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The structural-light technology is the use of designed patterns with special structure (such as discrete spotlight, stripe light, coded structural light, etc.), and then the pattern is projected onto the surface of a three-dimensional object, and another camera is used to observe the distortion of the image on the 3D physical surface. If the pattern of structural light projected on the surface of the object is a plane, then the pattern of the structural light in the observed image is similar to the pattern of the projection, without deformation, but only produces some scale changes according to the distance. But, if the surface is not flat, then observed the structure of the light pattern will be because the surface of different geometry generates different deformations, and according to the distance of difference, according to the known structure light pattern and the deformation observed shape can calculate according to the algorithm of 3D shape and depth information. The measurement system of structural light mainly consists of a structured light projection system, camera and image acquisition and processing system, as shown below.
FIG.1 Structured light measurement system
Structural light is based on optical triangulation. The specific structural light is projected onto the measured object through the projection device, and a three-dimensional image of the light bar with the same shape is formed on the surface of the measured object. The three-dimensional image of the light bar is detected by another camera, and then the two-dimensional distorted image of the light bar is obtained. The distortion of the bar depends on the relative position between the projector and the camera and the shape and size of the object. In general, the displacement shown along the strip is proportional to the height of the object's surface, twist together (similar to contour lines on a plane map) represents the change in the plane, and discontinuities represent physical gaps on the surface. When the position between the structure projector and the camera is immobilized, the 3D contour of the object surface can be reconstructed by the distorted 2D beam image. The recognition degree of different structural light modes is different, which needs to be determined by specific use scenes.
Structural light technology is greatly influenced by environmental light sources, which is more suitable for indoor application scenarios, and its frame rate is lower, so it is more suitable for static scenes or scenes with slow changes. Its advantage is that it can obtain a high-resolution depth image.
According to different beam modes projected by optical projectors, they can be divided into the following several structural light modes：
1.Point structured light mode
A beam of light from an optical projector is projected onto an object to produce a point of light that is imprinted by the camera's lens onto the camera's image plane, forming a two-dimensional point. The intersection of the camera's rays and the beam of light in space forms a simple triangular geometry. This trigonometric geometric constraint can be obtained by certain calibration, and the position of the light point in a coordinate system can be uniquely determined.
It is necessary to scan the object point by point for measurement, and the time required for image acquisition and image processing increases sharply with the increase of the object.
FIG.2 Structural light mode
2.Linear structured light mode
The linear structured light mode is to project a beam of light to the object. The light bar is modulated with the change of the surface depth of the object and may be modulated by the gap. In the image, the light bar is distorted and discontinuous, the degree of distortion is proportional to the depth, while the discontinuity shows the physical gap on the surface of the object. The task is to obtain the three-dimensional information of the object surface from the deformed light bar image information.
The linear structured light mode can be said to be an extension of the point structured light mode. The line-of-sight beam passing through the optical center of the camera intersects with the laser plane in space, producing many points of intersection. The points of intersection on the object surface are numerous points of light on the light bar, thus forming the triangular geometry constraint in a similar point structured light mode. Compared with the point structured light mode, the linear structured light mode has more measurement information and its complexity does not increase.
FIG.3 Linear structured light mode
3. Multiline structured light mode
Multiline structural light mode is an optical projector that projects multiple beams on the surface of an object. On the one hand, the purpose is to process multiple light bars in an image to improve the processing efficiency of the image; on the other hand, the measurement quantity of multiple light coverage is realized to increase the information and obtain the depth information of the object surface in a wider range. The efficiency and range of multi-line structured light modes are improved, and the calibration complexity and light bar recognition are introduced.
FIG.4 Multi-line structured light mode
4. Plane structured light mode
Using the plane structure light mode. A two-dimensional light pattern of a certain structure onto the surface of the object and the three-dimensional contour measurement of the object can be obtained without scanning. The measurement speed is fast, and the most common method is to project grating fringes to the surface of the object.
FIG.5 Structural light modes
In order to determine the corresponding relationship between object surface points and image pixel points, the projected structured light pattern needs to be encoded. There are several common coding methods.
(1) direct coding
According to the image grayscale or color information coding, this method needs a wide spectral range, and all points need to be coded to achieve a high resolution. However, due to the environmental impact, the measurement accuracy is poor.
FIG.6 direct coding
(2) Airspace coding
This method is based on the distribution of all points in a window adjacent to the surrounding identification code, mainly suitable for moving objects. Incorrect window decoding may occur if the discontinuous object surface is obscured during coding.
GIF.7 spatial-multiplexing coding
(3) Time-domain coding
This scheme requires projecting different coded lights of N continuous sequences, and the receiving end recognizes them according to receiving N continuous sequence images. Each coding point has a high measurement accuracy, and a higher resolution depth map can be obtained. It is suitable for static scenes because each coding point needs to calculate N consecutive projections, which requires a large amount of calculation and is not suitable for dynamic scenes.
GIF.8 time-multiplexing coding