Detecting a position of a sheet of light in an image

• US 9,417,058 B1
• Filed: 10/24/2014
• Issued: 08/16/2016
• Est. Priority Date: 10/24/2014
• Status: Active Grant

• Current Assignee: Matrox Electronic Systems Ltd.
• Original Assignee: Matrox Electronic Systems Ltd.
• Inventors: Lina, Arnaud
• Primary Examiner(s): Carter, Aaron W
• Application Number:
  US14/522,766
• Time in Patent Office: 662 Days
• Field of Search: None
• US Class Current: 1/1
• CPC Class Codes: G01J 1/42 using electric radiation detectors optical or mechanical part G01J1/04; by comparison with a reference light or electric value G01J1/10
• CPC Class Codes: G06T 7/13 Edge detection
• CPC Class Codes: G06T 7/521 from laser ranging, e.g. using interferometry; from the projection of structured light
• CPC Class Codes: G06T 7/73 using feature-based methods

Abstract

In a method and system for detecting a position of a sheet of light in an image, two parameters are used, namely the width of the widest peak to be detected and the intensity difference of the least-contrasted peak to be detected. From these two parameters, a size S, a distance D, and a threshold T are determined. A region of interest (ROI) is determined based on the intensity profile associated with a line of pixels of the image, distance D and threshold T. A derivative filter of size S is applied to the intensity profile to produce a slope of the intensity profile. In the determined ROI, one or more zero-crossings in the slope of the intensity profile are detected. From the detected zero-crossings, a zero-crossing is selected and the position of the selected zero-crossing is returned as the detected position of the sheet of light for the line of pixels.

Claims

1. A method for detecting a position of a sheet of light in an image, the method comprising:

receiving an image of the sheet of light as projected onto an object, the image comprising a line of pixels with an associated intensity profile;

setting a size S of a derivative filter to be at least as large as a width W of a widest peak to be detected in the intensity profile;

determining a region of interest for detection by locating intensity differences in the intensity profile that are at least as large as a threshold value;

applying the derivative filter of size S to the intensity profile to obtain a slope of the intensity profile;

detecting one or more zero-crossings in the slope of the intensity profile inside the region of interest, each one of the one or more detected zero-crossings having a corresponding pixel position in the intensity profile;

selecting a zero-crossing from the one or more detected zero-crossings; and

outputting the corresponding pixel position of the selected zero-crossing as the detected position of the sheet of light for the line of pixels.

2. The method of claim 1, wherein determining a region of interest for detection comprises:

setting the threshold value to be smaller than an intensity difference of a least-contrasted peak to be detected; and

locating each of the intensity differences in the intensity profile over a distance D, where the distance D is set to be at least as large as half of the width W of the widest peak to be detected.

3. The method of claim 2, wherein determining a region of interest for detection comprises:

scanning the intensity profile in one direction;

determining an entry point of the region of interest by locating a first increase in intensity over the distance D that is at least as large as the threshold value; and

determining an exit point of the region of interest by locating a first decrease in intensity over the distance D that is at most equal to the threshold value.

4. The method of claim 1, wherein outputting the corresponding pixel position of the selected zero-crossing comprises estimating a sub-pixel position using interpolation of at least two pixel positions.

5. The method of claim 4, wherein the interpolation of at least two pixel positions is a linear interpolation.

6. The method of claim 1, wherein the derivative filter is a uniform derivative filter.

7. The method of claim 1, wherein the sheet of light is generated using a laser.

8. The method of claim 1, wherein each one of the one or more detected zero-crossings has a corresponding pixel intensity in the intensity profile and wherein the step of selecting a zero-crossing from the one or more detected zero-crossings comprises selecting the zero-crossing having the greatest pixel intensity.

9. The method of claim 1, wherein each one of the one or more detected zero-crossings has a corresponding pixel intensity in the intensity profile, and wherein the method further comprises outputting the corresponding pixel intensity of the selected zero-crossing.

10. The method of claim 1, further comprising outputting a width of the region of interest as an estimate of a width of the sheet of light.

11. The method of claim 1, wherein the size S of the derivative filter is even and wherein the derivative filter is anti-symmetric.

12. The method of claim 1, wherein the image comprises a plurality of lines of pixels, each one of the plurality of lines of pixels having an associated intensity profile, and wherein the method detects a position of the sheet of light for each one of the plurality of lines of pixels.

