Niels Jørgen Christensen

Photon maps in bidirectional Monte Carlo ray tracing of complex objects

Abstract This paper describes a bidirectional Monte Carlo ray tracing method simulating global illumination in models containing complex objects that do not have to be tessellated. The two pass method combines a first pass light ray tracing (ray casting) with a second pass optimized Monte Carlo ray tracing. In the first pass, the light emitted from the light sources hit objects in the scene and may be reflected or transmitted, a kind of backward path tracing. This step handles all kinds of reflections and not only the pecular to diffuse reflections. This turns out to be a valuable optimization. At every object-interaction, energy is stored on the surface of the object. For simple objects an illumination map is used. For complex objects e.g., procedurally based objects like fractals, energy is stored in a photon map. This new concept makes it possible to treat caustics upon such objects without having to parameterize the surface of the objects. The second pass, Monte Carlo ray tracing from the eye, visualizes the scene based upon the result from the first pass. We use the irradiance gradient method to model diffuse reflections seen directly from the eye. All secondary reflections are taken from the photon maps or the illumination maps. Only the caustic part of the ray casting step is visualized directly.

Volume sculpting using the level-set method

In this paper, we propose the use of the level-set method as the underlying technology of a volume sculpting system. The main motivation is that this leads to a very generic technique for deformation of volumetric solids. In addition, our method preserves a distance field volume representation. A scaling window is used to adapt the level-set method to local deformations and to allow the user to control the intensity of the tool. Level-set based tools have been implemented in an interactive sculpting system, and we show sculptures created using the system.

Simulating photon mapping for real-time applications

This paper introduces a novel method for simulating photon mapping for real-time applications. First we introduce a new method for selectively redistributing photons. Then we describe a method for selectively updating the indirect illumination. The indirect illumination is calculated using a new GPU acceleratedfinal gathering method and the illumination is then stored in light maps. Caustic photons are traced on the CPU and then drawn using points in the framebuffer, and finally filtered using the GPU. Both diffuse and non-diffuse surfaces can be handled by calculating the direct illumination on the GPU and the photon tracing on the CPU. We achieve real-time frame rates for dynamic scenes.

Stability Mechanisms of a Thermophilic Laccase Probed by Molecular Dynamics

Laccases are highly stable, industrially important enzymes capable of oxidizing a large range of substrates. Causes for their stability are, as for other proteins, poorly understood. In this work, multiple-seed molecular dynamics (MD) was applied to a Trametes versicolor laccase in response to variable ionic strengths, temperatures, and glycosylation status. Near-physiological conditions provided excellent agreement with the crystal structure (average RMSD ∼0.92 Å) and residual agreement with experimental B-factors. The persistence of backbone hydrogen bonds was identified as a key descriptor of structural response to environment, whereas solvent-accessibility, radius of gyration, and fluctuations were only locally relevant. Backbone hydrogen bonds decreased systematically with temperature in all simulations (∼9 per 50 K), probing structural changes associated with enthalpy-entropy compensation. Approaching T opt (∼350 K) from 300 K, this change correlated with a beginning “unzipping” of critical β-sheets. 0 M ionic strength triggered partial denucleation of the C-terminal (known experimentally to be sensitive) at 400 K, suggesting a general salt stabilization effect. In contrast, F− (but not Cl−) specifically impaired secondary structure by formation of strong hydrogen bonds with backbone NH, providing a mechanism for experimentally observed small anion destabilization, potentially remedied by site-directed mutagenesis at critical intrusion sites. N-glycosylation was found to support structural integrity by increasing persistent backbone hydrogen bonds by ∼4 across simulations, mainly via prevention of F− intrusion. Hydrogen-bond loss in distinct loop regions and ends of critical β-sheets suggest potential strategies for laboratory optimization of these industrially important enzymes.

Fast texture-based form factor calculations for radiosity using graphics hardware

Abstract This paper describes a method that speeds up hemicube rendering by using texture maps to discretize a scene instead of geometric meshing. The method avoids subdivision of input surfaces and takes advantage of hardware texture mapping in order to accelerate form factor calculations. By using the method, we are able to compute finite-element progressive radiosity significantly faster than when using traditional meshing.

INTERACTIVE MODELLING OF SHAPES USING THE LEVEL–SET METHOD

In this paper, we propose a technique for intuitive, interactive modelling of 3D shapes. The technique is based on the Level–Set Method which has the virtue of easily handling changes to the topology of the represented solid. Furthermore, this method also leads to sculpting operations that are very simple and intuitive from a user perspective. A final virtue is that the LSM makes it easy to maintain a distance field representation of the represented solid. This has a number of benefits such as simplification of the rendering scheme and the curvature computation. A number of LSM speed functions which are suitable for shape modelling are proposed. However, normally these would result in tools that would affect the entire model. To facilitate local changes to the model, we introduce a windowing scheme which constrains the LSM to affect only a small part of the model. The LSM based sculpting tools have been incorporated in our sculpting system which also includes facilities for volumetric CSG and several techniques for visualization.

Single-pass wireframe rendering

The depth offset is required because the procedure for rasterizing lines is not exactly the same as the one for rasterizing polygons. Consequently, when rasterizing a polygon edge as a line, a given fragment may have a depth value that is different from when the corresponding polygon itself is rasterized. This leads to stippling artefacts. However, adding an offset is not an ideal solution since this offset can result in disocclusions of lines that should be hidden. Moreover, there is usually stippling in any case near steep slopes in the mesh where a very large offset is sometimes required. The only real fix is a slope dependent offset but that tends to make disocclusion problems much worse. A few authors have proposed improved techniques, but either these are not intended for modern graphics hardware [Wang et al.] or they incur a performance hit [Herrel et al.].

A technique for volumetric CSG based on morphology

In this paper, a new technique for volumetric CSG is presented. The technique requires the input volumes to correspond to solids which fulfill a voxelization suitability criterion. Assume the CSG operation is union. The volumetric union ot two such volumes is defined in terms of the voxelization of the union of the two original solids. The theory behind the new technique is discussed, the algorithm and implementation are presented. Finally, we present images and timings.

Boundary correct real-time soft shadows

This paper describes a method to determine correct shadow boundaries from an area light source using umbra and penumbra volumes. The light source is approximated by a circular disk as this gives a fast way to extrude the volumes. The method also gives a crude estimate of the visibility of the area light source as seen from a point in the shadow region. Instead of rendering the volumes to the stencil buffer, we use an extended shadow map - a so-called D-buffer, which among other things stores distances from the center of the light source to the umbra and penumbra volumes. The method is suited for implementation on most programmable hardware. Though some crude approximations are used in the visibility function, the method can be used to produce soft shadows with correct boundaries in real time

Noninvasive particle sizing using camera-based diffuse reflectance spectroscopy

Diffuse reflectance measurements are useful for noninvasive inspection of optical properties such as reduced scattering and absorption coefficients. Spectroscopic analysis of these optical properties can be used for particle sizing. Systems based on optical fiber probes are commonly employed, but their low spatial resolution limits their validity ranges for the coefficients. To cover a wider range of coefficients, we use camera-based spectroscopic oblique incidence reflectometry. We develop a noninvasive technique for acquisition of apparent particle size distributions based on this approach. Our technique is validated using stable oil-in-water emulsions with a wide range of known particle size distributions. We also measure the apparent particle size distributions of complex dairy products. These results show that our tool, in contrast to those based on fiber probes, can deal with a range of optical properties wide enough to track apparent particle size distributions in a typical industrial process.