Range mapping using speckle decorrelation

1. A method for mapping, comprising:

projecting a primary speckle pattern from an illumination assembly into a target region;

capturing a plurality of reference images of the primary speckle pattern at different, respective distances from the illumination assembly in the target region;

capturing a test image of the primary speckle pattern that is projected onto a surface of an object in the target region;

comparing the test image to the reference images so as to identify a reference image in which the primary speckle pattern most closely matches the primary speckle pattern in the test image; and

estimating a location of the object based on a distance of the identified reference image from the illumination assembly.

2. The method according to claim 1 , and comprising finding respective offsets between the primary speckle pattern on multiple areas of the object in the test image and the primary speckle pattern in the identified reference image so as to derive a three-dimensional (3D) map of the object.

3. The method according to claim 1 , wherein at least some of the different, respective distances are separated axially by more than an axial length of the primary speckle pattern at the respective distances.

4. The method according to claim 1 , wherein comparing the test image to the reference images comprises computing a respective cross-correlation between the test image and each of at least some of the reference images, and selecting the reference image having the greatest respective cross-correlation with the test image.

5. The method according to claim 1 , wherein capturing the test image comprises capturing a succession of test images while the object is moving, and wherein estimating the location comprise tracking a movement of the object in the target region.

6. The method according to claim 5 , wherein the object is a part of a human body, and wherein tracking the movement comprises identifying a gesture made by the part of the human body and providing an input to a computer application responsively to the gesture.

7. Apparatus for mapping, comprising:

an illumination assembly, which is configured to project a primary speckle pattern into a target region;

an imaging assembly, which is configured to capture a plurality of reference images of the primary speckle pattern at different, respective distances from the illumination assembly in the target region, and to capture a test image of the primary speckle pattern that is projected onto a surface of an object in the target region; and

an image processor, which is coupled to compare the test image to the reference images so as to identify a reference image in which the primary speckle pattern most closely matches the primary speckle pattern in the test image, and to estimate a location of the object based on a distance of the identified reference image from the illumination assembly.

8. The apparatus according to claim 7 , wherein the image processor is arranged to find respective offsets between the primary speckle pattern on multiple areas of the object in the test image and the primary speckle pattern in the identified reference image so as to derive a three-dimensional (3D) map of the object.

9. The apparatus according to claim 7 , wherein at least some of the different, respective distances are separated axially by more than an axial length of the primary speckle pattern at the respective distances.

10. The apparatus according to claim 7 , wherein the image processor is arranged to compute a respective cross-correlation between the test image and each of at least some of the reference images, and to select the reference image having the greatest respective cross-correlation with the test image.

11. The apparatus according to claim 7 , wherein the imaging assembly is configured to capture a succession of test images while the object is moving, and wherein the image processor is arranged to process the test images so as to track a movement of the object in the target region.

12. The apparatus according to claim 11 , wherein the object is a part of a human body, and wherein the image processor is arranged to identify a gesture made by the part of the human body and to provide an input to a computer application responsively to the gesture.

13. A computer software product, comprising a computer-readable medium in which program instructions are stored, which instructions, when read by a computer, cause the computer to receive a plurality of reference images of the primary speckle pattern that is projected into a target region by an illumination assembly, wherein the reference images are captured at different, respective distances from the illumination assembly in the target region, and to receive a test image of the primary speckle pattern that is projected onto a surface of an object in the target region, and to compare the test image to the reference images so as to identify a reference image in which the primary speckle pattern most closely matches the primary speckle pattern in the test image, and to estimate a location of the object based on a distance of the identified reference image from the illumination assembly.

14. The product according to claim 13 , wherein the instructions cause the computer to find respective offsets between the primary speckle pattern on multiple areas of the object in the test image and the primary speckle pattern in the identified reference image so as to derive a three-dimensional (3D) map of the object.