结构光扫描仪（维基百科全翻译版）

原理（Principle）
图案的生成（Generation of light pattern）
标定（Calibration）
条纹图案分析（Analysis of stripe patterns ）
精度和范围（Precision and range）
定位（Navigation）
挑战（Challenges）
速度（Speed）
应用（Applications）
软件（Software）
参考文献（References）

关于结构光扫描仪，维基百科里有一个较好的概述，鉴于很多同学无法查看维基百科，所以此篇的内容为维基百科的翻译版。并加上一些个人的注解。

A structured-light 3D scanner is a 3D scanning device for measuring the three-dimensional shape of an object using projected light patterns and a camera system.¹

结构光3D扫描仪是一种3D扫描设备，用于使用投射的光图案和摄像头系统测量物体的三维形状。¹

原理（Principle）

Projecting a narrow band of light onto a three-dimensionally shaped surface produces a line of illumination that appears distorted from other perspectives than that of the projector, and can be used for geometric reconstruction of the surface shape (light section).

A faster and more versatile method is the projection of patterns consisting of many stripes at once, or of arbitrary fringes, as this allows for the acquisition of a multitude of samples simultaneously. Seen from different viewpoints, the pattern appears geometrically distorted due to the surface shape of the object.

Although many other variants of structured light projection are possible, patterns of parallel stripes are widely used. The picture shows the geometrical deformation of a single stripe projected onto a simple 3D surface. The displacement of the stripes allows for an exact retrieval of the 3D coordinates of any details on the object’s surface.

将窄带的光投影到三维形状的表面上会产生一条照明线，该照明线从投影器的其他角度来看存在一定的变形，这种变形可用于表面形状（光覆盖的部分）的几何重建。

更快更通用的方法是一次投影由多个条纹或任意条纹组成的图案，因为这样可以同时采集多个样本。从不同的角度观察，由于物体的表面形状，图案呈现出几何变形。

尽管结构化光投影可能存在很多变体，但是使用最广泛的是平行条纹的图案。图片显示了投影到简单3D曲面上的单个条纹的几何变形。通过条纹的位移可以对物体表面的任何细节的三维坐标进行精确跟踪。

图案的生成（Generation of light pattern）

Fringe pattern recording system with 2 cameras (avoiding obstructions)
Two major methods of stripe pattern generation have been established: Laser interference and projection.

The laser interference method works with two wide planar laser beam fronts. Their interference results in regular, equidistant line patterns. Different pattern sizes can be obtained by changing the angle between these beams. The method allows for the exact and easy generation of very fine patterns with unlimited depth of field. Disadvantages are high cost of implementation, difficulties providing the ideal beam geometry, and laser typical effects like speckle noise and the possible self interference with beam parts reflected from objects. Typically, there is no means of modulating individual stripes, such as with Gray codes.

The projection method uses incoherent light and basically works like a video projector. Patterns are usually generated by passing light through a digital spatial light modulator, typically based on one of the three currently most widespread digital projection technologies, transmissive liquid crystal, reflective liquid crystal on silicon (LCOS) or digital light processing (DLP; moving micro mirror) modulators, which have various comparative advantages and disadvantages for this application. Other methods of projection could be and have been used, however.

Patterns generated by digital display projectors have small discontinuities due to the pixel boundaries in the displays. Sufficiently small boundaries however can practically be neglected as they are evened out by the slightest defocus.

A typical measuring assembly consists of one projector and at least one camera. For many applications, two cameras on opposite sides of the projector have been established as useful.

Invisible (or imperceptible) structured light uses structured light without interfering with other computer vision tasks for which the projected pattern will be confusing. Example methods include the use of infrared light or of extremely high framerates alternating between two exact opposite patterns.²

比较知名的制作条纹图案的方法有两种：激光干涉法和投影法。

激光干涉法基于两个宽的平面激光束前端。它们的干涉可以产生规则等距的线图案。通过改变这些光束之间的角度可以获得不同的图案尺寸。该方法易于精确的生成具有无限景深的精细图案。缺点是成本高，难以提供理想的光束几何形状以及典型的激光效果，例如斑点噪声以及部分光束从物体的反射带来的自干扰。通常无法调制独特的条纹，比如格雷码。【注：激光法能生成比较精细的简单图案，比如多线激光，但是复杂的如格雷码图案就难以生成。移动式扫描仪采用激光法比较多。】

