Karsten Schwenk

A modern declarative surface shader for X3D

This paper introduces a modern, declarative surface shader for the X3D standard that allows for a compact, expressive, and implementation-independent specification of surface appearance. X3Ds Material node is portable, but its feature set has become inadequate over the last years. Explicit shader programs, on the other hand, offer the expressive power to specify advanced shading techniques, but are highly implementation-dependent. The motivation for our proposal is to bridge the gap between these two worlds -- to provide X3D with renderer-independent support for modern materials and to increase interoperability with DCC tools. At the core of our proposal is the CommonSurfaceShader node. This node provides no explicit shader code, only a slim declarative interface consisting of a set of parameters with clearly defined semantics. Implementation details are completely hidden and portability is maximized. It supports diffuse and glossy surface reflection, bump mapping, and perfect specular reflection and refraction. This feature set can capture the appearance of many common materials accurately and is easily mappable to the material descriptions of other software packages and file formats. To verify our claims, we have implemented and analyzed the proposed node in three different rendering pipelines: a renderer based on hardware-accelerated rasterization, an interactive ray tracer, and a path tracer.

Practical Noise Reduction for Progressive Stochastic Ray Tracing with Perceptual Control

A proposed method reduces noise in stochastic ray tracing for interactive progressive rendering. The method accumulates high-variance light paths in a separate buffer, which is filtered by a high-quality edge-preserving filter. Then, this method adds a combination of the noisy unfiltered samples and the less noisy (but biased) filtered samples to the low-variance samples to form the final image. A novel per-pixel blending operator combines both contributions in a way that respects a user-defined threshold on perceived noise. This method can provide fast, reliable previews, even in the presence of complex features such as specular surfaces and high-frequency textures. At the same time, its consistent in that the bias due to filtering vanishes in the limit.

CommonSurfaceShader revisited: improvements and experiences

We present an improved version of the CommonSurfaceShader node, a modern declarative surface shader for X3D. The new version is better suited for physically-based rendering, has support for anisotropic surfaces, and overcomes some of the limitations the original design had for layered materials. We also present a general discussion of how the node performed in practice.

Extending a distributed virtual reality system with exchangeable rendering back-ends

We present an approach to integrate multiple rendering back-ends under a common application layer for distributed systems. The primary goal was to find a practical and nonintrusive way to use potentially very different renderers in heterogeneous computing environments without impairing their strengths and without burdening the back-ends or the application with details of the cluster environment. Our approach is based on a mediator layer that handles multithreading, clustering, and the synchronization between the application’s and the back-end’s scene. We analyze the proposed approach with an implementation for a state-of-the-art distributed VR/AR system. In particular, we present two case studies and an example application.

Adapting Precomputed Radiance Transfer to Real-time Spectral Rendering

Spectral rendering takes the full visible spectrum into account when calculating light-surface interaction and can overcome the well-known deficiencies of rendering with tristimulus color models. We present a variant of the precomputed radiance transfer algorithm that is tailored towards real-time spectral rendering on modern graphics hardware. Our method renders diffuse, self-shadowing objects with spatially varying spectral reflectance properties under distant, dynamic, full-spectral illumination. To achieve real-time frame rates and practical memory requirements we split the light transfer function into an achromatic part that varies per vertex and a wavelengthdependent part that represents a spectral albedo texture map. As an additional optimization, we project reflectance and illuminant spectra into an orthonormal basis. One area of application for our research is virtual design applications that require relighting objects with high color fidelity at interactive frame rates.

Filtering noise in progressive stochastic ray tracing

We present an improved version of a state-of-the-art noise reduction technique for progressive stochastic rendering. Our additions make the method significantly faster at the cost of an acceptable loss in quality. Additionally, we improve the robustness of the method in the presence of difficult features like glossy reflection, caustics, and antialiased edges. We show with visual and numerical comparisons that our extensions improve the overall performance of the original approach and make it more broadly applicable.

A System Architecture for Flexible Rendering Back-ends in Distributed Virtual Reality Applications

We describe an approach to integrate multiple rendering back-ends under a common application layer for distributed systems. The primary goal was to find a practical and non-intrusive way to use potentially very different rendering back-ends in heterogeneous computing environments without impairing their strengths and without burdening the back-ends or the application with details of the cluster environment. Our approach is based on a mediator layer that can be plugged into the OpenSG infrastructure. This design allows us to elegantly extent OpenSGs low-level multithreading and clustering capabilities to our system. It also allows us to sync incremental changes very efficiently. We demonstrate results and problems with two case studies.

CommonVolumeShader: simple and portable specification of volumetric light transport in X3D

Rendering volumetric phenomena with believable appearance can add tremendous realism to virtual scenes. We introduce the CommonVolumeShader node, an extension of the X3D standard which has been specifically designed for physically-based rendering of participating media. CommonVolumeShader allows content authors to specify optical properties in a concise and purely declarative way and can accurately capture the appearance of many volumetric phenomena. We demonstrate results with implementations for an interactive ray tracer and a rasterization-based pipeline.

Radiance filtering for interactive path tracing

We present a method for noise reduction that is especially tailored to interactive progressive path tracing (PT). The idea is to exploit spatial coherence in the image and reuse information from neighboring pixels. However, in contrast to image filtering techniques (e.g. [Schwenk et al. 2012]), we do not simply filter pixel values or samples of outgoing radiance. Instead, we only reuse the incident indirect radiance of neighboring pixels in a radiance estimation step with a shrinking kernel similar to stochastic progressive photon mapping (SPPM) [Hachisuka and Jensen 2009]. This novel approach significantly reduces the variance in indirect lighting without blurring details in geometry or texture. In equal time comparisons we often achieve higher quality than previous approaches. The primary use case of our algorithm is to provide fast, reliable previews of global illumination. It is also consistent, retains the conceptual simplicity of PT, is orthogonal to importance and stratified sampling, and is easy to integrate into existing renderers.

Precomputed Radiance Transfer as a Variance Reduction Technique -- A Small Case Study

We present a small case study in which we evaluate the potential of the precomputed radiance transfer (PRT) algorithm as a means of variance reduction in path tracing. We describe how to use PRT solutions as correlated functions for both correlated and importance sampling in a straightforward way and study how each approach reduces noise in images rendered with path tracing.