結構光掃描儀（維基百科全翻譯版）

關於結構光掃描儀，維基百科裏有一個較好的概述，鑑於很多同學無法查看維基百科，所以此篇的內容爲維基百科的翻譯版。並加上一些個人的註解。

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

結構光3D掃描儀是一種3D掃描設備，用於使用投射的光圖案和攝像頭系統測量物體的三維形狀。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.

將窄帶的光投影到三維形狀的表面上會產生一條照明線，該照明線從投影器的其他角度來看存在一定的變形，這種變形可用於表面形狀（光覆蓋的部分）的幾何重建。

更快更通用的方法是一次投影由多個條紋或任意條紋組成的圖案，因爲這樣可以同時採集多個樣本。 從不同的角度觀察，由於物體的表面形狀，圖案呈現出幾何變形。

儘管結構化光投影可能存在很多變體，但是使用最廣泛的是平行條紋的圖案。圖片顯示了投影到簡單3D曲面上的單個條紋的幾何變形。 通過條紋的位移可以對物體表面的任何細節的三維座標進行精確跟蹤。

圖案的生成（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

比較知名的製作條紋圖案的方法有兩種：激光干涉法和投影法。

激光干涉法基於兩個寬的平面激光束前端。它們的干涉可以產生規則等距的線圖案。通過改變這些光束之間的角度可以獲得不同的圖案尺寸。該方法易於精確的生成具有無限景深的精細圖案。缺點是成本高，難以提供理想的光束幾何形狀以及典型的激光效果，例如斑點噪聲以及部分光束從物體的反射帶來的自干擾。通常無法調製獨特的條紋，比如格雷碼。【注：激光法能生成比較精細的簡單圖案，比如多線激光，但是複雜的如格雷碼圖案就難以生成。移動式掃描儀採用激光法比較多。】

投影法使用不相干的光，並且基本上像視頻投影儀一樣工作。 使光線通過數字化的空間光調製器（spatial light modulator）來生成圖案，通常基於三種當前最廣泛使用的數字投影技術之一：透射液晶，硅上反射液晶（LCOS）或數字光處理（DLP；移動微鏡） ）調製器，各有優劣勢。同時也存在其他投影方法。【注：投影法可以生成各式各樣的圖案，而固定式投影法結構光多采用格雷碼、相位碼圖案，移動式投影法結構光多采用隨機散斑圖案。】

由於顯示器中的像素邊界，數字顯示投影儀產生的圖案具有較小的不連續性。 但由於輕微的散焦使邊界差異變得比較均勻，從而可以忽略這種不連續性。【注：意思就是散焦讓像素的離散化變得不明顯了，實際圖案是連續的色彩，在圖像上看上去也是連續的色彩】

典型的測量組件由一臺投影儀和至少一臺攝像機組成。 在很多應用中，在投影器兩側各安裝一個相機比較常見。【注：單目立體和雙目立體，雙目立體最常見，也還有更多目的立體，比如Creaform公司的GoScan Spark三維掃描儀是三目立體】

不可見（或無法察覺）的結構化燈光使用不會干擾其他計算機視覺任務的結構化燈光，而不可見的投影圖案會讓使用者困惑其原理。 比如使用紅外光或具有極高幀頻的兩個正好相反的圖案交替使用。²【注：不可見的圖案應用也比較廣泛，比如Kincect使用的紅外結構光，IPhone裏使用的紅外TOF，相對來說不可見光的體驗感或許會好一些】

標定（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.

必須通過使用特殊的校準圖案和表面對測量設備進行校準，以補償光學器件和視角造成的幾何變形。 數學模型用於描述投影儀和照相機的成像特性。 從本質上講，該模型基於針孔相機的簡單幾何特性，還必須考慮到投影儀和相機鏡頭的幾何畸變和光學像差。 相機的參數及其在空間中的位置姿態可以使用攝影測量光束法平差通過一系列校準測量來確定。【注：相機的內外參和畸變參數，是校準的輸出，這是一切三維測量的前提。在攝影測量領域，內參就是像主點和主距，外參就是三個線元素Xs、Ys、Zs和三個角元素Phi、Omega、Kappa；在計算機視覺中，內參就是K矩陣，外參就是R矩陣和C矩陣】

條紋圖案分析（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.

