基于图像频谱分析三维网格模型数字浮雕生成算法

由3维模型生成2.5维数字浮雕曲面是最近国内外数字浮雕研究的热点,通过对三维网格模型频率域的能量进行压缩来得到几何高度被压缩的浮雕模型.算法通过对三维模型进行重新采样,获得视线方向上的规则化深度图,然后利用图像傅里叶变换对深度图进行频谱分析,最后将模型高度的压缩转换为频谱域能量系数的压缩来得到浮雕曲面模型.在压缩过程中,应用高通滤波器工具对低频能量进行压缩、应用对数压缩函数对高频边缘压缩,从而使得生成的浮雕达到较好的效果.实例表明在频率域对三维模型的能量进行压缩来生成浮雕是可行的,算法对细节不太丰富的模型同样具有较好的效果.     

细节保持的曲面浅浮雕算法

为保持曲面浮雕的细节特征,改善浮雕与背景曲面的过渡效果,提出一种细节保持的曲面浮雕算法.首先采用Canny算子在梯度域定位内外轮廓,获得连续的梯度域;其次提出非线性函数,并用其压缩梯度幅值实现形状压缩;再通过双边滤波算子来保持和增强浮雕细节;最终通过求解积分方程重建曲面浮雕.该算法将背景曲面作为积分方程的优化条件,使浮雕在轮廓处向曲面光滑过渡;在重建过程中利用Fourier变换的微分性质在频域实现方程的精确求解,无需设置迭代收敛条件;通过调整参数可方便地控制浮雕整体变化范围、细节锐化程度以及浮雕与背景曲面的过渡效果.实验结果表明,文中算法参数的几何意义直观,所得曲面浮雕细节清晰,边界过渡自然.     

不同类型高度图控制浅浮雕模型生成方法

目的 数字浅浮雕是在平面载体上塑造高低起伏形象的一种造型艺术,具有独特的结构及视觉效果,应用场景极为广泛。为了增加数字浮雕设计的多样性,实现浮雕的风格化应用,提出一种基于高度图的浅浮雕模型生成方法。方法 引入掩模理论,利用掩模操作对待处理的图像进行处理,融合图像处理技术,控制所要生成浮雕不同部位的高度,得到控制浮雕生成效果的高度图,借鉴已有的浅浮雕模型生成方法,利用基于高斯混合模型的滚动引导法向滤波（Gaussian mixture model based rolling guidance normal filtering,GRNF）进行法向分解,基于SfG （surface from gradients）的局部调整和全局重建的方法进行曲面重建,采用拉普拉斯算子及双边滤波器进行去噪平滑处理,最终生成不同高度风格的浅浮雕模型。结果 实验结果表明,本文方法能够生成带有不同视觉效果的浅浮雕模型,通过对细节特征及结构特征相关参数的调整,均能够生成轮廓清晰、细节丰富的浅浮雕模型。结论 本文提出的基于高度图的浅浮雕模型生成方法能够为浅浮雕的多样化设计提供新的思路和方法,在家居装饰行业中有重要的应用价值。     

A Multi-Frame Structure-from-Motion Algorithm under Perspective Projection

We present a fast, robust algorithm for multi-frame structure from motion from point features which works for general motion and large perspective effects. The algorithm is for point features but easily extends to a direct method based on image intensities. Experiments on synthetic and real sequences show that the algorithm gives results nearly as accurate as the maximum likelihood estimate in a couple of seconds on an IRIS 10000. The results are significantly better than those of an optimal two-image estimate. When the camera projection is close to scaled orthographic, the accuracy is comparable to that of the Tomasi/Kanade algorithm, and the algorithms are comparably fast. The algorithm incorporates a quantitative theoretical analysis of the bas-relief ambiguity and exemplifies how such an analysis can be exploited to improve reconstruction. Also, we demonstrate a structure-from-motion algorithm for partially calibrated cameras, with unknown focal length varying from image to image. Unlike the projective approach, this algorithm fully exploits the partial knowledge of the calibration. It is given by a simple modification of our algorithm for calibrated sequences and is insensitive to errors in calibrating the camera center. Theoretically, we show that unknown focal-length variations strengthen the effects of the bas-relief ambiguity. This paper includes extensive experimental studies of two-frame reconstruction and the Tomasi/Kanade approach in comparison to our algorithm. We find that two-frame algorithms are surprisingly robust and accurate, despite some problems with local minima. We demonstrate experimentally that a nearly optimal two-frame reconstruction can be computed quickly, by a minimization in the motion parameters alone. Lastly, we show that a well known problem with the Tomasi/Kanade algorithm is often not a significant one.     

结合深度图像和强度图像的人脸浅浮雕生成算法

针对单纯基于深度图像压缩方法生成的三维浮雕模型容易存在大量噪声,并且易丢失细节的问题,为了生成特征表达准确且具有艺术美感的人脸浅浮雕,提出一种基于三维深度图像和与之对应的二维强度图像的混合人脸浅浮雕的生成算法.首先对三维深度图像的高度场进行压缩生成浮雕基网格;然后提取二维强度图像的灰度信息、梯度信息和显著度信息,并基于显著度信息对基网格的高度场相应地叠加灰度和梯度信息,以保持脸部五官的细节;最后根据显著度信息使用Laplace算子对网格进行光顺处理,使脸部光滑.实验结果表明,与单纯使用基于深度图像压缩方法相比,文中算法生成的浅浮雕模型可以更好地保持五官的细节特征;与基于图像的浮雕生成方法相比,该算法避免了复杂交互操作,可以更好地保持人脸的整体轮廓和形状.     

