Shadow Volume Research Articles

Study on Outdoor Soft Shadow Generation Method for Mobile Devices

This paper proposes the method of soft shadow generation by extending the shadow volume to generate outdoor 3DCG images. In our method, two parameters, “brightness of shadow” and “soft shadow range”, are introduced to enable various soft shadows in different time zones. This algorithm aims to mount our method onto portable devices, so that the shadow texture for a soft shadow is generated and a soft shadow can be generated with the processing that does not depend on hardware specifications.

An Improved Physically‐Based Soft Shadow Volume Algorithm

Abstract We identify and analyze several performance problems in a state‐of‐the‐art physically‐based soft shadow volume algorithm, and present an improved method that alleviates these problems by replacing an overly conservative spatial acceleration structure by a more efficient one. The new technique consistently outperforms both the previous method and a ray tracing‐based reference solution in several realistic situations while retaining the correctness of the solution and other desirable characteristics of the previous method. These include the unintrusiveness of the original algorithm, meaning that our method can be used as a black‐box shadow solver in any offline renderer without requiring multiple passes over the image or other special accommodation. We achieve speedup factors from 1.6 to 12.3 when compared to the previous method. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Three‐Dimensional Graphics and Realism]: Shadowing

Minimizing CPU usage in soft shadow volumes algorithm

In this paper we present methods for reducing the CPU burden associated with the original implementation of the hardware-accelerated soft shadow volumes algorithm. We propose and implement two hard...

N‐Buffers for efficient depth map query

We introduce the N-buffer as a tool for multiresolution depth map representation. This neighborhood buffer encodes the value and position of local depth extrema at different scales in an image cube, in contrast to the image pyramid. The resulting increase in storage space is largely compensated by the following benefits: objects of any size can be culled in constant time against an occlusion map using four depth lookups; visibility-like queries can be performed in vertex and fragment programs; N-buffers can be computed very efficiently with graphics hardware. We present three applications of this datastructure, and in particular a novel approach for shadow volume acceleration.

Occlusion culling and z-fail for soft shadow volume algorithms

This paper presents a significant improvement of our previously proposed soft shadow volume algorithm for simulating soft shadows. By restructuring the algorithm, we can considerably simplify the computations, introduce efficient occlusion culling with speedups of 3–4 times, thus approaching real-time performance. We can also generalize the algorithm to produce correct shadows even when the eye is inside a shadowed region (using z-fail). We present and evaluate a three pass implementation of the restructured algorithm for near real-time rendering of soft shadows on a computer with a commodity graphics accelerator. However, preferably the rendering of the wedges should be implemented in hardware, and for this we suggest and evaluate a single pass algorithm.

Observations on Silhouette Sizes

Silhouettes have many applications in computer gr aphics such as non-photorealistic edge rendering, fur rendering, shadow volume creation, a nd anti-aliasing. The number of edges, s, in the silhouette of a model observed from a point is ther efore useful in analyzing such algorithms. This paper examines, from a theoretical viewpoi nt, a menagerie of objects with interesting silhouettes (including those with minimal and maximal silhouettes). It shows that the relationship between and s and the number of triangles in a model, f, is bounded above by s = O( f) and below by s = Ω(1), and that the expected value of s over all observation points at infinity is proport ional to the sum of the dihedral angles. In practice, the models used with silhouette-ba sed rendering algorithms are triangle meshes that a re manually constructed or result from scans of human- made objects. They consist of only surface geometry with few cracks; there is no internal deta il like the engine under a car's hood. Geometric and aesthetic constraints on these models appear to create an inherent relationship between f and s. Measurements of the actual silhouettes of real-worl d 3D models with polygon counts varied across six orders of magnitude show them to follow the relatio nship s ~ f 0.8 . Furthermore, the expected value of s at infinity is a good approximation of the expecte d silhouette size for a viewer at a finite location.

Shadow Volumes on Programmable Graphics Hardware

Abstract One of the best choices for fast, high quality shadows is the shadow volume algorithm. However, for real timeapplications the extraction of silhouette edges can significantly burden the CPU, especially with highly tessellatedinput geometry or when complex geometry shaders are applied. In this paper we show how this last, expensive part of the shadow volume method can be implemented on programmablegraphics hardware. This way, the originally hybrid shadow volumes algorithm can now be reformulatedas a purely hardware‐accelerated approach. The benefits of this implementation is not only the increase in speed. Firstly, all computations now run on thesame hardware resulting in consistent precision within all steps of the algorithm. Secondly, programmable vertextransformations are no longer problematic when applied to shadow casting objects. Categories and Subject Descriptors (according to ACM CCS): I.3.1 [Computer Graphics]: Hardware Architecture;I.3.3 [Computer Graphics]: Picture/Image Generation; I.3.7 [Computer Graphics]: Three‐Dimensional Graphicsand Realism