在觀察到的條紋圖案中包含若干深度信息。 任何單個條紋的位移都可以直接轉換爲3D座標。因此必須識別單個條帶。例如可以通過跟蹤或計數條帶（圖案識別方法）來實現。 另一種常見的方法是投影交替的條紋圖案，從而產生二進制格雷碼序列，該序列標識出擊中對象的每個單獨條紋的數量。 重要的深度信息也來自沿着物體表面的不同條紋寬度。 條紋寬度是表面部分的陡度的函數，即高程的一階導數。 條帶頻率和相位提供類似的深度信息，並且可以通過傅立葉變換進行分析。 基於同樣的目的近些年也在研究小波變換。【注：這裏討論的是固定式結構光技術，由一個投影儀和雙相機組成，物體不動，投影多幀特殊圖案在雙相機中依次成像，如格列碼和相位碼，基於格列碼的二進制序列以及相位碼的解相來計算表面的三維座標。】

在很多實際應用中，採用將模式識別，格雷碼和傅里葉變換相結合的一系列測量手段，來實現三維形狀的完整且準確的重構。

另一種利用照相機景深的方法也屬於條紋投影區域。³

也可以將投影圖案用作結構插入場景中，以進行攝影測量。【注：這裏應該說的是將投影圖案當做場景中的一種結構化紋理，來輔助攝影測量。】

精度和範圍（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

條紋投影方法的光學分辨率取決於所用條紋的寬度及其光學質量。它也受光波長的限制。

由於景深，相機分辨率和顯示分辨率的限制，條紋寬度的無限制減小沒有太大意義。 因此，相移方法被廣泛使用：用稍微偏移的條紋進行至少3次曝光，通常約10次曝光。 該方法的第一個理論推論依賴於具有正弦波形強度調製的條紋，但是該方法也適用於“矩形”調製條紋，如LCD或DLP顯示器所提供的。通過相移，可以達到1/10條紋間距級別的細節。【注：條紋寬度減小確實有利於細節提升，但是受限於景深、相機分辨率和顯示分辨率，條紋寬度減小到非常小也沒有意義，瓶頸不再是條紋寬度。所以多次曝光的相移法可以用來進一步提升細節，達到1/10之一甚至更高的細節度。當然多次曝光也會讓單次測量時間變長，實際應用會有所取捨。】

因此，當前的光學條紋圖案輪廓測定法可以實現低至光波長的細節分辨率，實踐中小於1微米，對於較大的條紋圖案，則可以達到大約條紋寬度的1/10。 對圖像的幾個像素進行插值可以產生可靠的高程分辨率，並且精度低至1/50像素。【注：精度一方面和硬件有關，即條紋寬度和光波長、景深、相機分辨率等；另一方面也和算法有關，子像素算法明顯會比整像素算法精度高很多。】

可以使用大條紋圖案來測量大尺寸對象。 實際應用中有測量幾米到幾十米的物體的實踐。【注：現在市面上的掃描儀掃描大尺寸物體已不是稀奇。數米的測量還是比較常見的。】

典型的精度數字爲：
• 2英尺（0.61m）寬的平面度，可以到10微米（0.00039英寸）。
• 馬達燃燒室的形狀達到2微米（7.9×10-5英寸）（高程），容積精度比容積計量好10倍。
• 2英寸（51毫米）大的物體，約1微米（3.9×10-5英寸）
• 葉片邊緣的半徑例如10微米（0.00039英寸），精度到±0.4μm
【注：這裏的精度說的是固定式結構光掃描儀，手持式的結構光掃描儀要相對低一些，一般最高到0.01mm也就是10微米，手持式的優點是便攜。】

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.

由於該方法一次只能從一個角度測量形狀，因此必須從不同角度的不同測量中組合完整的3D形狀。這可以通過在對象表面粘貼定位標誌點並隨後通過匹配這些標誌點來組合各個視角的數據來實現。通過將物體安裝在電動轉盤或CNC定位設備上，該過程可以自動化。 標誌點也可以應用在定位設備上，而不是對象本身。【注：要完整的測量數據，就必須從多個角度測量，並知道各角度之間的相對位置關係。特殊形狀的標記點（圓形用的最多）便可以作爲控制點來計算每一個角度下的掃描儀位置，只需要匹配標記點（標記點的提取和匹配相對來說簡單很多）就可以得到各角度的相對位置關係。標記點也可以安裝到掃描設備上，再通過一個跟蹤設別來跟蹤掃描設備的位置，這樣就不用在物體上貼標誌點了。】