实时数字浮雕建模

针对现有的数字浮雕生成方法速度较慢且浮雕细节不够丰富的缺点,提出了新的高细节实时可交互地生成数字浮雕的方法.首先提出了一种高细节动态范围压缩函数,并在理论上证明了它比目前的压缩函数能更好地保持细节;然后基于该函数建立了一个实时数字浮雕建模系统,其将建模、交互和渲染流程放到GPU上实现,并利用多目标渲染提高帧缓存的利用率,获得了实时建模速度.系统中还设计了一个交互控制界面,使用户能实时地调整刻画浮雕的各种参数.实验结果证明,该方法能快速、便捷地制作出比现有方法具有更高细节特征的数字浮雕.     

Digital relief generation from 3D models

It is difficult to extend image-based relief generation to high-relief generation, as the images contain insufficient height information. To generate reliefs from three-dimensional (3D) models, it is necessary to extract the height fields from the model, but this can only generate bas-reliefs. To overcome this problem, an efficient method is proposed to generate bas-reliefs and high-reliefs directly from 3D meshes. To produce relief features that are visually appropriate, the 3D meshes are first scaled. 3D unsharp masking is used to enhance the visual features in the 3D mesh, and average smoothing and Laplacian smoothing are implemented to achieve better smoothing results. A nonlinear variable scaling scheme is then employed to generate the final bas-reliefs and high-reliefs. Using the proposed method, relief models can be generated from arbitrary viewing positions with different gestures and combinations of multiple 3D models. The generated relief models can be printed by 3D printers. The proposed method provides a means of generating both high-reliefs and bas-reliefs in an efficient and effective way under the appropriate scaling factors.     

二次联合稀疏表示和低秩近似的浅浮雕优化

目的 针对低质量浅浮雕表面的噪声现象,提出一种二次联合局部自适应稀疏表示和非局部低秩矩阵近似的浅浮雕优化算法。方法 本文方法分两个阶段。第1阶段,将浅浮雕灰度图划分成大小相同的数据块,提取边界块并进行去噪,分别对数据块进行稀疏表示和低秩近似处理。一方面,通过字典学习获得过完备字典和稀疏编码;另一方面,利用K均值聚类算法（K-means）将事先构建的外部字典库划分成k类,从k个簇中心匹配每个数据块的相似块并组成相似矩阵,依次进行低秩近似和特征增强处理。最后通过最小二乘法求解,重建并聚合新建数据块以得到新的高度场。第2阶段与第1阶段的结构相似,主要区别在于改用重建高度场的非局部自身相似性来实现块匹配。结果 在不同图像压缩率下（70%,50%,30%）,对比本文方法与BM3D（block-matching and 3D filtering）、WNNM（weighted nuclear norm minimization）、STROLLR（sparsifying transform learning and low-rank）、TWSC（trilateral weighted sparse coding）4个平滑降噪方法的浅浮雕重建结果,发现BM3D和STROLLR方法的特征保持虽好,但平滑效果较差,WNNM方法出现模型破损现象,TWSC方法的平滑效果比BM3D和STROLLR方法更好,但特征也同时被光顺化。阴影恢复形状法（shape from shading,SFS）是一种基于图像的3D建模法,但是其重建结果比较粗糙。相比之下,本文方法生成的浅浮雕模型更加清晰直观,在浅浮雕的特征增强和平滑去噪方面都展现出更好的性能。结论 本文综合数据块的局部稀疏性和数据块之间的非局部相似性对粗糙的浅浮雕模型进行二次高度场重建。本文方法有效改善了现有浅浮雕模型的质量,提高了模型的整体视觉效果,为浅浮雕的优化提供了新方法。     

Digital three-dimensional visualization of cellular organelles studied by medium- and high-voltage electron microscopy

Tilted thick sections (one-half to several micrometres) of biological specimens observed with medium- to high-voltage electron microscopes are extremely useful for the study of the three-dimensional (3-D) structure of organelles. If high resolution in 3-D visualization and 3-D reconstruction is needed, many images corresponding to various angles of rotation and tilt must be recorded. This necessitates very time-consuming work—including eventual photographic processing—before good positioning of the object is defined. We have developed software which permits very rapid and precise determination of the tilt-axis, the registration of tilted views, 3-D measurements and 3-D visualization. Images are digitized either from negative films or directly with a camera fitted to the microscope. The application of the software is performed in minutes and allows for a rapid check of the quality of the tilt-series and of the features of interest of the object. Application of the software to the study of the 3-D structure of active components of the nucleolus stained with silver is shown.     

讲基于曲率特征的三维模型浮雕压缩算法

基于三维拓扑和几何信息的数字化浮雕生成,是浮雕设计领域的一个全新发展方向。根据浮雕艺术形式的特点,实现了一种基于三维测量数据的浮雕曲面生成算法。该算法将浮雕的生成抽象为三维模型在某个投影方向上深度场的压缩过程,以模型曲面的几何信息作为压缩对象,生成的浮雕曲面在深度方向上的立体感可由用户自由控制。实验表明该方法在保持模型拓扑结构一致性的基础上,包含了丰富的几何细节特征。算法可以利用现有的各种实体造型,生成灵活多样的浮雕艺术品,具有广泛的应用前景。