Depth mapping using optical elements having non-uniform focal characteristics

1. A method for mapping, comprising:

projecting a pattern onto an object via an astigmatic optical element having different, respective focal lengths in different meridional planes of the element,

wherein the astigmatic optical element causes the pattern that is projected on the object to be elongated with a direction of elongation that varies with a distance from the element;

capturing an image of the pattern on the object; and

processing the image so as to derive a three-dimensional (3D) map of the object responsively to the different focal lengths,

wherein processing the image comprises finding the distance to the object responsively to the direction of the elongation.

2. The method according to claim 1 , wherein the pattern comprises multiple spots, which are projected onto the object by the astigmatic optical element as ellipses, having respective major axes in the direction of elongation.

3. The method according to claim 2 , wherein the ellipses have respective minor axes, and the major and minor axes of each ellipse have respective lengths, and wherein finding the distance comprises comparing the respective lengths of the major and minor axes.

4. The method according to claim 1 , wherein the astigmatic optical element comprises a combination of at least two cylindrical lens surfaces in different, respective orientations.

5. The method according to claim 1 , wherein the astigmatic optical element comprises a diffractive optical element.

6. The method according to claim 1 , wherein the astigmatic optical element comprises an off-axis element.

7. A method for mapping, comprising:

capturing an image of an object, which has a surface with features having respective shapes at respective locations on the surface, using an optical objective that is configured to modify the shapes of the features in the image as a function of a distance of the respective locations from the objective,

wherein the optical objective causes the shapes of the features in the image to be elongated with a direction of elongation that varies with the distance of the respective locations of the features from the objective; and

processing the image so as to derive a three-dimensional (3D) map of the object responsively to the modified shapes of the features,

wherein processing the image comprises finding the distance to the respective locations responsively to the direction of the elongation.

8. The method according to claim 7 , and comprising projecting a pattern of multiple spots onto the surface of the object, wherein the features in the image comprise the spots, and wherein processing the image comprises analyzing the shapes of the spots in the image.

9. The method according to claim 7 , wherein the optical objective comprises an astigmatic optical element having different, respective focal lengths in different meridional planes of the element.

10. The method according to claim 7 , wherein the objective comprises a diffractive optical element.

11. An apparatus for mapping, comprising:

an illumination assembly, which comprises an astigmatic optical element having different, respective focal lengths in different meridional planes of the element and is configured to project a pattern onto an object via the astigmatic optical element,

wherein the astigmatic optical element causes the pattern that is projected on the object to be elongated with a direction of elongation that varies with a distance from the element;

an image capture assembly, which is configured to capture an image of the pattern on the object; and

an image processor, which is configured to process the image so as to derive a three-dimensional (3D) map of the object responsively to the different focal lengths,

wherein the image processor is configured to find the distance to the object responsively to the direction of the elongation.

12. The apparatus according to claim 11 , wherein the pattern comprises multiple spots, which are projected onto the object by the astigmatic optical element as ellipses, having respective major axes in the direction of elongation.

13. The apparatus according to claim 12 , wherein the ellipses have respective minor axes, and the major and minor axes of each ellipse have respective lengths, and wherein the image processor is configured to find the distance by comparing the respective lengths of the major and minor axes.

14. The apparatus according to claim 11 , wherein the astigmatic optical element comprises a combination of at least two cylindrical lenses in different, respective orientations.

15. The apparatus according to claim 11 , wherein the astigmatic optical element comprises a diffractive optical element.

16. The apparatus according to claim 11 , wherein the astigmatic optical element comprises an off-axis element.

17. An apparatus for mapping, comprising:

an image capture assembly, which is configured to capture an image of an object, which has a surface with features having respective shapes at respective locations on the surface, and comprises an optical objective that is configured to modify the shapes of the features in the image as a function of a distance of the respective locations from the objective,

wherein the optical objective causes the shapes of the features in the image to be elongated with a direction of elongation that varies with the distance of the respective locations of the features from the objective; and

an image processor, which is configured to process the image so as to derive a three-dimensional (3D) map of the object responsively to the modified shapes of the features,

wherein the image processor is configured to find the distance to the respective locations responsively to the direction of the elongation.

18. The apparatus according to claim 17 , and comprising an illumination assembly, which is configured to project a pattern of multiple spots onto the surface of the object, wherein the features in the image comprise the spots, and wherein the image processor is configured to derive the 3D map responsively to the shapes of the spots in the image.

19. The apparatus according to claim 17 , wherein the optical objective comprises an astigmatic optical element having different, respective focal lengths in different meridional planes of the element.

20. The apparatus according to claim 17 , wherein the optical objective comprises a diffractive optical element.