Depth from focal gradient analysis using object texture removal by albedo normalization

• US 5,878,152 A
• Filed: 05/21/1997
• Issued: 03/02/1999
• Est. Priority Date: 05/21/1997
• Status: Expired due to Term

• Current Assignee: Cognex Corporation
• Original Assignee: Cognex Corporation
• Inventors: Sussman, Michael
• Primary Examiner(s): ROGERS, SCOTT A
• Application Number:
  US08/859,854
• Time in Patent Office: 650 Days
• Field of Search: 382/106, 382/108, 382/141, 382/255, 382/254, 382/144, 382/146, 382/147, 382/149, 382/150, 348/128, 348/131, 348/135, 345/429, 345/430
• US Class Current: 382/106
• CPC Class Codes: G01B 11/2513 with several lines being projected in more than one direction, e.g. grids, patterns
• CPC Class Codes: G06T 2207/10012 Stereo images
• CPC Class Codes: G06T 2207/10148 Varying focus
• CPC Class Codes: G06T 2207/10152 Varying illumination
• CPC Class Codes: G06T 2207/30152 Solder
• CPC Class Codes: G06T 7/40 Analysis of texture depth or shape recovery from texture G06T7/529
• CPC Class Codes: G06T 7/521 from laser ranging, e.g. using interferometry; from the projection of structured light
• CPC Class Codes: G06T 7/571 from focus

Abstract

The invention provides a method and apparatus for obtaining a range image of an object. The method includes the act of "albedo normalization", i.e., removing the effects of object reflectance using a structured illumination image of the object and a uniform illumination image of the object to provide an albedo-normalized image. This image is then processed using a focus measure to provide a focal image, which image is then used to provide a range image. The invention substantially removes the effects of object reflectance from an image acquired using structured illumination, so that only the structured illumination pattern and its degree of focus/defocus remains. Albedo normalization is achieved by dividing an image of an object taken under structured illumination by a corresponding image of the object taken under uniform illumination. The albedo normalization act removes the primary source of noise in range images obtained using a depth from defocus or depth from focus of structured illumination technique, by removing spurious image frequencies from the image before processing by a focus measure. The albedo normalization act permits the depth from defocus and depth from focus techniques to be used for one or more focal positions, and over a broad range of materials of interest in machine vision.

Claims

1. A method for providing a range image of an object, the method comprising the acts of:

acquiring a structured illumination image of the object;

acquiring a uniform illumination image of the object;

performing albedo normalization using the structured illumination image of the object and the uniform illumination image of the object to provide an albedo-normalized image;

processing the albedo-normalized image using a focus measure to provide a focal image;

using the focal image to provide a range image.

2. The method of claim 1, wherein the act of processing the albedo-normalized image using a focus measure to provide a focal image includes the act of using an RMS operation.

3. The method of claim 1, wherein the act of processing the albedo-normalized image using a focus measure to provide a focal image includes the act of using a Laplacian-like convolution.

4. The method of claim 1, wherein the act of using the focal image to provide a range image includes the act of using a table look-up calibration employing a look-up table.

5. The method of claim 4, wherein the act of using a table look-up calibration includes the acts of:

collecting focus measure data at known Z-positions; and

inverting a look-up table using interpolation.

6. The method of claim 1, wherein the act of performing albedo normalization is achieved by dividing the structured illumination image of the object by the uniform illumination image of the object.

7. Apparatus for providing a range image of an object, the apparatus comprising:

structured illumination image acquisition means for acquiring a structured illumination image of the object;

uniform illumination image acquisition means for acquiring a uniform illumination image of the object;

albedo-normalization means, connected to the structured illumination image acquisition means and the uniform illumination image acquisition means, for using the structured illumination image of the object and the uniform illumination image of the object to provide an albedo-normalized image; and

processing means, connected to the albedo-normalization means, for processing the albedo-normalized image using a focus measure to provide a focal image that conveys relative range information.

