The Research On Transfer Functions Of Direct Volume Rendering

Direct volume rendering has proven to be a powerful and effective technique for three-dimensional (3D) visualization of volumetric datasets. Transfer functions play an important role in direct volume rendering as they give data optical properties (typically color and opacity) to make it more visible:opacity values can be used to decide which parts of the dataset is visible and color values can be used to visually distinguish different objects. Two-dimensional (2D) transfer functions are the most commonly used in direct volume rendering because they are intuitive and user friendly to use. However, there are still some difficult issues need to be studied further:boundary disperses seriously, unconspicuous boundary information, overlapping regions of adjacent arches and objects overlapping in the histogram.First, there are often unconspicuous boundary information and overlapping regions of adjacent arches in the space, which makes it difficult for users to design good trans-fer functions for exploring and visualizing tissues of interest. To deal with these prob-lems, we proposes a transfer function space based on volume charge density and electric force magnitude. Compared with the IGM space, the proposed space significantly re-duces the number of overlapping regions of adjacent arches and produces more compact arches. Thus, users can design appropriate transfer functions for tissues of interest and obtain meaningful visualizations。To deal with boundary disperses seriously and incom-plete boundary information,even adjacent arches are intersected, a novel transfer function space is proposed to better highlight and differentiate different materials in realistic vol-ume datasets.The results of various datasets in volume rendering show that boundary of different materials exhibits a trapezoidal shape in the proposed IS space, and boundary information is much better brought out in comparison to the IGM space. Thus the IS space provides much more intuitive clues than the IGM space in order that transfer functions can be more easily designed. Meanwhile, more details of materials of interest are visible in the rendering images.Second, when different objects with the same attribute values can be mapped to the same position in the 2D transfer function space, resulting in objects overlapping in the rendered image, making it difficult to distinguish them. To deal with these problems, we propose a novel method by using twice watershed transform. Firstly, we use water-shed transform for voxels belonging to the transfer function set by users to distinguish different interesting tissues which are not spatial connectivity. And then we use water-shed transform on the intensity of voxels to separate those connective tissues. However, watershed transform is too sensitive to the intensity of voxels, we present an interactive method for separating these overlapping structures using the watershed transform. The over-segmentation problem of the watershed transform can be efficiently resolved by re-vising intensity values of the datasets. To reduce dependence on transfer functions de-signed by users and automatic identify those overlapping objects, we present an automatic method based on the watershed transform to overcome object occlusion. The method be-gins with a user defined initial 2D transfer function, followed by procedures that utilize either local intensity properties of materials or spatial information in the volumetric data to distinguish different objects. Unlike traditional 2D transfer functions, our method can not only distinguish different objects, but also eliminate object occlusions. In addition, our method can achieve similar effects of high dimensional transfer functions without the added complexity associated with high dimensional transfer functions. The method can be easily implemented in most visualization systems and can be used in conjunction with most traditional 2D transfer functions to improve their classification capability.