【作者】 吴一帆；

【导师】 杨旭波；

【作者基本信息】 上海交通大学 ， 计算机科学与技术， 2014， 硕士

【摘要】 人体皮肤渲染技术在电影制作、电子游戏、医学等领域有着重要的作用。随着人们对渲染真实的人体皮肤乃至虚拟人物的需求越来越高,皮肤渲染渐渐成为计算机图形学领域的重要课题。完全模拟光线在皮肤中的传输开销太大,如何在尽量保持真实感的情况下减少计算开销是皮肤渲染方面的重点发展方向;研究皮肤参数对肤色、通透性的影响,不仅有助于渲染更真实的皮肤,而且在其他领域也有重要的作用。本文以可调节参数的人体皮肤渲染技术为主题,目标是实现一个结合人体皮肤生物学参数的皮肤渲染引擎,研究不同参数条件下皮肤的外观特征,体现生物学参数对皮肤外观的影响。为了实现这一目标,本文分三个方面进行了研究,从离线渲染到实时渲染,完成了一套解决方案。首先,现有的Multipole漫射模型(diffusion model)并不能很好地处理厚度小于两个平均自由程(mean free path)的薄层介质,其中包括了本文将要研究的皮肤。针对这一问题,本文提出一种易于实现、不影响性能的扩展方法,通过将光线分解为直接透射光和漫射光两部分,解决了这一问题。通过和Monte-Carlo方法的比较,本文验证了扩展方法在处理小于两个平均自由程厚度的薄层时的准确性。随后,本文将扩展方法应用于皮肤渲染。本文从微观和亚观层面的皮肤结构出发,通过对各种皮肤结构光学性质的分析,建立了一个适合参数调整的皮肤模型。在将扩展方法应用于皮肤渲染的同时,本文针对渲染过程,借鉴了实时渲染的一些思路,提出了表面点生成和纹理应用相关方面的改进。通过该皮肤模型,本文比较了不同参数下皮肤的外观的变化,并使用该模型,匹配了高加索人种、亚洲人种、非洲人种等皮肤,体现了该模型较为广泛的适用范围。离线渲染的一大劣势是其渲染速度。为了能够在调整皮肤参数时,实时获得反馈,本文结合实时渲染方法,使用预计算插值的思路,通过对一系列细节的分析完善了预计算插值方法。本文使用预计算插值方法,最终实现了实时调整皮肤参数。与离线渲染效果的对比以及性能测试、录制的视频表明,本文实现的实时调整皮肤参数方法较为准确和高效。本文文末分析了研究的不足之处,展望了未来研究的方向和应用前景。和已有研究相比,本文创新点主要在于:(1)提出简单易实现的Multipole漫射模型的扩展;(2)详细分析了皮肤外观,特别是颜色渗出效果随参数变化的趋势;(3)实现了实时调整皮肤参数。更多还原

【Abstract】 Human skin rendering techniques play important roles in the fields of movie making, video games and medical science. With growing need of rendering photorealistic human skin or even virtual characters, skin rendering has gradually become an important subject in computer graphics. It needs too much computation power to fully simulate light transport in skin, thus reducing the amount of computation, at minimal cost of photorealism, is an important topic in skin rendering. Studying how parameters affect skin appearance not only helps render more realistic skin, but also means a lot in other related fields.This thesis focuses on the topic of human skin rendering techniques with adjustable parameters. The objective is to implement a skin rendering engine that respects biological parameters of human skin, and to study how skin parameters affect its appearance. To achieve the objective, three aspects are studied in the thesis, making up a complete solution for both offline rendering and real-time rendering.First, the Multipole diffusion model is not well suited for materials thinner than 2 mean free paths, which includes skin we are going to study. To address this issue, the thesis proposes a simple-to-implement extension that does not affect performance. The extension solves the issue by separating incoming light into two parts, one for direct transmitting light and one for diffused light. Through comparison with Monte-Carlo methods, the accuracy of the extension method is validated for materials thinner than 2 mean free paths.After that, the thesis focuses on applying the extension method to skin rendering. Through investigation of skin structure from microscale to mesoscale, a skin model suitable for parameter adjusting is built. As for rendering, some improvements for surface point generation and texture usage, inspired by real-time techniques, are proposed. Using the skin model, appearance of skin with different parameters are compared. Caucasian skin, Asian skin and African skin are rendered, which highlights the applicability of the model.The major shortcoming of offline rendering is its speed. In order to get real-time responses while adjusting skin parameters, a “pre-compute and interpolation” method is proposed in conjunction with real-time rendering techniques. Using the pre-compute method, real-time adjusting of skin parameters is achieved. The accuracy and efficiency of the method is proven by comparisons with offline rendering results, performance tests and a video recording.At the end of the thesis the limitations of the research are analyzed. Prospect of future work and potential applications are discussed.Compared with previous research, the innovations of the thesis are as follows:(1) An easy-to-implement extension to the Multipole diffusion model is proposed;(2) Analyzes in detail how skin appearance, especially the color bleeding effect, varies with skin parameters.(3) Achieves real-time adjusting of skin parameters.更多还原