Patent Application
20050046872
The present invention relates generally to structured light profiling of a part and more specifically to methods and systems for image processing to obtain a three dimensional profile of a part using a structured light source.
In structured light applications, a three dimensional profile of a part is obtained by using a laser source and at least one video camera. A laser source emanates a laser beam and spreads into multiple (one or more) laser stripes which strikes a surface of a part. These stripes are viewed from one or more video cameras from an angle other than the illumination angle. Typically, applications for structured light profiling include obtaining shape profile information about the part or generating a 3D contour map of the part.
The limitations of using laser stripes to obtain accurate profile information are mainly attributed to sampling error and the noise associated with the laser because the center of a laser stripe may not be imaged at the center of the pixel of the camera and may not be the detected intensity peak. Sampling error occurs while locating the center of the laser stripe on the image. There are image processing techniques such as maximum intensity, intensity center, Gaussian fitting and zero-crossing which attempt to extract the relevant information from the laser stripe. The associated problem with several of these techniques is that it gives the location of the highest peak, which is not the true center of the stripe. Current correction techniques for this error include neighborhood averaging over neighboring pixels and doing a weighted average or using fitting methods, but these techniques also fail to adequately address the sampling error.
The noise associated with the laser primarily takes the form of laser speckle, which is the oscillation of the intensity profile for a laser when it is reflected from the surface of the part and is caused by coherency of the laser. One way to reduce the speckle noise is by choosing an appropriate viewing system. By changing the size of the aperture, the size of the speckle changes, the larger the aperture, the smaller the size of the speckle. However, in this case there's a depth of field tradeoff.
Therefore there is need for an improved image processing technique for reducing the speckle noise and also the sampling error in structured light applications.
Briefly, in accordance with one aspect of the present invention, an image processing method for structured light profiling includes sampling an image of a structured light pattern to obtain an intensity distribution. A number of sets of sampled points are then selected from the intensity distribution, where each of the respective sets includes a number of sampled points. Each of the sets of sampled points is then fitted to a respective distribution function. Finally, the distribution functions are filtered to select a representative distribution function for the intensity distribution.
In accordance with another aspect of the present invention, a system for obtaining a three dimensional profile of a part using a structured light pattern includes a source of structured light positioned at a predetermined distance from a part, where the source projects a beam of structured light to illuminate the part. The system also includes at least one imaging device configured to acquire an image of a structured light pattern of the part, where the at least one imaging device is positioned such that an angle of view of the imaging device is different from an angle of illumination of the source. A processor is coupled to the at least one imaging device, and the processor is configured for a variety of tasks including sampling the image of a structured light pattern to obtain an intensity distribution; selecting a number of sets of sampled points from the intensity distribution, where each of the sets includes at least three sampled points; fitting each of the sets of sampled points to respective Gaussian distribution functions; extracting a center for each of the Gaussian distribution functions; filtering the Gaussian distribution functions by using the centers to select a representative distribution function for the intensity distribution; and reconstructing a three dimensional profile of the part, using the representative distribution function.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As would be appreciated by those skilled in the art, the intensity profile for a flat surface in the case of illumination with Gaussian profile by source 12 and any viewing angle of the imaging device 20, for any tilt angle (θ) of the surface of the part 16, remains a Gaussian distribution. For such a symmetric intensity profile on a flat surface, it will be well appreciated by those skilled in the art that sampling of the symmetric point and applying zero-crossing which is a second derivative on the profile, will yield the center corresponding to the maximum intensity.
However, in the case of a complex part, where the surface is not flat, the intensity profile is no longer symmetric. In case of a curved surface, the curvature of the surface may be approximated by a cylinder to analyze the intensity profiles for a Gaussian distribution. It is found that the degree of asymmetry δ (the maximum distance difference between two points with the same intensity to the maximum intensity point) is small and hence the effect on image due to this asymmetry is small. Therefore the intensity profile near a cylindrical neighborhood in a curved surface can also be assumed to be a Gaussian distribution. In this case, considering that the maximum intensity point is sampled, the image bias after applying zero-crossing is also small, of the order of 10−2. This is shown by a set of graphs in
However, if the maximum intensity point is not sampled, i.e. there is sampling error, zero-crossing does not give accurate results and leads to a larger image bias as shown by the set of graphical representations in
In order to reduce sampling error and speckle noise, an imaging processing method is provided.
In case there is a speckle which can be observed as an outlier on the intensity profile, the corresponding center using the outlier can be easily discarded and the speckle noise can be addressed. This is indicated in
As noted above, the flowcharts illustrated in
The various embodiments and aspects of the invention described above comprise an ordered listing of executable instructions for implementing logical functions. The ordered listing can be embodied in any computer-readable medium for use by or in connection with a computer-based system that can retrieve the instructions and execute them. In the context of this application, the computer-readable medium can be any means that can contain, store, communicate, propagate, transmit or transport the instructions. The computer readable medium can be an electronic, a magnetic, an optical, an electromagnetic, or an infrared system, apparatus, or device. An illustrative, but non-exhaustive list of computer-readable mediums can include an electrical connection (electronic) having one or more wires, a portable computer diskette (magnetic), a random access memory (RAM) (magnetic), a read-only memory (ROM) (magnetic), an erasable programmable read-only memory (EPROM or Flash memory) (magnetic), an optical fiber (optical), and a portable compact disc read-only memory (CDROM) (optical).
Note that the computer readable medium may comprise paper or another suitable medium upon which the instructions are printed. For instance, the instructions can be electronically captured via optical scanning of the paper or other medium, then compiled, interpreted or otherwise processed in a suitable manner if necessary, and then stored in a computer memory.
The various aspects of the technique described hereinabove have utility in industrial as well as medical environments. The methods can be used for non-contact measurement of complex parts e.g. aircraft parts for inspection, in the extrusion process in the steel industry and other high temperature manufacturing environments where contact measurement of a part is difficult. These methods are also useful in medical fields for surgery planning, where these may be used for profiling the different parts of a human body to have precision in surgery.
Although only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
