中文摘要：

目的 光线投射法是一种重要的直接体绘制算法,但其效果取决于复杂的传递函数.为此提出基于结构特征的自适应光线投射算法,从而使得利用简单传递函数即可很好地揭示体数据特征.方法 首先分析光线方向标量值的变化趋势获取结构特征——特征段;然后基于若干意义明确启发式规则（特征段的次序、尺度、重要度）自动计算特征段的可见度,根据特征段可见度调节每个采样点的不透明度;最后基于调节后的不透明度完成绘制.结果 使用合成数据、医学真实采样数据和工业CT（computed tomography）数据进行测试,结果表明本文算法在展示体数据的内部结构特征,尤其是细小结构方面优于其他类似算法;本文算法速度比DVR（direct volume rendering）慢,但仍可满足交互需求.另外,本文算法还提供多个形象直观、意义明确的参数供用户调节,进一步增加了本文算法的灵活性.结论 本文提出的自适应光线投射算法,允许用户使用简单传递函数和调节意义明显的参数即可有效揭示体数据特征,进一步提高了光线透射法的直观性.

英文摘要：

Objective The ray casting method is an important direct volume rendering method, but it depends on complex transfer functions. This paper presents a structural feature-based adaptive ray casting algorithm that can show inner structural features with simple transfer functions. Method We analyze scalar value along a ray to extract structural features, including feature segments. Subsequently, several heuristic rules （scale, order, and importance of feature segment） are used to determine the visibility of the feature segments. Furthermore, the opacity ofsample points iscalculated. Finally, the calculated opacity isused to perform volume rendering. Result Synthesis data, medical data, and industry CT are used in our experiments. Results show that our method is superior to other similar methods for structure exhibition, specifically for tiny structural features. Furthermore, our method allows the user to adjust several intuitive parameters. This possibility adds flexibility to our method. The presented algorithm is relatively slower than DVR but is fast enough for interactive rendering. Conclusion The proposed structure-based adaptive ray casting algorithm allows users to reveal the inner features of volume data with a simple transfer function and intuitive parameters. Therefore, the proposed method can further improve the intuitiveness of the ray casting algorithm.