Tom Cuypers

Reflectance model for diffraction

We present a novel method of simulating wave effects in graphics using ray-based renderers with a new function: the Wave BSDF (Bidirectional Scattering Distribution Function). Reflections from neighboring surface patches represented by local BSDFs are mutually independent. However, in many surfaces with wavelength-scale microstructures, interference and diffraction requires a joint analysis of reflected wavefronts from neighboring patches. We demonstrate a simple method to compute the BSDF for the entire microstructure, which can be used independently for each patch. This allows us to use traditional ray-based rendering pipelines to synthesize wave effects. We exploit the Wigner Distribution Function (WDF) to create transmissive, reflective, and emissive BSDFs for various diffraction phenomena in a physically accurate way. In contrast to previous methods for computing interference, we circumvent the need to explicitly keep track of the phase of the wave by using BSDFs that include positive as well as negative coefficients. We describe and compare the theory in relation to well-understood concepts in rendering and demonstrate a straightforward implementation. In conjunction with standard raytracers, such as PBRT, we demonstrate wave effects for a range of scenarios such as multibounce diffraction materials, holograms, and reflection of high-frequency surfaces.

Depth from sliding projections

In this paper we present a novel method for 3D structure acquisition, based on structured light. Unlike classical structured light methods, in which a static projector illuminates a scene with time-varying illumination patterns, our technique makes use of a moving projector emitting a static striped illumination pattern. This projector is translated at a constant velocity, in the direction of the projectors horizontal axis. Illuminating the object in this manner allows us to perform a per pixel analysis, in which we decompose the recorded illumination sequence into a corresponding set of frequency components. The dominant frequency in this set can be directly converted into a corresponding depth value. This per pixel analysis allows us to preserve sharp edges in the depth image. Unlike classical structured light methods, the quality of our results is not limited by projector or camera resolution, but is solely dependent on the temporal sampling density of the captured image sequence. Additional benefits include a significant robustness against common problems encountered with structured light methods, such as occlusions, specular reflections, subsurface scattering, interreflections, and to a certain extent projector defocus.

Gloss and Normal Map Acquisition of Mesostructures Using Gray Codes

We propose a technique for gloss and normal map acquisition of fine-scale specular surface details, or mesostructure. Our main goal is to provide an efficient, easily applicable, but sufficiently accurate method to acquire mesostructures. We therefore employ a setup consisting of inexpensive and accessible components, including a regular computer screen and a digital still camera. We extend the Gray code based normal map acquisition approach of Francken et al. [1] which utilizes a similar setup. The quality of the original method is retained and without requiring any extra input data we are able to extract per pixel glossiness information. In the paper we show the theoretical background of the method as well as results on real-world specular mesostructures.

Fast Normal Map Acquisition Using an LCD Screen Emitting Gradient Patterns

We propose an efficient technique for normal map acquisition, using a cheap and easy to build setup. Our setup consists solely of off-the-shelf components, such as an LCD screen, a digital camera and a linear polarizer filter. The LCD screen is employed as a linearly polarized light source emitting gradient patterns, whereas the digital camera is used to capture the incident illumination reflected off the scanned objects surface. Also, by exploiting the fact that light emitted by an LCD screen has the property of being linearly polarized, we use the filter to surpress any specular highlights. Based on the observed Lambertian reflection of only four different light patterns, we are able to obtain a detailed normal map of the scanned surface. Overall, our techniques produces convincing results, even on weak specular materials.

Validity of Wigner Distribution Function for ray-based imaging

In this work we provide an introduction to the Wigner Distribution Function (WDF) using geometric optics principles. The WDF provides a useful model of wave-fields, allowing simulation of diffraction and interference effects. We attempt to explain these Fourier optics concepts to computational photography researchers by clarifying the relationship between the WDF and position-angle representations. We demonstrate how the WDF can be used to simulate diffraction effects using a light field representation and discuss its validity in the near-field, far-field, and under the paraxial approximation. Finally, we demonstrate that although the WDF representation contains negative values, any projection always yields a non-negative intensity value.

Mesostructure from specularity using gradient illumination

We present a method to efficiently acquire specular mesostructure normal maps, only making use of off-the-shelf components, such as a digital still camera, an LCD screen and a linear polarizing filter. Where current methods require a specialized setup, or a considerable number of input images, we only need a cheap setup to maintain a similar level of quality. We verify the presented theory on real world examples, and provide a ground truth evaluation on photorealistic synthetic data.

Depth from Encoded Sliding Projections

We present a novel method for 3D shape acquisition, based on mobile structured light. Unlike classical structured light methods, in which a static projector illuminates the scene with dynamic illumination patterns, mobile structured light employs a moving projector translated at a constant velocity in the direction of the projectors horizontal axis, emitting static or dynamic illumination. For our approach, a time multiplexed mix of two signals is used: (1) a wave pattern, enabling the recovery of point-projector distances for each point observed by the camera, and (2) a 2D De Bruijn pattern, used to uniquely encode a sparse subset of projector pixels. Based on this information, retrieved on a per (camera) pixel basis, we are able to estimate a sparse reconstruction of the scene. As this sparse set of 2D-3D camera-scene correspondences is sufficient to recover the camera location and orientation within the scene, we are able to convert the dense set of point-projector distances into a dense set of camera depths, effectively providing us with a dense reconstruction of the observed scene. We have verified our technique using both synthetic and real-world data. Our experiments display the same level of robustness as previous mobile structured light methods, combined with the ability to accurately estimate dense scene structure and accurate camera/projector motion without the need for prior calibration.

Shadow multiplexing for real-time silhouette extraction

In this work we propose a real-time implementation for efficient extraction of multi-viewpoint silhouettes using a single camera. The method is based on our previously presented proof-of-concept shadow multiplexing method. We replace the cameras of a typical multi-camera setup with colored light sources and capture the multiplexed shadows. Because we only use a single camera, our setup is much cheaper than a classical setup, no camera synchronization is required, and less data has to be captured and processed. In addition, silhouette extraction is simple as we are segmenting the shadows instead of the texture of objects and background. Demultiplexing runs at 40 fps on current graphics hardware. Therefore this technique is suitable for real-time applications such as collision detection. We evaluate our method on both a real and a virtual setup, and show that our technique works for a large variety of objects and materials.

WBSDF for simulating wave effects of light and audio

Diffraction is a common phenomenon in nature when dealing with small scale occluders. It can be observed on biological surfaces, such as feathers and butterfly wings, and man--made objects like rainbow holograms. In acoustics, the effect of diffraction is even more significant due to the much longer wavelength of sound waves. In order to simulate effects such as interference and diffraction within a ray--based framework, the phase of light or sound waves needs to be integrated.