A geometry-based soft shadow volume algorithm using graphics hardware

Most previous soft shadow algorithms have either suffered from aliasing, been too slow, or could only use a limited set of shadow casters and/or receivers. Therefore, we present a strengthened soft shadow volume algorithm that deals with these problems. Our critical improvements include robust penumbra wedge construction, geometry-based visibility computation, and also simplified computation through a four-dimensional texture lookup. This enables us to implement the algorithm using programmable graphics hardware, and it results in images that most often are indistinguishable from images created as the average of 1024 hard shadow images. Furthermore, our algorithm can use both arbitrary shadow casters and receivers. Also, one version of our algorithm completely avoids sampling artifacts which is rare for soft shadow algorithms. As a bonus, the four-dimensional texture lookup allows for small textured light sources, and, even video textures can be used as light sources. Our algorithm has been implemented in pure software, and also using the GeForce FX emulator with pixel shaders. Our software implementation renders soft shadows at 0.5--5 frames per second for the images in this paper. With actual hardware, we expect that our algorithm will render soft shadows in real time. An important performance measure is bandwidth usage. For the same image quality, an algorithm using the accumulated hard shadow images uses almost two orders of magnitude more bandwidth than our algorithm.

Shadow silhouette maps

The most popular techniques for interactive rendering of hard shadows are shadow maps and shadow volumes . Shadow maps work well in regions that are completely in light or in shadow but result in objectionable artifacts near shadow boundaries. In contrast, shadow volumes generate precise shadow boundaries but require high fill rates. In this paper, we propose the method of silhouette maps , in which a shadow depth map is augmented by storing the location of points on the geometric silhouette. This allows the shader to construct a piecewise linear approximation to the true shadow silhouette, improving the visual quality over the piecewise constant approximation of conventional shadow maps. We demonstrate an implementation of our approach running on programmable graphics hardware in real-time.

On the Degree of Vertices in a Shadow Volume Silhouette

In shadow volume rendering, the shadow volume silhouette edges are used to create primitives that model the shadow volume. A common misconception is that the vertices on such silhouettes can only be connected to two silhouette edges, i.e., have degree two. Furthermore, some believe that such a vertex can have any degree. In this short note, we present a geometric proof that shows that the degree of a silhouette vertex must be even, and not necessarily two.

Optimized Shadow Mapping Using the Stencil Buffer

Abstract Shadow maps and shadow volumes are common techniques for computing real-time shadows. We optimize the performance of a hardware-accelerated shadow mapping algorithm by rasterizing the light frustum into the stencil buffer, in a manner similar to the shadow volume algorithm. The pixel shader code that performs shadow tests and illumination computations is applied only to the pixels that are inside the light frustum. We also use deferred shading to further limit the operations to visible pixels. Our technique can be easily plugged into existing applications, and is especially useful for dynamic scenes that contain several local light sources. In our test scenarios, the overall frame rate was up to 2.2 times higher than for our comparison methods.

Dose effect of guidewire position in intravascular brachytherapy

It has been reported that the dose effects of metallic guidewires are significant in intravascular brachytherapy (IVBT) using a beta source. The purpose of this work is to investigate the dependence of these dose effects on guidewire position. The EGS4 Monte Carlo codes were used to perform the dose calculations for the 90Sr (NOVOSTE), 32P (Guidant) and 192Ir (BEST Ind.) sources with and without a guidewire in place. Guidewires were placed at various distances from the central axes of the sources. Due to the attenuation by the guidewires, a dose reduction of up to 70% behind a guidewire was observed for the beta sources, while the dose perturbation was found to be negligible for the gamma source. The dose reduction for the beta sources was found to be dependent on the guidewire location. For example, the dose reduction was 10% higher for a stainless steel guidewire located at 0.5 mm than that for the guidewire at 2 mm from the central axis of the source. The portion of the target volume affected (shadowed) dosimetrically by the guidewire was reduced when the guidewire was positioned farther away from the source. The shadow volume (in which the dose reduction occurs) can be reduced by up to 45% as the guidewire is moved away from the source axis from 0.5 mm to 2 mm. The dosimetric perturbations due to the presence of a metallic guidewire as well as their dependence on guidewire location should be considered in designing a new IVBT delivery device, in analysing the treatment efficacy, and/or in dose prescription for a beta source.

AN OBJECT-ORIENTED APPROACH TO SHADOW GENERATION IN COMPUTER GRAPHICS

This paper presents an object-oriented shadow generation algorithm for a large number of convex polyhedra (objects) in a 3-D scene. In the past, the shadow volume binary space partitioning (SVBSP) tree algorithm is known to be efficient in a static environment in which the point light source can be changed. However, if a scene consists of many objects and each object consists of many polygons, the time complexity for generating and traversing an SVBSP tree increases rapidly because the SVBSP tree algorithm only deals with polygons which are components of objects. Furthermore, the SVBSP tree algorithm suffers from polygon-splitting problems resulting in a high cost as the number of polygons increases. Our approach is object-oriented in the sense that an object is used as the basic logical unit instead of polygons. In the preprocessing stage the object ordering for shadow generation is determined for all possible light source positions. At the run time, the shadow detection algorithm is executed, and if necessary, shadow fragments are generated. We also present an approach to retrieving, updating, and displaying a 3-D static or dynamic world which consists of a large number of objects.

