Ray casting is a widely recognized method for high quality volume rendering. It traverses and samples the volume data ray by ray in image space. Traditionally, the algorithm is implemented in CPU on PC platform,resulting in slow speed and poor interacfivity. This paper introduces a new technique named View Dependent Layer Sampling (VDLS), which supports a hardware implementation of ray casting by Graphics Processing Unit (GPU).VDLS organizes the ray sampling points into a set of layers which can be efficiently represented by two view dependent geometric buffers as two dynamic textures. Based on the structure of VDLS, the six steps involved in the ray casting algorithm including ray generation, ray traversal, interpolation, classification, shading and composition can be fully accomplished in GPU, taking advantage of its programmability and flexibility. In addition, two speedup techniques exploiting object space and image space coherence are proposed for fast culling of the lapsed rays.Several advanced features regarding illumination and composition are further discussed, with which VDLS is capable of reconfiguring the well-known geometric hardware engine for volume ray casting. The novel approach of GPU supported ray casting can render up to 150 million interpolated, post shaded and composed ray samples per second for perspective view. Experimental results suggest that the proposed framework can be regarded as an alternative for on-the-fly visualization and exploitation of discrete scalar data in medical visualization, physical phenomena simulation and material testing applications.%光线投射是一种高质量的体绘制方法.它以图像空间为序,逐根光线遍历和采样体数据.因此,传统上,它只能在CPU上实现,因而速度慢,交互性不好.提出了一个新的视点相关的层次采样VDLS (view dependent layer sampling)结构,VDLS将光线上的所有采样点重新组织成一系列层,并简化为两个视点相关的几何缓冲器,进而在GPU(graphics processing unit)中用两个动态纹理表示.利用GPU的可编程性,光线投射算法的6个步骤(光线生成、光线遍历、插值、分类、着色和颜色合成)得以完全在GPU中实现.在此基础上,提出两个基于体空间和图像空间连贯性的加速技巧,快速剔除无效的光线.结合其他与渲染和颜色合成有关的技巧,VDLS将面向多边形绘制的图形引擎转化为体光线投射算法引擎,在透视投影方式下,每秒能处理1.5亿个插值、后分类与着色的光线采样点.实验结果表明,提出的方法能用于医学可视化、真实物理现象模拟、材质检测中灰度体数据快速交互的可视化与漫游.
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