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Captured reflectance fields tend to provide a relatively coarse
sampling of the incident light directions. As a result, sharp
illumination features, such as highlights or shadow boundaries, are
poorly reconstructed during relighting; highlights are disconnected,
and shadows show banding artefacts. In this paper, we propose a novel
interpolation technique for 4D reflectance fields that reconstructs
plausible images even for non-observed light
directions. Given a sparsely sampled reflectance field, we can
effectively synthesize images as they would have been obtained from denser
sampling.
The processing pipeline consists of three steps: (1) segmentation of
regions where intermediate lighting cannot be obtained by blending,
(2) appropriate flow algorithms for highlights and shadows, plus (3) a final
reconstruction technique that uses image-based priors to faithfully
correct errors that might be introduced by the segmentation or flow
step. The algorithm reliably reproduces scenes that contain specular
highlights, interreflections, shadows or caustics.
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Translucent objects pose a difcult problem for traditional
structured light 3D scanning techniques. Subsurface
scattering corrupts the range estimation in two ways: by
drastically reducing the signal-to-noise ratio and by shifting
the intensity peak beneath the surface to a point which
does not coincide with the point of incidence. In this paper
we analyze and compare two descattering methods in order
to obtain reliable 3D coordinates for translucent objects.
By using polarization-difference imaging, subsurface scattering
can be ltered out because multiple scattering randomizes
the polarization direction of light while the surface
reectance partially keeps the polarization direction of the
illumination. The descattered reectance can be used for reliable
3D reconstruction using traditional optical 3D scanning
techniques, such as structured light. Phase-shifting is
another effective descattering technique if the frequency of
the projected pattern is sufciently high. We demonstrate
the performance of these two techniques and the combination
of them on scanning real-world translucent objects.
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In this project we address the problem of reconstructing the 3D structure of planetary nebulae from 2D observations. Assuming axial symmetry, our method jointly reconstructs the distribution of dust and ionized gas in the nebulae from observations at two different wavelengths. In an inverse rendering framework we optimize for the emission and absorption densities which are correlated to the gas and dust distribution present in the nebulae. First, the density distribution of the dust component is estimated based on an infrared image, which traces only the dust distribution due to its intrinsic temperature. In a second step, we optimize for the gas distribution by comparing the rendering of the nebula to the visible wavelength image. Using our method we recover both gas and dust density distributions present in the nebula by exploiting the distinct absorption or emission parameters at different wavelengths.
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Multiperspective images generated from a collection of
photographs or a videostream can be used to effectively
summarize long, roughly planar scenes such as city
streets. The final image can span a larger field of view than any
single input image. However, common projections used to make these
images, including the cross-slits and pushbroom projections, may
suffer from depth-related distortions in non-planar scenes. In this
paper, we propose a metric for evaluating the distortion in these
images due to deviation from the standard perspective
projection. Minimizing this error metric we can automatically define
the picture surface and viewpoints of a multiperspective image to
reduce the distortion artifacts. This optimization requires only a
coarse estimate of scene geometry, which can be provided as a depth
map or in the form of a 2D spatial importance map defining interesting
parts of the scene. These maps can be automatically constructed in
many cases, allowing rapid generation of images of very long scenes.
