91,426 research outputs found
The Double Sphere Camera Model
Vision-based motion estimation and 3D reconstruction, which have numerous
applications (e.g., autonomous driving, navigation systems for airborne devices
and augmented reality) are receiving significant research attention. To
increase the accuracy and robustness, several researchers have recently
demonstrated the benefit of using large field-of-view cameras for such
applications. In this paper, we provide an extensive review of existing models
for large field-of-view cameras. For each model we provide projection and
unprojection functions and the subspace of points that result in valid
projection. Then, we propose the Double Sphere camera model that well fits with
large field-of-view lenses, is computationally inexpensive and has a
closed-form inverse. We evaluate the model using a calibration dataset with
several different lenses and compare the models using the metrics that are
relevant for Visual Odometry, i.e., reprojection error, as well as computation
time for projection and unprojection functions and their Jacobians. We also
provide qualitative results and discuss the performance of all models
Gravitational Lensing by Spinning Black Holes in Astrophysics, and in the Movie Interstellar
Interstellar is the first Hollywood movie to attempt depicting a black hole
as it would actually be seen by somebody nearby. For this we developed a code
called DNGR (Double Negative Gravitational Renderer) to solve the equations for
ray-bundle (light-beam) propagation through the curved spacetime of a spinning
(Kerr) black hole, and to render IMAX-quality, rapidly changing images. Our
ray-bundle techniques were crucial for achieving IMAX-quality smoothness
without flickering.
This paper has four purposes: (i) To describe DNGR for physicists and CGI
practitioners . (ii) To present the equations we use, when the camera is in
arbitrary motion at an arbitrary location near a Kerr black hole, for mapping
light sources to camera images via elliptical ray bundles. (iii) To describe
new insights, from DNGR, into gravitational lensing when the camera is near the
spinning black hole, rather than far away as in almost all prior studies. (iv)
To describe how the images of the black hole Gargantua and its accretion disk,
in the movie \emph{Interstellar}, were generated with DNGR. There are no new
astrophysical insights in this accretion-disk section of the paper, but disk
novices may find it pedagogically interesting, and movie buffs may find its
discussions of Interstellar interesting.Comment: 46 pages, 17 figure
Visualizing Interstellar's Wormhole
Christopher Nolan's science fiction movie Interstellar offers a variety of
opportunities for students in elementary courses on general relativity theory.
This paper describes such opportunities, including: (i) At the motivational
level, the manner in which elementary relativity concepts underlie the wormhole
visualizations seen in the movie. (ii) At the briefest computational level,
instructive calculations with simple but intriguing wormhole metrics,
including, e.g., constructing embedding diagrams for the three-parameter
wormhole that was used by our visual effects team and Christopher Nolan in
scoping out possible wormhole geometries for the movie. (iii) Combining the
proper reference frame of a camera with solutions of the geodesic equation, to
construct a light-ray-tracing map backward in time from a camera's local sky to
a wormhole's two celestial spheres. (iv) Implementing this map, for example in
Mathematica, Maple or Matlab, and using that implementation to construct images
of what a camera sees when near or inside a wormhole. (v) With the student's
implementation, exploring how the wormhole's three parameters influence what
the camera sees---which is precisely how Christopher Nolan, using our
implementation, chose the parameters for \emph{Interstellar}'s wormhole. (vi)
Using the student's implementation, exploring the wormhole's Einstein ring, and
particularly the peculiar motions of star images near the ring; and exploring
what it looks like to travel through a wormhole.Comment: 14 pages and 13 figures. In press at American Journal of Physics.
Minor revisions; primarily insertion of a new, long reference 15 at the end
of Section II.
bRing: An observatory dedicated to monitoring the Pictoris b Hill sphere transit
Aims. We describe the design and first light observations from the
Pictoris b Ring ("bRing") project. The primary goal is to detect photometric
variability from the young star Pictoris due to circumplanetary
material surrounding the directly imaged young extrasolar gas giant planet
\bpb. Methods. Over a nine month period centred on September 2017, the Hill
sphere of the planet will cross in front of the star, providing a unique
opportunity to directly probe the circumplanetary environment of a directly
imaged planet through photometric and spectroscopic variations. We have built
and installed the first of two bRing monitoring stations (one in South Africa
and the other in Australia) that will measure the flux of Pictoris,
with a photometric precision of over 5 minutes. Each station uses two
wide field cameras to cover the declination of the star at all elevations.
Detection of photometric fluctuations will trigger spectroscopic observations
with large aperture telescopes in order to determine the gas and dust
composition in a system at the end of the planet-forming era. Results. The
first three months of operation demonstrate that bRing can obtain better than
0.5\% photometry on Pictoris in five minutes and is sensitive to
nightly trends enabling the detection of any transiting material within the
Hill sphere of the exoplanet
Couette-Poiseuille flow experiment with zero mean advection velocity: Subcritical transition to turbulence
We present a new experimental set-up that creates a shear flow with zero mean
advection velocity achieved by counterbalancing the nonzero streamwise pressure
gradient by moving boundaries, which generates plane Couette-Poiseuille flow.
We carry out the first experimental results in the transitional regime for this
flow. Using flow visualization we characterize the subcritical transition to
turbulence in Couette-Poiseuille flow and show the existence of turbulent spots
generated by a permanent perturbation. Due to the zero mean advection velocity
of the base profile, these turbulent structures are nearly stationary. We
distinguish two regions of the turbulent spot: the active, turbulent core,
which is characterized by waviness of the streaks similar to traveling waves,
and the surrounding region, which includes in addition the weak undisturbed
streaks and oblique waves at the laminar-turbulent interface. We also study the
dependence of the size of these two regions on Reynolds number. Finally, we
show that the traveling waves move in the downstream (Poiseuille).Comment: 17 pages, 15 figure
Efficient Online Surface Correction for Real-time Large-Scale 3D Reconstruction
State-of-the-art methods for large-scale 3D reconstruction from RGB-D sensors
usually reduce drift in camera tracking by globally optimizing the estimated
camera poses in real-time without simultaneously updating the reconstructed
surface on pose changes. We propose an efficient on-the-fly surface correction
method for globally consistent dense 3D reconstruction of large-scale scenes.
Our approach uses a dense Visual RGB-D SLAM system that estimates the camera
motion in real-time on a CPU and refines it in a global pose graph
optimization. Consecutive RGB-D frames are locally fused into keyframes, which
are incorporated into a sparse voxel hashed Signed Distance Field (SDF) on the
GPU. On pose graph updates, the SDF volume is corrected on-the-fly using a
novel keyframe re-integration strategy with reduced GPU-host streaming. We
demonstrate in an extensive quantitative evaluation that our method is up to
93% more runtime efficient compared to the state-of-the-art and requires
significantly less memory, with only negligible loss of surface quality.
Overall, our system requires only a single GPU and allows for real-time surface
correction of large environments.Comment: British Machine Vision Conference (BMVC), London, September 201
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