13. A field-programmable gate array (FPGA) configured for detecting a position of a sheet of light in an image, the FPGA having logic programmed for:

receiving an image of the sheet of light as projected onto an object, the image comprising a line of pixels with an associated intensity profile;

setting a size S of a derivative filter to be at least as large as a width W of a widest peak to be detected in the intensity profile;

determining a region of interest for detection by locating intensity differences in the intensity profile that are at least as large as a threshold value;

applying the derivative filter of size S to the intensity profile to obtain a slope of the intensity profile;

detecting one or more zero-crossings in the slope of the intensity profile inside the region of interest, each one of the one or more detected zero-crossings having a corresponding pixel position in the intensity profile;

selecting a zero-crossing from the one or more detected zero-crossings; and

outputting the corresponding pixel position of the selected zero-crossing as the detected position of the sheet of light for the line of pixels.

14. The FPGA of claim 13, wherein determining a region of interest for detection comprises:

setting the threshold value to be smaller than an intensity difference of a least-contrasted peak to be detected; and

locating each of the intensity differences in the intensity profile over a distance D, where the distance D is set to be at least as large as half of the width W of the widest peak to be detected.

15. The FPGA of claim 14, wherein determining a region of interest for detection comprises:

scanning the intensity profile in one direction;

determining an entry point of the region of interest by locating a first increase in intensity over the distance D that is at least as large as the threshold value; and

determining an exit point of the region of interest by locating a first decrease in intensity over the distance D that is at most equal to the threshold value.

16. The FPGA of claim 13, wherein outputting the corresponding pixel position of the selected zero-crossing comprises estimating a sub-pixel position using interpolation of at least two pixel positions.

17. The FPGA of claim 16, wherein the interpolation of at least two pixel positions is a linear interpolation.

18. The FPGA of claim 13, wherein the derivative filter is a uniform derivative filter.

19. The FPGA of claim 13, wherein the sheet of light is generated using a laser.

20. The FPGA of claim 13, wherein each one of the one or more detected zero-crossings has a corresponding pixel intensity in the intensity profile and wherein the step of selecting a zero-crossing from the one or more detected zero-crossings comprises selecting the zero-crossing having the greatest pixel intensity.

21. The FPGA of claim 13, wherein each one of the one or more detected zero-crossings has a corresponding pixel intensity in the intensity profile, and wherein the logic is further configured for outputting the corresponding pixel intensity of the selected zero-crossing.

22. The FPGA of claim 13, further comprising outputting a width of the region of interest as an estimate of a width of the sheet of light.

23. The FPGA of claim 13, wherein the size S of the derivative filter is even and wherein the derivative filter is anti-symmetric.

24. The FPGA of claim 13, wherein the image comprises a plurality of lines of pixels, each one of the plurality of lines of pixels having an associated intensity profile, and wherein the logic is further configured for detecting a position of the sheet of light for each one of the plurality of lines of pixels.

25. A system for detecting a position of a sheet of light in an image, the system comprising:

a memory;

a processor coupled to the memory; and

an application stored in the memory and executable by the processor for;

receiving an image of the sheet of light as projected onto an object, the image comprising a line of pixels with an associated intensity profile;

setting a size S of a derivative filter to be at least as large as a width W of a widest peak to be detected in the intensity profile;

determining a region of interest for detection by locating intensity differences in the intensity profile that are at least as large as a threshold value;

applying the derivative filter of size S to the intensity profile to obtain a slope of the intensity profile;

detecting one or more zero-crossings in the slope of the intensity profile inside the region of interest, each one of the one or more detected zero-crossings having a corresponding pixel position in the intensity profile;

selecting a zero-crossing from the one or more detected zero-crossings; and

outputting the corresponding pixel position of the selected zero-crossing as the detected position of the sheet of light for the line of pixels.

26. A non-transitory computer readable medium having stored thereon program code for execution by a processor for detecting a position of a sheet of light in an image, the program code comprising instructions for:

receiving an image of the sheet of light as projected onto an object, the image comprising a line of pixels with an associated intensity profile;

setting a size S of a derivative filter to be at least as large as a width W of a widest peak to be detected in the intensity profile;

determining a region of interest for detection by locating intensity differences in the intensity profile that are at least as large as a threshold value;

applying the derivative filter of size S to the intensity profile to obtain a slope of the intensity profile;

detecting one or more zero-crossings in the slope of the intensity profile inside the region of interest, each one of the one or more detected zero-crossings having a corresponding pixel position in the intensity profile;

selecting a zero-crossing from the one or more detected zero-crossings; and

outputting the corresponding pixel position of the selected zero-crossing as the detected position of the sheet of light for the line of pixels.