投影法使用不相干的光，并且基本上像视频投影仪一样工作。使光线通过数字化的空间光调制器（spatial light modulator）来生成图案，通常基于三种当前最广泛使用的数字投影技术之一：透射液晶，硅上反射液晶（LCOS）或数字光处理（DLP；移动微镜））调制器，各有优劣势。同时也存在其他投影方法。【注：投影法可以生成各式各样的图案，而固定式投影法结构光多采用格雷码、相位码图案，移动式投影法结构光多采用随机散斑图案。】

由于显示器中的像素边界，数字显示投影仪产生的图案具有较小的不连续性。但由于轻微的散焦使边界差异变得比较均匀，从而可以忽略这种不连续性。【注：意思就是散焦让像素的离散化变得不明显了，实际图案是连续的色彩，在图像上看上去也是连续的色彩】

典型的测量组件由一台投影仪和至少一台摄像机组成。在很多应用中，在投影器两侧各安装一个相机比较常见。【注：单目立体和双目立体，双目立体最常见，也还有更多目的立体，比如Creaform公司的GoScan Spark三维扫描仪是三目立体】

不可见（或无法察觉）的结构化灯光使用不会干扰其他计算机视觉任务的结构化灯光，而不可见的投影图案会让使用者困惑其原理。比如使用红外光或具有极高帧频的两个正好相反的图案交替使用。²【注：不可见的图案应用也比较广泛，比如Kincect使用的红外结构光，IPhone里使用的红外TOF，相对来说不可见光的体验感或许会好一些】

标定（Calibration）

Geometric distortions by optics and perspective must be compensated by a calibration of the measuring equipment, using special calibration patterns and surfaces. A mathematical model is used for describing the imaging properties of projector and cameras. Essentially based on the simple geometric properties of a pinhole camera, the model also has to take into account the geometric distortions and optical aberration of projector and camera lenses. The parameters of the camera as well as its orientation in space can be determined by a series of calibration measurements, using photogrammetric bundle adjustment.

必须通过使用特殊的校准图案和表面对测量设备进行校准，以补偿光学器件和视角造成的几何变形。数学模型用于描述投影仪和照相机的成像特性。从本质上讲，该模型基于针孔相机的简单几何特性，还必须考虑到投影仪和相机镜头的几何畸变和光学像差。相机的参数及其在空间中的位置姿态可以使用摄影测量光束法平差通过一系列校准测量来确定。【注：相机的内外参和畸变参数，是校准的输出，这是一切三维测量的前提。在摄影测量领域，内参就是像主点和主距，外参就是三个线元素Xs、Ys、Zs和三个角元素Phi、Omega、Kappa；在计算机视觉中，内参就是K矩阵，外参就是R矩阵和C矩阵】

条纹图案分析（Analysis of stripe patterns ）

There are several depth cues contained in the observed stripe patterns. The displacement of any single stripe can directly be converted into 3D coordinates. For this purpose, the individual stripe has to be identified, which can for example be accomplished by tracing or counting stripes (pattern recognition method). Another common method projects alternating stripe patterns, resulting in binary Gray code sequences identifying the number of each individual stripe hitting the object. An important depth cue also results from the varying stripe widths along the object surface. Stripe width is a function of the steepness of a surface part, i.e. the first derivative of the elevation. Stripe frequency and phase deliver similar cues and can be analyzed by a Fourier transform. Finally, the wavelet transform has recently been discussed for the same purpose.

In many practical implementations, series of measurements combining pattern recognition, Gray codes and Fourier transform are obtained for a complete and unambiguous reconstruction of shapes.

Another method also belonging to the area of fringe projection has been demonstrated, utilizing the depth of field of the camera.³

It is also possible to use projected patterns primarily as a means of structure insertion into scenes, for an essentially photogrammetric acquisition.