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We present a novel technique called symmetric photography to capture real world reflectance fields. The technique models the 8D reflectance field as a transport matrix between the 4D incident light field and the 4D exitant light field. It is a challenging task to acquire this transport matrix due to its large size. Fortunately, the transport matrix is symmetric and often data-sparse. Symmetry enables us to measure the light transport from two sides simultaneously, from the illumination directions and the view directions. Data-sparseness refers to the fact that sub-blocks of the matrix can be well approximated using low-rank representations. We introduce the use of hierarchical tensors as the underlying data structure to capture this data-sparseness, specifically through local rank-1 factorizations of the transport matrix. Besides providing an efficient representation for storage, it enables fast acquisition of the approximated transport matrix and fast rendering of images from the captured matrix. Our prototype acquisition system consists of an array of mirrors and a pair of coaxial projector and camera. We demonstrate the effectiveness of our system with scenes rendered from reflectance fields that were captured by our system. In these renderings we can change the viewpoint as well as relight using arbitrary incident light fields. |
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We present a novel photographic technique called dual
photography, which exploits Helmholtz reciprocity to
interchange the lights and cameras in a scene. With a video projector
providing structured illumination, reciprocity permits us to generate
pictures from the viewpoint of the projector, even though no camera
was present at that location. The technique is completely image-based,
requiring no knowledge of scene geometry or surface properties, and by
its nature automatically includes all transport paths, including
shadows, interre ections and caustics. In its simplest form, the
technique can be used to take photographs without a camera; we
demonstrate this by capturing a photograph using a projector and a
photo-resistor. If the photo-resistor is replaced by a camera, we can
produce a 4D dataset that allows for relighting with 2D incident
illumination. Using an array of cameras we can produce a 6D slice of
the 8D reflectance field that allows for relighting with arbitrary light
elds. Since an array of cameras can operate in parallel without
interference, whereas an array of light sources cannot, dual
photography is fundamentally a more ef cient way to capture such a 6D
dataset than a system based on multiple projectors and one camera. As
an example, we show how dual photography can be used to capture and
relight scenes.
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Translucent objects are characterized by diffuse light scattering beneath the object's surface. Light enters and leaves an object at possibly distinct surface locations. This paper presents the first method to acquire this transport behavior for arbitrary inhomogeneous objects. Individual surface points are illuminated in our DISCO measurement facility and the object's impulse response is recorded with a high-dynamic range video camera. The acquired data is resampled into a hierarchical model of the object's light scattering properties. Missing values are consistently interpolated resulting in measurement-based, complete and accurate representations of real translucent objects which can be rendered with various algorithms. |
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Measuring reflection properties of a 3D object involves capturing images for numerous viewing and lighting directions. We present a method to select advantageous measurement directions based on analyzing the estimation of the bi-directional reflectance distribution function (BRDF). The selected directions minimize the uncertainty in the estimated parameters of the BRDF. As a result, few measurements suffice to produce models that describe the reflectance behavior well. Moreover, the uncertainty measure can be computed fast on modern graphics cards by exploiting their capability to render into a floating-point frame buffer. This forms the basis of an acquisition planner capable of guiding experts and non-experts alike through the BRDF acquisition process. We demonstrate that spatially varying reflection properties can be captured more efficiently for real-world applications using our acquisition planner. |
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The use of realistic models for all components of images synthesis is a fundamental prerequisite for photorealistic rendering. The generation of these models in a manual process often becomes infeasible as the demand for visual complexity increases steadily. In this sketch we concentrate on the acquisition of realistic materials. In particular, we describe an acquisition method for shift-variant BRDFs, i.e., a specific BRDF for each surface point. |
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First author: Katja Daubert
Realistic modeling and high-performance rendering of cloth and
clothing is a challenging problem. Often these materials are seen at
distances where individual stitches and knits can be made out and
need to be accounted for. Modeling of the geometry at this level of
detail fails due to sheer complexity, while simple texture mapping
techniques do not produce the desired quality. In this paper, we describe an efficient and realistic approach that
takes into account view-dependent effects such as small
displacements causing occlusion and shadows, as well as illumination
effects.
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Concurrent rendering applications crave for more and more realistic models including both geometry and texture. Within this project methods have been developed to register a 2D image with a 3D model of an object, which allows to map the image precisely onto the model. Multiple images can be combined to texture the entire surface, blending between the images. |
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Supervisor: Wolfgang HeidrichThe goal of this project (my diploma thesis from February 1999 to July 1999) was to represent view-dependend reflecting and refracting properties of objects by a light field data structure in such a way that the object may be placed in arbitrary environments. Additionally, hardware accelerated rendering of vector-quantized light fields has been investigated. |
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Supervisors:
Marc Stamminger,
Wolfgang Heidrich During my senior thesis (Studienarbeit) the applicability of Affine Arithmetic in order to bound quantities within an hierarchical radiosity algorithm has been investigated. |
