stratigraphy of the Mars North Polar Layered Deposits

The Mars north polar layered deposits (NPLD) consist primarily of water ice and dust, and represent the bulk of the north polar cap [Byrne, 2009]. The NPLD likely hold the most extensive record of recent climate during a period of major insolation variation tied to quasiperiodic orbital changes [Thomas et al., 1992; Laskar et al., 2002]. Data from the High Resolution Imaging Science Experiment (HiRISE) permits definition of layers at sub-meter scale and a renewed analysis of possible orbital expression in NPLD thickness. In this study, imagederived digital elevation models are generated and used to produce thickness measurements in accordance with bed orientation and topographic expression. Stratigraphic columns have been produced for two new sites within the NPLD, and contain several sets of finely bedded units 1-2 m thick; dark, prominent marker beds 3-5 m thick; and undifferentiated units. This work suggests that 1-2 m beds are indeed common in the NPLD; that marker beds are also common, but occur at variable separation distances; and that models of NPLD formation need to incorporate new phenomena to generate the observed scale of bedding. As yet, no distinct orbital fingerprint can be identified

A vector-based method for bank-material tracking in coupled models of meandering and landscape evolution

Sinuous channels commonly migrate laterally and interact with banks of different strengths—an interplay that links geomorphology and life and shapes diverse landscapes from the seafloor to planetary surfaces. To investigate feedbacks between meandering rivers and landscapes over geomorphic timescales, numerical models typically represent bank properties using grids; however, this approach produces results inherently dependent on grid resolution. Herein we assess existing techniques for tracking landscape and bank-strength evolution in numerical models of meandering channels and show that grid-based models implicitly include unintended thresholds for bank migration that can control simulated landscape evolution. Building on stratigraphic modeling techniques, we develop a vector-based method for land surface- and subsurface-material tracking that overcomes the resolution-dependence inherent in grid-based techniques by allowing high-fidelity representation of bank-material properties for curvilinear banks and low channel lateral migration rates. We illustrate four specific applications of the new technique: (1) the effect of resistant mud-rich deposits in abandoned meander cutoff loops on meander belt evolution; (2) the stratigraphic architecture of aggrading, alluvial meandering channels that interact with cohesive-bank and floodplain material; (3) the evolution of an incising, meandering river with mixed bedrock and alluvial banks within a confined bedrock valley; and (4) the effect of a bank-height dependent lateral-erosion rate for a meandering river in an aggrading floodplain. In all cases the vector-based approach overcomes numerical artifacts with the grid-based model. Because of its geometric flexibility, the vector-based material tracking approach provides new opportunities for exploring the coevolution of meandering rivers and surrounding landscapes over geologic timescales

Numerical simulations of bedrock valley evolution by meandering rivers with variable bank material

Bedrock river valleys are fundamental components of many landscapes, and their morphologies—from slot canyons with incised meanders to wide valleys with strath terraces—may record environmental history. Several formation mechanisms for particular valley types have been proposed that involve changes in climatic and tectonic forcing, but the uniqueness of valley evolution pathways and the long-term stability of valley morphology under constant forcing are unknown and are not predicted in existing numerical models for vertically incising rivers. Because rivers often migrate more rapidly through alluvium than through bedrock, we explore the hypothesis that the distribution of bank materials strongly influences river meandering kinematics and can explain the diversity of bedrock river valley morphology. Simulations using a numerical model of river meandering with vector-based bank-material tracking indicate that channel lateral erosion rate in sediment and bedrock, vertical erosion rate, and initial alluvial-belt width explain first-order differences in bedrock valley type; that bedrock-bound channels can evolve under steady forcing from alluvial states; and that weak bedrock and low vertical incision rates favor wide, shallow valleys, while resistant bedrock and high vertical incision rates favor narrow, deep valleys. During vertical incision, sustained planation of the valley floor is favored when bedrock boundaries restrict channel migration to a zone of thin sediment fill. The inherent unsteadiness of river meandering in space and time is enhanced by evolving spatial contrasts in bank strength between sediment and bedrock and can account for several valley features—including strath terraces and underfit valleys—commonly ascribed to external drivers

Three-dimensional segmentation of computed tomography data using Drishti Paint: new tools and developments

