The Application Of NPR Techniques In Volume Rendering

In the traditional volume data visualization, most of the techniques are used to obtain photo-realistic effect. However, one special challenge of most rendering approaches is the specification of parameters such that the resulting images are in fact providing useful insights into the objects of interest. In recent years, researchers have begun to incorporate NPR techniques into volume visualization process in order to extent its information display functions. In this way, there is no physics based rendering model, and we are not confined by the destination of photo-realistic result. For specific need of observation and analysis, utilizing NPR principles to flexibly control the appearance of the result image and to have the interested structure information presented.Volume rendering is an important approach in volume visualization. In this paper, application of NPR techniques in volume rendering is divided into two categories: One is to enhance feature areas of the volume data structure so as to improve rendering result; the other is to implement some NPR effects in volume rendering for special observation requirement.In the enhancement techniques, for depth cues to be acquired in the rendering result, depth factor are added into the original boundary enhancement equation; and gradient magnitude factor is added into the silhouette enhancement process so that result effect becomes more reasonable. Another advantage of these improved methods is the flexibility of rendering multiple volume data sets. There is detailed analysis and experimental proof of this flexibility. Besides, since the sample variance can reflect sample value distribution, an enhancement algorithm based on variance statistic is proposed. In the result image, areas with great local variance, which generally have complex structure, will be emphasized.In implementing NPR effects, distance color changing method is taken in the contour rendering to solve its original problem of depth cues lacking. Also contour visualization with integration based blending mode is accomplished in order to enrich structure information in the rendering result. An extensive technique of tone shading, namely view shading, is introduced in this paper, in which color of an object is interpolated between cool tone and warm tone, based on the angle between surface norm vector and view vector. It is capable of enhancing the silhouette areas.