Shadow volume reconstruction from depth maps

Current graphics hardware can be used to generate shadows using either the shadow volume or shadow map techniques. However, the shadow volume technique requires access to a representation of the scence as a polygonal model, and handling the near plane clip correctly and efficiently is difficult; conversely, accurate shadow maps require high-precision texture map data representations, but these are not widely supported. We present a hybird of the shadow map and shadow volume approaches which does not have these difficulties and leverages high-performance polygon rendering. The scene is rendered from the point of view of the light source and a sampled depth map is recovered. Edge detection and a template-based reconstruction technique are used to generate a global shadow volume boundary surface, after which the pixels in shadow can be marked using only a one-bit stencil buffer and a single-pass rendering of the shadow volume boundary polygons. The simple form of our template-based reconstruction scheme simplifies capping the shadow volume after the near plane clip.

Linear-Time Algorithms for Finding the Shadow Volumes from a Convex Area Light Source

An area light source in three-dimensional space shines past a scene polygon to generate two types of shadow volumes for each scene polygon, i.e., one with partial occlusion and the other with complete occlusion. These are called penumbra and umbra, respectively. In this paper we propose linear-time algorithms for computing the penumbra and the umbra of a scene polygon from an area light source, respectively.

Pipeline rendering: interactive refractions, reflections and shadows

A coordinated use of hardware-provided bitplanes and rendering pipelines can create ray-trace quality illumination effects in real time. We provide recursive reflections through the use of secondary viewpoints, and present a method for using a homogeneous 2D projective image mapping to extend this method for refractive surface. We extend the traditional use of shadow volumes to provide reflected and refracted shadows as well as reflected and refracted lighting. A shadow blending technique is demonstrated, and the shadow and lighting effects are incorporated into our recursive viewpoint paradigm. Finally, we incorporate material properties including a translucency model to provide a general framework for creating physically approximate renderings. These techniques are immediately applicable to area such as 3D modelling, animation and interactive environments to produce more realistic images in real time.

A comparison of three shadow volume algorithms

This paper describes and compares three different approaches to computing shadows. Each is based on the idea of shadow volumes: basic algorithm, Shadow Volume BSP Tree algorithm, and Shadow Tiling, based on 2D space subdivision. Binary Space Partition trees are used to organise the polygons in the scene in a front-toback order from the point of view of the light source, and then shadows on a polygon are computed by clipping the polygon to the shadow volumes of polygons closer to the light source. The three algorithms differ in their approach to minimising the number of comparisons of a polygon with the shadow volumes of its predecessors, and one of the algorithms represents the total shadow volume itelf by a BSP tree. The algorithms are compared analytically and statistically.

A shadow algorithm for CSG

An algorithm is presented for constructing shadow volumes for CSG objects. For each primitive solid, the algorithm calculates a tree of shadow volumes to model the shadows generated by the shadow-generating parts of the boundary of the primitive. The shadow of the complete CSG object is the union of these shadow trees. An implementation of the algorithm is described for a scan-line display algorithm for CSG objects with polygonal primitives.

Near real-time shadow generation using BSP trees

This paper describes an object-space shadow generation algorithm for static polygonal environments illuminated by movable point light sources. The algorithm can be easily implemented on any graphics system that provides fast polygon scan-conversion and achieves near real-time performance for environments of modest size. It combines elements of two kinds of current shadow generation algorithms: two-pass object-space approaches and shadow volume approaches. For each light source a Binary Space Partitioning (BSP) tree is constructed that represents the shadow volume of the polygons facing it. As each polygon's contribution to a light source's shadow volume is determined, the polygon's shadowed and lit fragments are computed by filtering it down the shadow volume BSP tree. The polygonal scene with its computed shadows can be rendered with any polygon-based visible-surface algorithm. Since the shadow volumes and shadows are computed in object space, they can be used for further analysis of the scene. Pseudocode is provided, along with pictures and timings from an interactive implementation.

Smooth appearance for polygonal surfaces

Current shading methods for polygonal approximations to smooth surfaces reveal the polygon edges by lack of smoothness in the shading, highlights, texture, reflection lines, intersections, profiles, and shadows. These defects are eliminated here by interpolating the perspective depth in screen coordinates with aC1 piecewise-polynomial function, defined over triangles in the screen plane. The three eye-space components of the surface normal, and the two texture parameters, are separately interpolated with similarC1 functions. For smooth intersections, the depth comparison test of Watkins' scan-line algorithm is replaced by a simple test which uses closed-form solutions to polynomials of degree 2, 3, or 4. When the scan lines arise from scan planes containing the eye and the light source, a version of the shadow volume algorithm can also generate smooth cast shadows.