Computed tomography (CT) has become very widely used in scientific and medical research and industry for its non-destructive and high-resolution means of detecting internal structure. Three-dimensional segmentation of computed tomography data sheds light on internal features of target objects. Three-dimensional segmentation of CT data is supported by various well-established software programs, but the powerful functionalities and capabilities of open-source software have not been fully revealed. Here, we present a new release of the open-source volume exploration, rendering and three-dimensional segmentation software, Drishti v. 2.7. We introduce a new tool for thresholding volume data (i.e. gradient thresholding) and a protocol for performing three-dimensional segmentation using the 3D Freeform Painter tool. These new tools and workflow enable more accurate and precise digital reconstruction, three-dimensional modelling and three-dimensional printing results. We use scan data of a fossil fish as a case study, but our procedure is widely applicable in biological, medical and industrial research.This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (grant no. XDB26000000) and the National Natural Science Foundation of China (grant no. 41872023). Y.H. was supported by a Postgraduate Research Scholarship at the Research School of Physics, Australian National University. The development of Drishti is supported by National Computational Infrastructure, Australian National University. CT scans and three-dimensional printing are supported by the Department of Applied Mathematics, Research School of Physics and ANU CT Lab, with funding support from Prof. T. Senden and Australian Research Council Discovery Grant DP160102460

Numerical model predictions of autogenic fluvial terraces and comparison to climate change expectations

Terraces eroded into sediment (alluvial) and bedrock (strath) preserve an important history of river activity. River terraces are thought to form when a river switches from a period of slow vertical incision and valley widening to fast vertical incision and terrace abandonment. Consequently, terraces are often interpreted to reflect changing external drivers including tectonics, sea level, and climate. In contrast, the intrinsic unsteadiness of lateral migration in rivers may generate terraces even under constant rates of vertical incision without external forcing. To explore this mechanism, we simulate landscape evolution by a vertically incising, meandering river and isolate the age and geometry of autogenic river terraces. Modeled autogenic terraces form for a wide range of lateral and vertical incision rates and are often paired and longitudinally extensive for intermediate ratios of vertical-to-lateral erosion rate. Autogenic terraces have a characteristic reoccurrence time that scales with the time for relief generation. There is a preservation bias against older terraces due to reworking of previously visited parts of the valley. Evolving, spatial differences in bank strength between bedrock and sediment reduce terrace formation frequency and length, favor pairing, and can explain sublinear terrace margins at valley boundaries. Age differences and geometries for modeled autogenic terraces are consistent, in cases, with natural terraces and overlap with metrics commonly attributed to terrace formation due to climate change. We suggest a new phase space of terrace properties that may allow differentiation of autogenic terraces from terraces formed by external drivers

On the morphodynamics of a wide class of large-scale meandering rivers: Insights gained by coupling LES with sediment-dynamics

In meandering rivers, interactions between flow, sediment transport, and bed topography affect diverse processes, including bedform development and channel migration. Predicting how these interactions affect the spatial patterns and magnitudes of bed deformation in meandering rivers is essential for various river engineering and geoscience problems. Computational fluid dynamics simulations can predict river morphodynamics at fine temporal and spatial scales but have traditionally been challenged by the large scale of natural rivers. We conducted coupled large-eddy simulation (LES) and bed morphodynamics simulations to create a unique database of hydro-morphodynamic datasets for 42 meandering rivers with a variety of planform shapes and large-scale geometrical features that mimic natural meanders. For each simulated river, the database includes (i) bed morphology, (ii) three-dimensional mean velocity field, and (iii) bed shear stress distribution under bankfull flow conditions. The calculated morphodynamics results at dynamic equilibrium revealed the formation of scour and deposition patterns near the outer and inner banks, respectively, while the location of point bars and scour regions around the apexes of the meander bends is found to vary as a function of the radius of curvature of the bends to the width ratio. A new mechanism is proposed that explains this seemingly paradoxical finding. The high-fidelity simulation results generated in this work provide researchers and scientists with a rich numerical database for morphodynamics and bed shear stress distributions in large-scale meandering rivers to enable systematic investigation of the underlying phenomena and support a range of river engineering applications

Deltaic deposits at Aeolis Dorsa: Sedimentary evidence for a standing body of water on the northern plains of Mars

A fundamental long-standing question regarding Mars history is whether the flat and low-lying northern plains ever hosted an ocean. The best opportunity to solve this problem is provided by stratigraphic observations of sedimentary deposits onlapping the crustal dichotomy. Here, we use high-resolution imagery and topography to analyze a branching network of inverted channel and channel lobe deposits in the Aeolis Dorsa region, just north of the dichotomy boundary. Observations of stacked, cross-cutting channel bodies and stratal geometries indicate that these landforms represent exhumed distributary channel deposits. Observations of depositional trunk feeder channel bodies, a lack of evidence for past topographic confinement, channel avulsions at similar elevations, and the presence of a strong break in dip slope between topset and foreset beds suggest that this distributary system was most likely a delta, rather than an alluvial fan or submarine fan. Sediment transport calculations using both measured and derived channel geometries indicate a minimum delta deposition time on the order of 400  years. The location of this delta within a thick and widespread clastic wedge abutting the crustal dichotomy boundary, unconfined by any observable craters, suggests a standing body of water potentially 105 km2 in extent or greater and is spatially consistent with hypotheses for a northern ocean