This tutorial survey paper reviews several different models for light interaction with volume densities of absorbing, glowing, reflecting, and/or scattering material. They are, in order of increasing realism, absorption only, emission only, emission and absorption combined, single scattering of external illumination without shadows, single scattering with shadows, and multiple scattering. For each model the paper provides the physical assumptions, describes the applications for which it is appropriate, derives the differential or integral equations for light transport, presents calculation methods for solving them, and shows output images for a data set representing a cloud. Special attention is given to calculation methods for the multiple scattering model. >

Optical models for direct volume rendering

https://cg.informatik.uni-freiburg.de/intern/seminar/surfaceReconstruction_survey%20of%20marching%20cubes.pdf

A survey of the marching cubes algorithm

OpenGLand its extensionsprovide accessto advancedper-pixel operationsavailable in the rasterizationstageand in the frame buffer hardware of modern graphicsworkstations. With these mechanisms, completelynew renderingalgorithmscanbedesigned andimplementedin a very particularway. In this paperwe extend theideaof extensi vely usinggraphicshardwarefor therenderingof volumetricdatasetsin variousways. First, we introducethe conceptof clipping geometriesby meansof stencilbuffer operations, and we exploit pixel texturesfor the mappingof volume datato sphericaldomains.We show waysto use3D texturesfor the renderingof lightedandshadediso-surfacesin real-timewithout extractingany polygonalrepresentation. Second,wedemonstratethat even for volumedataon unstructuredgrids, whereonly software solutionsexist up to now, bothmethods,iso-surfaceextractionand directvolumerendering,canbeacceleratedto new ratesof interactivity by simplepolygondrawing andframebuffer operations. CR Categories: I.3.7 [ComputerGraphics]:Three-Dimensional GraphicsandRealism—Graphics Hardware, 3D Textures, Volume Rendering, Unstructured Grids

Efficiently using graphics hardware in volume rendering applications

A new and easy-to-implement method for direct volume rendering that uses 3D texture maps for acceleration, and incorporates directional lighting, is described. The implementation, called Voltx, produces high-quality images at nearly interactive speeds on workstations with hardware support for three-dimensional texture maps. Previously reported methods did not incorporate a light model, and did not address issues of multiple texture maps for large volumes. Our research shows that these extensions impact performance by about a factor of ten. Voltx supports orthographic, perspective, and stereo views. This paper describes the theory and implementation of this technique, and compares it to the shear-warp factorization approach. A rectilinear data set is converted into a three-dimensional texture map containing color and opacity information. Quantized normal vectors and a lookup table provide efficiency. A new tesselation of the sphere is described, which serves as the basis for normal-vector quantization. A new gradient-based shading criterion is described, in which the gradient magnitude is interpreted in the context of the field-data value and the material classification parameters, and not in isolation. In the rendering phase, the texture map is applied to a stack of parallel planes, which effectively cut the texture into many slabs. The slabs are composited to form an image.

/pdf/direct-volume-rendering-with-shading-via-three-dimensional-rc936emieo.pdf

Direct volume rendering with shading via three-dimensional textures

Direct volume rendering offers the opportunity to visualize all of a three-dimensional sample volume in one image. However, processing such images can be very expensive and good quality high-resolution images are far from interactive. Projection approaches to direct volume rendering process the volume region by region as opposed to ray-casting methods that process it ray by ray. Projection approaches have generated interest because they use coherence to provide greater speed than ray casting and generate the image in a layered, informative fashion. This paper discusses two topics: First, it introduces a projection approach for directly rendering rectilinear, parallel-projected sample volumes that takes advantage of coherence across cells and the identical shape of their projection. Second, it considers the repercussions of various methods of integration in depth and interpolation across the scan plane. Some of these methods take advantage of Gouraud-shading hardware, with advantages in speed but potential disadvantages in image quality.

A coherent projection approach for direct volume rendering

One method of directly rendering a three-dimensional volume of scalar data is to project each cell in a volume onto the screen. Rasterizing a volume cell is more complex than rasterizing a polygon....

A polygonal approximation to direct scalar volume rendering

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A polygonal approximation to direct scalar volume rendering