8. The apparatus of claim 7, wherein the albedo normalization means includes means for dividing the structured illumination image of the object by the uniform illumination image of the object.

9. The apparatus of claim 7, further including calibration means, connected to the processing means, for using the focal image to provide an absolute range image.

10. Apparatus for providing a range image of an object, the apparatus comprising:

a structured illumination image acquirer that provides a structured illumination image of the object;

a uniform illumination image acquirer that provides a uniform illumination image of the object;

an albedo-normalizer, connected to the structured illumination image acquirer and the uniform illumination image acquirer, capable of using the structured illumination image of the object and the uniform illumination image of the object to provide an albedo-normalized image; and

a focus measure processor, connected to the albedo-normalizer, capable of processing the albedo-normalized image using a focus measure to provide a focal image that conveys relative range information.

11. The apparatus of claim 10, wherein the albedo-normalizer includes an image divider that divides the structured illumination image of the object by the uniform illumination image of the object.

12. The apparatus of claim 10, further including a calibration processor, connected to the focus measure processor, capable of using the focal image to provide an absolute range image.

13. A method for providing a range image of an object, the method comprising the acts of:

acquiring a first structured illumination image of the object at a first focal position;

acquiring a first uniform illumination image of the object at the first focal position;

performing albedo normalization using the first structured illumination image of the object and the first uniform illumination image of the object to provide a first albedo-normalized image;

processing the first albedo-normalized image using a focus measure to provide a first focal image; and

performing range normalization using the first focal image and a second focal image acquired at a second focal position to provide a relative normalized range image.

14. The method of claim 13, further including the act of calibration to obtain an absolute normalized range image from the relative normalized range image.

15. The method of claim 13, wherein the act of processing the first albedo-normalized image using a focus measure to provide a first focal image includes the act of using an RMS operation.

16. The method of claim 13, wherein the act of processing the first albedo-normalized image using a first focus measure to provide a focal image includes the act of using a Laplacian-like convolution.

17. The method of claim 13, wherein the act of performing albedo normalization is achieved by dividing the first structured illumination image of the object by the first uniform illumination image of the object.

18. Apparatus for providing a range image of an object, the apparatus comprising:

first structured illumination image acquisition means for acquiring a first structured illumination image of the object at a first focal position;

first uniform illumination image acquisition means for acquiring a first uniform illumination image of the object at the first focal position;

albedo-normalization means, connected to the first structured illumination image acquisition means and the first uniform illumination image acquisition means, for using the first structured illumination image of the object and the first uniform illumination image of the object to perform albedo normalization so as to provide a first albedo-normalized image;

first processing means, connected to the first albedo-normalization means, for processing the first albedo-normalized image using a focus measure to provide a first focal image;

range normalization means, connected to the first processing means, for performing range normalization using the first focal image and a second focal image at a second focal position to provide a normalized range image.

19. The apparatus of claim 18, wherein said albedo-normalization means includes means for dividing the first structured illumination image of the object by the first uniform illumination image of the object.

20. Apparatus for providing a range image of an object, the apparatus comprising:

a first structured illumination image acquirer that provides a first structured illumination image of the object at a first focal position;

a first uniform illumination image acquirer that provides a first uniform illumination image of the object at the first focal position;

a first albedo-normalizer, connected to the first structured illumination image acquirer and the first uniform illumination image acquirer, capable of using the first structured illumination image of the object and the first uniform illumination image of the object to provide a first albedo-normalized image;

a first focus measure processor, connected to the first albedo-normalizer, capable of processing the first albedo-normalized image using a first focus measure to provide a first focal image;

a range image processor, connected to the focus measure processor, capable of using the first focal image and a second focal image at a second focal position to provide a normalized range image.

21. The apparatus of claim 20, wherein said albedo-normalizer includes an image divider capable of dividing the first structured illumination image of the object by the first uniform illumination image of the object.