19,233 research outputs found
Worst-Case Efficient Sorting with QuickMergesort
The two most prominent solutions for the sorting problem are Quicksort and
Mergesort. While Quicksort is very fast on average, Mergesort additionally
gives worst-case guarantees, but needs extra space for a linear number of
elements. Worst-case efficient in-place sorting, however, remains a challenge:
the standard solution, Heapsort, suffers from a bad cache behavior and is also
not overly fast for in-cache instances.
In this work we present median-of-medians QuickMergesort (MoMQuickMergesort),
a new variant of QuickMergesort, which combines Quicksort with Mergesort
allowing the latter to be implemented in place. Our new variant applies the
median-of-medians algorithm for selecting pivots in order to circumvent the
quadratic worst case. Indeed, we show that it uses at most
comparisons for large enough.
We experimentally confirm the theoretical estimates and show that the new
algorithm outperforms Heapsort by far and is only around 10% slower than
Introsort (std::sort implementation of stdlibc++), which has a rather poor
guarantee for the worst case. We also simulate the worst case, which is only
around 10% slower than the average case. In particular, the new algorithm is a
natural candidate to replace Heapsort as a worst-case stopper in Introsort
Plane-Based Optimization of Geometry and Texture for RGB-D Reconstruction of Indoor Scenes
We present a novel approach to reconstruct RGB-D indoor scene with plane
primitives. Our approach takes as input a RGB-D sequence and a dense coarse
mesh reconstructed by some 3D reconstruction method on the sequence, and
generate a lightweight, low-polygonal mesh with clear face textures and sharp
features without losing geometry details from the original scene. To achieve
this, we firstly partition the input mesh with plane primitives, simplify it
into a lightweight mesh next, then optimize plane parameters, camera poses and
texture colors to maximize the photometric consistency across frames, and
finally optimize mesh geometry to maximize consistency between geometry and
planes. Compared to existing planar reconstruction methods which only cover
large planar regions in the scene, our method builds the entire scene by
adaptive planes without losing geometry details and preserves sharp features in
the final mesh. We demonstrate the effectiveness of our approach by applying it
onto several RGB-D scans and comparing it to other state-of-the-art
reconstruction methods.Comment: in International Conference on 3D Vision 2018; Models and Code: see
https://github.com/chaowang15/plane-opt-rgbd. arXiv admin note: text overlap
with arXiv:1905.0885
A top-down scenario for the formation of massive Tidal Dwarf Galaxies
Among those objects formed out of collisional debris during galaxy mergers,
the prominent gaseous accumulations observed near the tip of some long tidal
tails are the most likely to survive long enough to form genuine recycled
galaxies. Using simple numerical models, Bournaud, Duc & Masset (2003) claimed
that tidal objects as massive as 10^9 Msun could only form, in these
simulations, within extended dark matter (DM) haloes. We present here a new set
of simulations of galaxy collisions to further investigate the structure of
tidal tails. First of all, we checked that massive objects are still produced
in full N-body codes that include feedback and a large number of particles.
Using a simpler N-body code with rigid haloes, we noticed that dissipation and
self-gravity in the tails, although important, are not the key factors.
Exploiting toy models, we found that, for truncated DM haloes, material is
stretched along the tail, while, within extended haloes, the tidal field can
efficiently carry away from the disk a large fraction of the gas, while
maintaining its surface density to a high value. This creates a density
enhancement near the tip of the tail. Only later-on, self-gravity takes over;
the gas clouds collapse and start forming stars. Thus, such objects were
fundamentally formed following a kinematical process, according to a top-down
scenario, contrary to the less massive Super Star Clusters that are also
present around mergers. This conclusion leads us to introduce a restrictive
definition for Tidal Dwarf Galaxies (TDGs) and their progenitors, considering
only the most massive ones, initially mostly made of gas, that were able to
pile up in the tidal tails. More simulations will be necessary to precisely
determine the fate of these proto--TDGs and estimate their number.Comment: 13 pages, 11 figures, accepted for publication in A&
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