收集的3D數據可用於恢復CAD數據（計算機輔助設計）、逆向工程；手工作品或雕塑、自然物體或人工製品的三維建模。

挑戰（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

與所有光學方法一樣，反射或透明表面難度較大。反射會導致光線原理相機或直接反射到其光學元件中。在這兩種情況下，都可以超出攝像機的動態範圍。透明或半透明的表面也會造成很大的困難。在這些情況下，僅出於測量目的，用薄的不透明清漆塗覆表面是一種常見的做法。一種最新方法是通過在光源（例如，投影儀）和要掃描的對象之間插入一維漫射器來處理高反射和鏡面反射的對象。⁴ 有的學者已經提出了替代性的光學技術來處理完全透明和鏡面物體。⁵【注：幾乎所有掃描儀在面對高度反光物體和透明物體時都會比較困難，噴粉處理是比較常用的做法，但也有一些物體是要求不能噴粉的，比如文物。現在有一些新的研究，但是還不是特別成熟。】

兩次反射和相互反射會導致條紋圖案被多餘的光線覆蓋，從而難以被準確的檢測識別。 因此，高反射的腔體和凹形物體很難處理。 由於次表面散射現象，也難以處理諸如皮膚，大理石，蠟，植物和人體組織之類的半透明材料。 最近，計算機視覺界已經在努力通過重新設計照明模式來處理這種光學上覆雜的場景。⁶ 這些方法已顯示出對傳統困難物體（例如高鏡面金屬凹度和半透明蠟燭）的3D掃描結果的希望。⁷【注：這裏說的是多面相互反射的現象和光打到表面會離散的表面，這兩種都是比較困難的，相對來說前一種更困難一些，後一種會導致精度降低】

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.

儘管在大多數結構光方法中必須爲每張圖片拍攝幾種模式，但是高速結構光掃描可用於許多應用程序，例如：
• 在生產過程中對零件進行在線精密檢查。
• 衛生保健應用，例如實時測量人體形狀或人體皮膚的微觀結構。

基於運動圖像的應用也有被開發，例如用於三維電視的空間場景數據的獲取。
【注：速度是掃描儀產業化的一個關鍵參數，提高作業效率對產業經濟來說至關重要】

應用（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

這部分我就不翻譯了！

軟件（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

參考文獻（References）

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1. Borko Furht (2008). Encyclopedia of Multimedia (2nd ed.). Springer. p. 222. ISBN 978-0-387-74724-8. ↩︎ ↩︎

2. 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. ↩︎ ↩︎

3. “Tiefenscannende Streifenprojektion (DSFP) mit 3D-Kalibrierung”. University of Stuttgart (in German). Archived from the original on 9 April 2013. ↩︎ ↩︎

4. Shree K. Nayar and Mohit Gupta, Diffuse Structured Light, Proc. IEEE International Conference on Computational Photography, 2012 ↩︎ ↩︎

5. 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. ↩︎ ↩︎

6. 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. ↩︎ ↩︎

7. Mohit Gupta; Shree K. Nayar (2012). “Micro Phase Shifting”. Proc. CVPR. ↩︎ ↩︎

8. “ATOS - Industrial 3D Scanning Technology”. GOM GmbH. Retrieved 9 July 2018. ↩︎

9. W J Walecki, F Szondy and M M Hilali, “Fast in-line surface topography metrology enabling stress calculation for solar cell manufacturing for throughput in excess of 2000 wafers per hour” 2008 Meas. Sci. Technol. 19 025302 (6pp) doi:10.1088/0957-0233/19/2/025302 ↩︎

10. https://ghosthuntersequipment.com/products/kinect-sls-camera ↩︎

11. Kyriakos Herakleous & Charalambos Poullis (2014). “3DUNDERWORLD-SLS: An Open-Source Structured-Light Scanning System for Rapid Geometry Acquisition”. arXiv:1406.6595 [cs.CV]. ↩︎

12. Hesam H. (2015). “DIY 3D scanner based on structured light and stereo vision in Python language”. ↩︎

13. J. Wilm; et al. (2014). “SLStudio: Open-source framework for real-time structured light”. ↩︎