Ring location

1. A method for localizing, in a digital image, at least two circles, one of which is entirely included in the other, the method comprising:

estimating a position of a center of one of the two circles;

centering a grid in a vicinity of the estimated position of the center, the grid being of reduced dimensions as compared to the image dimensions;

determining a first radius and first coordinates of the center of the one of the two circles;

determining a second radius and second coordinates of the center of another of the two circles based at least in part on the first radius and/or first coordinates of the center of the one of the two circles; and

outputting an indication of the localization of the at least two circles.

2. The method of claim 1 , using a single grid having a minimum pitch.

3. The method of claim 2 , wherein one of the two circles is larger than another of the two circles, and wherein the maximum size of the grid is smaller than a size of the larger circle.

4. The method of claim 3 , wherein the maximum size of the grid is smaller than a size of the circle forming the limit of strongest contrast.

5. The method of claim 3 , wherein the maximum size of the grid is smaller than a size of the smaller circle.

6. The method of claim 1 , wherein the step of estimating the position of the center includes:

a) predefining a set of possible radiuses of the one of the two circles;

b) sizing two one-dimension accumulators in a form of a column array of at most a size of an image in abscissa and of a line array of at most the image size in ordinate;

c) successively, for each image pixel:

(i) successively selecting each possible radius;

(ii) evaluating the position of a possible center of a circle of the selected radius and a considered pixel of which is on the periphery; and

(iii) incrementing the accumulators at the abscissa and ordinate of the possible center; and

d) selecting, as coordinates of the located center, the abscissa and the ordinate corresponding to the maximum of the accumulators.

7. The method of claim 6 , wherein incrementing the accumulators includes changing the accumulators by an increment of one.

8. The method of claim 6 , wherein incrementing the accumulators includes changing the accumulators by an increment which is weighted according to the significance of the gradient at the considered pixel.

9. A method for localizing, in a digital image, a ring defined by the inclusion of a first circle of relatively small radius in a second circle of relatively large radius, consisting of localizing the first and second circles according to the method of claim 1 .

10. The method of claim 9 , wherein the ring represents an iris of an eye, the first center represents a pupil of the eye and the second circle represents a limit between the iris and a cornea of the eye.

11. The method of claim 10 , wherein the pupil is the one of two circles.

12. A method of determining a position of a center of a circle in an image, the method comprising:

estimating a position of a center of a circle in an image, the image having an image area;

centering a grid around the estimated position of the center, the grid having a grid area that is less than the image area

determining the actual position of the center of the circle within the grid; and

providing an indication of the actual position of the center of the circle.

13. The method of claim 12 , wherein the circle has a circle area and the circle area is smaller than the image area and is larger than the grid area.

14. The method of claim 12 , further comprising determining an actual radius of the circle.

15. The method of claim 12 , wherein said step of estimating a position of a center of a circle includes:

a) determining a set of possible radiuses;

b) determining a first one-dimension accumulator representing at most the number of lines of the image and a second one-dimension accumulator representing at most the number of columns of the image;

c) selecting one radius from the set of possible radiuses;

d) determining a gradient of a possible center of the circle;

e) incrementing the first and second accumulators at the possible center;

f) repeating steps c, d and e for each radius of the set of possible radiuses;

g) repeating steps c, d, e and f for each possible center of the circle; and

h) determining that the estimated position of the center of the circle is where the first and second accumulators are at a maximum.

16. The method of claim 15 , wherein said step of determining a gradient of a possible center of the circle includes comparing a first level of gray of the possible center with a second level of gray of a neighboring point.

17. A method of determining positions of centers of first and second circles in an image, the method comprising:

estimating a position of a center of a first circle in an image, the image having an image area;

centering a first grid around the estimated position of the center, the first grid having a first grid area that is less than the image area;

determining the actual position of the center of the first circle within the first grid;

centering a second grid around the actual position of the center of the first circle, the second grid having a second grid area that is less than the image area;

determining the actual position of the center of the second circle within the second grid; and

outputting an indication of the positions of centers of the first and second circles in the image.

18. The method of claim 17 , wherein the first circle is smaller than the second circle.

19. The method of claim 18 , wherein the first circle is located within the second circle.