在观察到的条纹图案中包含若干深度信息。任何单个条纹的位移都可以直接转换为3D坐标。因此必须识别单个条带。例如可以通过跟踪或计数条带（图案识别方法）来实现。另一种常见的方法是投影交替的条纹图案，从而产生二进制格雷码序列，该序列标识出击中对象的每个单独条纹的数量。重要的深度信息也来自沿着物体表面的不同条纹宽度。条纹宽度是表面部分的陡度的函数，即高程的一阶导数。条带频率和相位提供类似的深度信息，并且可以通过傅立叶变换进行分析。基于同样的目的近些年也在研究小波变换。【注：这里讨论的是固定式结构光技术，由一个投影仪和双相机组成，物体不动，投影多帧特殊图案在双相机中依次成像，如格列码和相位码，基于格列码的二进制序列以及相位码的解相来计算表面的三维坐标。】

在很多实际应用中，采用将模式识别，格雷码和傅里叶变换相结合的一系列测量手段，来实现三维形状的完整且准确的重构。

另一种利用照相机景深的方法也属于条纹投影区域。³

也可以将投影图案用作结构插入场景中，以进行摄影测量。【注：这里应该说的是将投影图案当做场景中的一种结构化纹理，来辅助摄影测量。】

精度和范围（Precision and range）

The optical resolution of fringe projection methods depends on the width of the stripes used and their optical quality. It is also limited by the wavelength of light.

An extreme reduction of stripe width proves inefficient due to limitations in depth of field, camera resolution and display resolution. Therefore, the phase shift method has been widely established: A number of at least 3, typically about 10 exposures are taken with slightly shifted stripes. The first theoretical deductions of this method relied on stripes with a sine wave shaped intensity modulation, but the methods work with “rectangular” modulated stripes, as delivered from LCD or DLP displays as well. By phase shifting, surface detail of e.g. 1/10 the stripe pitch can be resolved.

Current optical stripe pattern profilometry hence allows for detail resolutions down to the wavelength of light, below 1 micrometer in practice or, with larger stripe patterns, to approx. 1/10 of the stripe width. Concerning level accuracy, interpolating over several pixels of the acquired camera image can yield a reliable height resolution and also accuracy, down to 1/50 pixel.

Arbitrarily large objects can be measured with accordingly large stripe patterns and setups. Practical applications are documented involving objects several meters in size.

Typical accuracy figures are:

• Planarity of a 2-foot (0.61 m) wide surface, to 10 micrometres (0.00039 in).
• Shape of a motor combustion chamber to 2 micrometres (7.9×10−5 in) (elevation), yielding a volume accuracy 10 times better than with volumetric dosing.
• Shape of an object 2 inches (51 mm) large, to about 1 micrometre (3.9×10−5 in)
• Radius of a blade edge of e.g. 10 micrometres (0.00039 in), to ±0.4 μm

条纹投影方法的光学分辨率取决于所用条纹的宽度及其光学质量。它也受光波长的限制。

由于景深，相机分辨率和显示分辨率的限制，条纹宽度的无限制减小没有太大意义。因此，相移方法被广泛使用：用稍微偏移的条纹进行至少3次曝光，通常约10次曝光。该方法的第一个理论推论依赖于具有正弦波形强度调制的条纹，但是该方法也适用于“矩形”调制条纹，如LCD或DLP显示器所提供的。通过相移，可以达到1/10条纹间距级别的细节。【注：条纹宽度减小确实有利于细节提升，但是受限于景深、相机分辨率和显示分辨率，条纹宽度减小到非常小也没有意义，瓶颈不再是条纹宽度。所以多次曝光的相移法可以用来进一步提升细节，达到1/10之一甚至更高的细节度。当然多次曝光也会让单次测量时间变长，实际应用会有所取舍。】

因此，当前的光学条纹图案轮廓测定法可以实现低至光波长的细节分辨率，实践中小于1微米，对于较大的条纹图案，则可以达到大约条纹宽度的1/10。对图像的几个像素进行插值可以产生可靠的高程分辨率，并且精度低至1/50像素。【注：精度一方面和硬件有关，即条纹宽度和光波长、景深、相机分辨率等；另一方面也和算法有关，子像素算法明显会比整像素算法精度高很多。】

可以使用大条纹图案来测量大尺寸对象。实际应用中有测量几米到几十米的物体的实践。【注：现在市面上的扫描仪扫描大尺寸物体已不是稀奇。数米的测量还是比较常见的。】

典型的精度数字为：
• 2英尺（0.61m）宽的平面度，可以到10微米（0.00039英寸）。
• 马达燃烧室的形状达到2微米（7.9×10-5英寸）（高程），容积精度比容积计量好10倍。
• 2英寸（51毫米）大的物体，约1微米（3.9×10-5英寸）
• 叶片边缘的半径例如10微米（0.00039英寸），精度到±0.4μm
【注：这里的精度说的是固定式结构光扫描仪，手持式的结构光扫描仪要相对低一些，一般最高到0.01mm也就是10微米，手持式的优点是便携。】

定位（Navigation）

As the method can measure shapes from only one perspective at a time, complete 3D shapes have to be combined from different measurements in different angles. This can be accomplished by attaching marker points to the object and combining perspectives afterwards by matching these markers. The process can be automated, by mounting the object on a motorized turntable or CNC positioning device. Markers can as well be applied on a positioning device instead of the object itself.

