Method and apparatus for detecting a pupil
A method for detecting a pupil in an image of an eye, comprises analyzing the image, exploiting an expected shape of the pupil, to identify portions of the image that are candidate portions of the pupil. A first region of the image, which corresponds to a lower part of the pupil, is analyzed in preference to a second region of the image, which corresponds to an upper part of the pupil, so as to reduce or avoid errors arising from artifacts that tend to be present in said second region of the image. A computer or processor program and an apparatus for performing the method are also disclosed.
1. A method of detecting a pupil in an image of an eye, comprising:
identifying pixels having characteristics consistent with a pupil;
searching line by line for a longest line in the image, the longest line being a line having the largest number of identified pixels having characteristics consistent with the pupil among lines extending in a predetermined direction in the image; and
using the longest line to determine a size and/or position of the pupil,
wherein the searching is conducted in a first region of the image, which corresponds to a lower part of the pupil, in preference to a second region of the image, which corresponds to an upper part of the pupil, and
wherein the searching continues as long as the number of identified pixels in the line is increasing.
2. A method as claimed in claim 1 , comprising using a length of the longest line to determine a diameter or radius of the pupil.
3. A method as claimed in claim 1 , comprising using a centre position of the longest line to determine a centre of the pupil.
4. A method as claimed in claim 1 , wherein the predetermined direction is the horizontal direction and each said line is a horizontal row.
5. A method as claimed in claim 1 , wherein said longest line is the line having the largest number of consecutive such identified pixels among said lines extending in said predetermined direction in the image.
6. Method as claimed in claim 5 , comprising interrupting the search when the number of such identified pixels is significantly larger than the number in previous line and designating the length of the longest line as a length of the penultimate searched line.
7. Method as claimed in claim 1 , wherein searching for the longest line in the image is directed only to the longest fragment of the line.
8. A method as claimed in claim 4 , wherein the search is conducted horizontal row by horizontal row, starting in the first region.
9. A method as claimed in claim 8 , comprising starting the search in a horizontal row of the first region that is farthest from the second region and going horizontal row by horizontal row towards the second region.
10. A method as claimed in claim 1 , wherein the image is a split image captured by an image capturing device having a split prism rangefinder, and the said predetermined direction coincides with the direction of a split line between the two fields of the split image.
11. A method as claimed in claim 10 , wherein said longest line is the line having the largest number of consecutive such identified pixels among said lines extending in said predetermined direction in the image.
12. A method of detecting a pupil in an image of an eye, comprising:
analysing the image which is split into first and second regions, the first region of the image corresponding to a lower part of the pupil and the second region of the image corresponding to an upper part of the pupil,
wherein the analysis comprising:
i. starting from a lowest row of the second region, and working upwards row by row, identifying portions of the image that are candidate portions of the pupil;
ii. identifying a row at which the identified portions start to decrease in length; and
iii. determining a location of the pupil based on the identified row.
13. An apparatus for detecting a pupil in an image of an eye, comprising:
an identifying unit configured to identify pixels having characteristics consistent with a pupil;
a search unit configured to search line by line for a longest line in the image, the longest line being the line having the largest number of identified pixels having characteristics consistent with the pupil among lines extending in a predetermined direction in the image;
a determination unit configured to use the longest line to determine a size and/or position of the pupil,
wherein the search unit is configured to conduct the search in a first region of the image, which corresponds to a lower part of the pupil, in preference to a second region of the image, which corresponds to an upper part of the pupil, and to continue the search as long as the number of identified pixels is increasing.