目的 浮雕是雕塑艺术的一种，根据其空间结构和用途的不同分为高浮雕、浅浮雕和凹浮雕三类。随着数字化技术和3D打印技术的发展，数字化浮雕的生成技术已经成为近年来计算机图形学领域的研究热点之一，从三维模型生成浮雕以其真实自然的效果成为浮雕生成的主要方法之一。为了使即将进入该领域的学者尽快了解该方法的现状和发展趋势，本文对三种类型的浮雕生成技术进行了系统的综述。 方法 介绍了三种类型的浮雕生成技术，着重比较分析了基于三维网格模型的数字浅浮雕生成过程中的关键技术，存在问题及解决方案。针对复杂三维网格模型在生成数字凹浮雕过程中存在的部分细节信息丢失、特征线类型体现形式不完善、线条与形体间的过渡尚未解决、生成浮雕效果不自然等具体问题，提出了适用于三维复杂网格模型生成数字凹浮雕的研究方案。同时，从角色动画序列出发，对最优浮雕的生成技术进行了探讨，探讨结合信息熵理论计算选择最佳动作及观察视角的场景，还原艺术家的创作过程，为适用于面向三维打印的用户浮雕产品定制服务提供了可行的解决方案。结果 基于三维模型的浮雕生成方法是生成数字浮雕的一种重要方法，如何通过压缩和细节保持相关算法得到效果自然的浮雕模型一直是研究者们研究的热点问题。 结论 虽然由三维模型生成数字浮雕是一种行之有效方法，但是仍存在细节信息丢失、线条过度不自然、特征线类型不完善等几个值得继续研究的问题，另外一个值得研究的问题就是如何智能的从三维动画序列生成浮雕。
Survey and Prospect of 3D Model-based Digital Relief Generation
Wang Meili,yangliying,Geng Nan,He Dongjian(College of Information Engineering Northwest A F University;College of Information Engineering Northwest A F University,Yangling;College of Mechanical and Electronic Engineering Northwest A F University,Yangling)
Objective Relief art is a kind of sculpture arts which can be categorized into high relief, bas relief and sunken relief according to its space structure and application. High relief refers to the relief whose height occupies above 50% of the original mesh depth; bas-relief refers to the relief whose height occupies below 50% of the original mesh depth; sunken relief is always obtained by carving object into a background plane. In the modern industrial production, relief has a broad application in producing nameplates, coins, architectural decorations and so on. Currently, there are two main methods to generate digital reliefs which are 2D image based method and 3D mesh based method. Most 2D image based methods usually generate digital relief by feature extraction and 3D reconstruction technologies. However, 2D images cannot convey the depth of 3D mesh. 3D mesh based method always obtain digital relief by compression and detail reservation technologies. Digital relief generation based on 3D mesh is a recently emerged research topic with the development of digitalized technology and 3D printing. It becomes one of the hot topics to generate digital relief from 3D models in computer graphics fields. Therefore, this paper proposes abovementioned literature review to support scholars in gaining further insights into the frontier development of the topic. Method Three types of digital relief generation techniques were systematically analyzed, and the key technologies, problems and solutions in the process of digital bas relief generation were analyzed and compared. Although digital high relief and bas-relief generation has been widely investigated, for the sunken relief, there are still spaces to be explored. Currently, there are some drawbacks in generating sunken reliefs with complex model, such as, the detail information is missing, line types has not achieved a good embodied form, the transition between the lines and the body has not worked out, the generated sunken relief is not vivid and natural. This paper gives a solution for generating digital sunken reliefs from 3D complex models. To investigate the technology of optimal relief generation, this paper starts from the character animation sequence, puts forward to calculate the optimal action and perspective of the scene with the information entropy theory aiming to recover the creation process of artists. The investigation work will provide a feasible solution for user customization and 3D printing relief production services. Result It is one of the main methods to generate a piece of relief from 3D models. For high relief and bas-relief, many researchers have studied several methods and we categorize those methods into two classes. One is based on the human visual information and relief is generated by compressing the 3D model. Although this algorithm is simple, most relief details are lost during compression. The other one is based on some geometrical operations. This algorithm is better to reserve details, but it always needs to solve poisson equation which needs lots of calculations and is low efficiency. Furthermore, this algorithm is easy to make local deformation and sometimes some human interactions such as adjusting parameters are necessary to achieve desired effect. Now how to obtain a vivid effect through compression and details reservation has been a hot topic. Sunken relief always conveys information by lines. But exiting methods have not considered the manifestations of three types of lines which are called the main line, the inner line, the fine shadow line respectively. Simple line arrangement could not show a complete sunken relief. And transition between the lines is also necessary to generate a vivid and natural sunken relief. For these problems, some research is consequential, such as, investigate different embodied form of different type of lines and set different engraving depth and location for different type of lines to make it smooth of the transition between the lines and the body. On the basis of relief generation, there are two important researches, that is, the choice of best attitude and perspective. And how to achieve an optimal relief through intelligent algorithms is worthy to be discussed. This will reduce the difficulty for artists and sculptors in the process of carving. Choosing a best posture from an animation sequence is similar to the problem of key frame extraction. And a best perspective can provide more important information than other perspectives. For that, using the information entropy theory is a feasible?scheme for optimal relief generation from an animation sequence. In addition, current digital relief generation methods cannot composite models taking into account spatial structure and the importance of the characters. Combination models relief should not be generated by simply compositing several models together. Conclusion Generating digital relief from 3D models is an effective method. However how to obtain more details and more line types and how to make the transition between lines are also some problems, and further to intelligently generate the 3D bas relief automatically from 3D animation sequence.