Johannes Hirche

Adaptive view dependent tessellation of displacement maps

Displacement Mapping is an effective technique for encoding the high levels of detail found in todays triangle based surface models. Extending the hardware rendering pipeline to be capable of handling displacement maps as geometric primitives, will allow highly detailed models to be constructed without requiring large numbers of triangles to be passed from the CPU to the graphics pipeline. We present a new approach based on recursive tessellation that adapts to the surface complexity described by the displacement map. We also ensure that the resolution of the displaced mesh is tessellated with respect to the current view point. Our tessellation scheme performs all tests only on triangle edges to avoid generating cracks on the displaced surface. The main decision for vertex insertion is based on two comparisons involving the average height surrounding the vertices and the normals at the vertices. Individually, the tests will fail to tessellate a mesh satisfactorily, but their combination achieves good results. We propose several additions to the typical hardware rendering pipeline in order to achieve displacement map rendering in hardware. The mesh tessellation is placed within the rendering pipeline so that we can take advantage of the pre-existing vertex transformation units to perform the setup calculations for our view dependent test. Our method adds only simple arithmetic and comparison operations to the graphics pipeline and makes use of existing units for calculations wherever possible.

Hardware support for adaptive subdivision surface rendering

Adaptive subdivision of triangular meshes is highly desirable for surface generation algorithms including adaptive displacement mapping in which a highly detailed model can be constructed from a coarse triangle mesh and a displacement map. The communication requirements between the CPU and the graphics pipeline can be reduced if more detailed and complex surfaces are generated, as in displacement mapping, by an adaptive tessellation unit which is part of the graphics pipeline. Generating subdivision surfaces requires a large amount of memory in whicmultiple arbitrary accesses are required to neighbouring vertices to calculate the new vertices. In this paper we present a meshing scheme and new architecture for the implementation of adaptive subdivision of triangular meshes that allows for quick access using a small memory making it feasible in hardware, while at the same time allowing for new vertices to be adaptively inserted. The architecutre is regular and characterized by an efficient data management that minimizes the data storage and avoids the wait cycles that would be associated with the multiple data accesses required for traditional subdivision. This architecture is presented as an improvement for adaptive displacement mapping algorithms, but could also be used for adaptive subdivision surface generation in hardware.

VIZARD II: a reconfigurable interactive volume rendering system

This paper presents a reconfigurable, hardware accelerated, volume rendering system for high quality perspective ray casting. The volume rendering accelerator performs ray casting by calculating the path of the ray through the volume using a programmable Xilinx Virtex FPGA which provides fast design changes and low cost development. Volume datasets are stored on the card in low profile DIMMs with standard connectors allowing both, large datasets up to 1 GByte with 32 bit per voxel, and easy upgrades to larger memory capacities. Per-sample Phong shading and post-classification is performed in hardware, giving immediate feedback to changes in the visualization of a dataset. Adding new features, such as pre-integrated classification, can be accomplished using the existing card without expensive and time consuming redesigns. The card can also be used for medical image reconstruction by reconfiguring the FPGA broadening its usefulness for end users. For the first time, users are able to generate high quality perspective images as required for applications such as virtual endoscopy and colonoscopy, and stereoscopic image generation.

VoxelCache: a cache-based memory architecture for volume graphics

This paper presents a cache-based memory architecture for volume graphics. We describe the memory organization and cache logic to implement a voxel cache based on 43 voxel blocks. We show an efficient prefetching scheme that increases the cache hit ratio to more than 98% in most cases. The performance of the memory system with different types of external memory is demonstrated by a cycle accurate C++ simulation. The VoxelCache memory architecture is designed to be easily adapted to different memory technologies, because all volume graphics specific parts of the memory system are encapsulated inside the on-chip cache. The design is targeted at implementation on off-the-shelf reconfigurable hardware.

A meshing scheme for efficient hardware implementation of butterfly subdivision using displacement mapping

Displacement mapping is an effective technique for encoding the high levels of detail of surface models using coarse triangle meshes and displacement maps. These maps are 2D representations containing the distances between the coarse mesh and the surface to represent. Displacement maps have been used in many applications such as ray tracing, image warping, and volume rendering. In this article, we propose modifications to our previous grouping strategy, a new subdivision strategy based on the Modified Butterfly algorithm and new heuristics for the adaptive subdivision procedure, and, finally, the corresponding modifications on our hardware proposal. A meshing scheme and an adaptive subdivision strategy based on displacement mapping reduce the bottleneck between the CPU and graphics pipeline common in high-performance graphics systems.

Efficient space leaping for ray casting architectures

One of the most severe problems for ray casting architectures is the waste of computation cycles and I/O bandwidth, due to redundant sampling of empty space. While several techniques exist for software implementations to skip these empty regions, few are suitable for hardware implementation. The few which have been presented either require a tremendous amount of logic or are not feasible for high frequency designs (i.e. running at 100 MHz) where latency is the one of the biggest issues. In this paper, we present an efficient space leaping mechanism which requires only a small amount of SRAM (4 Kbit for a 2563 volume) and can be easily integrated into ray casting architectures. For each sub-cube of the volume, a bit is stored in an occupancy map, which can be generated in real-time, using the VIZARD II architecture. Hence, space leaping can be classification dependent achieving yet another significant speed-up over skipping only the empty space (voxel = 0). Using a set of real-world datasets, we show that frame-rates well above 15 frames per second can be accomplished for the VIZARD II architecture.

VIZARD II: An FPGA-based Interactive Volume Rendering System

In this paper we present a volume rendering system that implements a Direct Volume Rendering algorithm on a Xilinx FPGA being capable of visualizing 3D-datasets with highest image quality at interactive frame rate. The volume renderer utilizes a cache optimized memory scheme for maximum memory bandwidth and a fully pipelined architecture of the computational expensive rendering calculations. The used ray-casting algorithm was adapted in critical parts to fit the specific need of an efficient hardware usage, with respect to available resources and computational power, without limiting rendering features.Using a FPGA approach offers full flexibility to the implementation of the algorithm making it easy to adapt and extend new features to the rendering pipeline without the need of time consuming redesigns, especially important in a scientific environment.