The 3D data gathered can be used to retrieve CAD (computer aided design) data and models from existing components (reverse engineering), hand formed samples or sculptures, natural objects or artifacts.

由于该方法一次只能从一个角度测量形状，因此必须从不同角度的不同测量中组合完整的3D形状。这可以通过在对象表面粘贴定位标志点并随后通过匹配这些标志点来组合各个视角的数据来实现。通过将物体安装在电动转盘或CNC定位设备上，该过程可以自动化。标志点也可以应用在定位设备上，而不是对象本身。【注：要完整的测量数据，就必须从多个角度测量，并知道各角度之间的相对位置关系。特殊形状的标记点（圆形用的最多）便可以作为控制点来计算每一个角度下的扫描仪位置，只需要匹配标记点（标记点的提取和匹配相对来说简单很多）就可以得到各角度的相对位置关系。标记点也可以安装到扫描设备上，再通过一个跟踪设别来跟踪扫描设备的位置，这样就不用在物体上贴标志点了。】

收集的3D数据可用于恢复CAD数据（计算机辅助设计）、逆向工程；手工作品或雕塑、自然物体或人工制品的三维建模。

挑战（Challenges）

As with all optical methods, reflective or transparent surfaces raise difficulties. Reflections cause light to be reflected either away from the camera or right into its optics. In both cases, the dynamic range of the camera can be exceeded. Transparent or semi-transparent surfaces also cause major difficulties. In these cases, coating the surfaces with a thin opaque lacquer just for measuring purposes is a common practice. A recent method handles highly reflective and specular objects by inserting a 1-dimensional diffuser between the light source (e.g., projector) and the object to be scanned.⁴ Alternative optical techniques have been proposed for handling perfectly transparent and specular objects.⁵

Double reflections and inter-reflections can cause the stripe pattern to be overlaid with unwanted light, entirely eliminating the chance for proper detection. Reflective cavities and concave objects are therefore difficult to handle. It is also hard to handle translucent materials, such as skin, marble, wax, plants and human tissue because of the phenomenon of sub-surface scattering. Recently, there has been an effort in the computer vision community to handle such optically complex scenes by re-designing the illumination patterns.⁶ These methods have shown promising 3D scanning results for traditionally difficult objects, such as highly specular metal concavities and translucent wax candles.⁷

与所有光学方法一样，反射或透明表面难度较大。反射会导致光线原理相机或直接反射到其光学元件中。在这两种情况下，都可以超出摄像机的动态范围。透明或半透明的表面也会造成很大的困难。在这些情况下，仅出于测量目的，用薄的不透明清漆涂覆表面是一种常见的做法。一种最新方法是通过在光源（例如，投影仪）和要扫描的对象之间插入一维漫射器来处理高反射和镜面反射的对象。⁴ 有的学者已经提出了替代性的光学技术来处理完全透明和镜面物体。⁵【注：几乎所有扫描仪在面对高度反光物体和透明物体时都会比较困难，喷粉处理是比较常用的做法，但也有一些物体是要求不能喷粉的，比如文物。现在有一些新的研究，但是还不是特别成熟。】

两次反射和相互反射会导致条纹图案被多余的光线覆盖，从而难以被准确的检测识别。因此，高反射的腔体和凹形物体很难处理。由于次表面散射现象，也难以处理诸如皮肤，大理石，蜡，植物和人体组织之类的半透明材料。最近，计算机视觉界已经在努力通过重新设计照明模式来处理这种光学上复杂的场景。⁶ 这些方法已显示出对传统困难物体（例如高镜面金属凹度和半透明蜡烛）的3D扫描结果的希望。⁷【注：这里说的是多面相互反射的现象和光打到表面会离散的表面，这两种都是比较困难的，相对来说前一种更困难一些，后一种会导致精度降低】

速度（Speed）

Although several patterns have to be taken per picture in most structured light variants, high-speed implementations are available for a number of applications, for example:

• Inline precision inspection of components during the production process.
• Health care applications, such as live measuring of human body shapes or the micro structures of human skin.

Motion picture applications have been proposed, for example the acquisition of spatial scene data for three-dimensional television.

尽管在大多数结构光方法中必须为每张图片拍摄几种模式，但是高速结构光扫描可用于许多应用程序，例如：
• 在生产过程中对零件进行在线精密检查。
• 卫生保健应用，例如实时测量人体形状或人体皮肤的微观结构。

基于运动图像的应用也有被开发，例如用于三维电视的空间场景数据的获取。
【注：速度是扫描仪产业化的一个关键参数，提高作业效率对产业经济来说至关重要】

应用（Applications）

• Industrial Optical Metrology Systems (ATOS) from GOM GmbH utilize Structured Light technology to achieve high accuracy and scalability in measurements. These systems feature self-monitoring for calibration status, transformation accuracy, environmental changes, and part movement to ensure high-quality measuring data.⁸
• Google Project Tango SLAM (Simultaneous localization and mapping) using depth technologies, including Structured Light, Time of Flight, and Stereo. Time of Flight require the use of an infrared (IR) projector and IR sensor; Stereo does not.
• A technology by PrimeSense, used in an early version of Microsoft Kinect, used a pattern of projected infrared points to generate a dense 3D image. (Later on, the Microsoft Kinect switched to using a time-of-flight camera instead of structured light.)
• Occipital

• Structure Sensor uses a pattern of projected infrared points, calibrated to minimize distortion to generate a dense 3D image.
• Structure Core uses a stereo camera that matches against a random pattern of projected infrared points to generate a dense 3D image.

• Intel RealSense camera projects a series of infrared patterns to obtain the 3D structure.
• Face ID system works by projecting more than 30,000 infrared dots onto a face and producing a 3D facial map.
• VicoVR sensor uses a pattern of infrared points for skeletal tracking.
• Chiaro Technologies uses a single engineered pattern of infrared points called >• Symbolic Light to stream 3D point clouds for industrial applications
• Made to measure fashion retailing
• 3D-Automated Optical Inspection
• Precision shape measurement for production control (e.g. turbine blades)
• Reverse engineering (obtaining precision CAD data from existing objects)
• Volume measurement (e.g. combustion chamber volume in motors)
• Classification of grinding materials and tools
• Precision structure measurement of ground surfaces
• Radius determination of cutting tool blades
• Precision measurement of planarity
• Documenting objects of cultural heritage
• Capturing environments for augmented reality gaming
• Skin surface measurement for cosmetics and medicine
• Body shape measurement
• Forensic science inspections
• Road pavement structure and roughness
• Wrinkle measurement on cloth and leather
• Structured Illumination Microscopy
• Measurement of topography of solar cells⁹
• GHOST HUNTING ¹⁰

这部分我就不翻译了！

软件（Software）

• 3DUNDERWORLD SLS - OPEN SOURCE¹¹
• DIY 3D scanner based on structured light and stereo vision in Python language¹²
• SLStudio—Open Source Real Time Structured Light¹³

参考文献（References）

Borko Furht (2008). Encyclopedia of Multimedia (2nd ed.). Springer. p. 222. ISBN 978-0-387-74724-8. ↩︎ ↩︎
Fofi, David; T. Sliwa; Y. Voisin (January 2004). “A Comparative Survey on Invisible Structured Light” (PDF). SPIE Electronic Imaging — Machine Vision Applications in Industrial Inspection XII. San Jose, USA. pp. 90–97. ↩︎ ↩︎
“Tiefenscannende Streifenprojektion (DSFP) mit 3D-Kalibrierung”. University of Stuttgart (in German). Archived from the original on 9 April 2013. ↩︎ ↩︎
Shree K. Nayar and Mohit Gupta, Diffuse Structured Light, Proc. IEEE International Conference on Computational Photography, 2012 ↩︎ ↩︎
Eron Steger & Kiriakos N. Kutulakos (2008). “A Theory of Refractive and Specular 3D Shape by Light-Path Triangulation”. Int. J. Computer Vision, vol. 76, no. 1. ↩︎ ↩︎
Mohit Gupta, Amit Agrawal, Ashok Veeraraghavan and Srinivasa G. Narasimhan (2011). “Measuring Shape in the Presence of Inter-reflections, Sub-surface Scattering and Defocus”. Proc. CVPR. ↩︎ ↩︎
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