Calibrating a camera system

• US 8,593,524 B2
• Filed: 12/14/2007
• Issued: 11/26/2013
• Est. Priority Date: 12/18/2006
• Status: Expired due to Fees

• Current Assignee: Koninklijke Philips Electronics N.V. (Koninklijke Philips N.V.)
• Original Assignee: Koninklijke Philips N.V.
• Inventors: Kleihorst, Richard Petrus
• Primary Examiner(s): Harold, Jeffrey
• Assistant Examiner(s): TEITELBAUM, MICHAEL E
• Application Number:
  US12/518,650
• Publication Number:
  US 20100020178A1
• Time in Patent Office: 2,174 Days
• Field of Search: 382/154
• US Class Current: 348/175
• CPC Class Codes: G06T 2207/10012 Stereo images
• CPC Class Codes: G06T 7/85 Stereo camera calibration

Abstract

A method of calibrating a camera system which is capable of recording at least two images simultaneously is described. The method comprises the step of processing a first pair of images, wherein the processing step comprises transforming at least one of a first and a second image of the first pair through a projective map on a plane, the map depending on one or more parameter values, so as to align an epipolar line in the first image with a corresponding epipolar line in the second image of the first pair, resulting in a second pair of images. The method further comprises estimating respective values for a disparity, which values associate respective first pixels along a first epipolar line in a first image of the second pair with respective second pixels along a corresponding second epipolar line in a second image of the second pair, and displacing respective pixels of the first epipolar line in the first image of the second image pair in accordance with a respective value of the disparity for the respective pixels. The method further comprises determining a measure of error between the first and the second image of the second image pair, dependent on the one or more parameters, determining an update for the one or more parameter values for reducing a value of the measure of error, and updating the one or more parameter values, using the update.

Claims

1. A method of calibrating a camera system which is capable of recording at least two images simultaneously, the method comprising:

processing a first pair of images, wherein the processing step comprises;

transforming at least one of a first and a second image of the first pair through a projective map on a plane, the map depending on one or more parameter values, so as to align an epipolar line in the first image with a corresponding epipolar line in the second image of the first pair, resulting in a second pair of images;

estimating respective values for a disparity, which values associate respective first pixels along a first epipolar line in a first image of the second pair with respective second pixels along a corresponding second epipolar line in a second image of the second pair;

displacing respective pixels of the first epipolar line in the first image of the second image pair in accordance with a respective value of the disparity for the respective pixels;

determining a single measure of error as the sum of squares of individual error terms, where each individual error term comprises a difference between a first window of pixels at coordinates x,y on the first epipolar line in the first image and a second window of pixels on the second epipolar line in the second image, wherein the second window of pixels are displaced in the x-direction over the value of the estimated disparity d(x,y) for the second image;

determining an update for the one or more parameter values for reducing a value of the measure of error; and

updating the one or more parameter values, using the update.

2. The method of claim 1, wherein the transforming step comprises rectifying a single one of the first and the second image of the first pair through a projective map on a plane of the other one of the first and the second image of the first pair.

3. The method of claim 1, wherein the transforming step comprises rectifying both the first and the second image of the first pair through an individual projective map on one and the same plane so as to horizontally align the respective epipolar lines in the images of the second pair.

4. The method of claim 1, comprising the step of subjecting a subsequent pair of images to the processing operation.

5. The method of claim 1, wherein the camera system comprises a stereo camera system.

7. A non-transitory computer storage medium comprising program code means stored on a computer-readable medium for performing the method of claim 1 when said program code is executed on a computer.

6. Software stored on a non-transitory computer storage medium for calibrating a camera system which is capable of recording at least two images simultaneously, the software comprising instructions for processing a first pair of images, the instructions being operative to carry out the following steps:

transforming at least one of a first and a second image of the first pair through a projective map on a plane, the map depending on one or more parameter values, so as to align an epipolar line in the first image of the first pair with a corresponding epipolar line in the second image of the first pair, resulting in a second pair of images;

estimating respective values for a disparity, which values associate respective first pixels along the first epipolar line in a first image of the second pair with respective second pixels along a corresponding second epipolar line in a second image of the second pair;

displacing respective pixels of the first epipolar line in the first image of the second pair in accordance with a respective value of the disparity for the respective pixels;

determining a single measure of error as the sum of squares of individual error terms, where each individual error term comprises a difference between a first window of pixels at coordinates x,y on the first epipolar line in the first image and a second window of pixels on the second epipolar line in the second image, wherein the second window of pixels are displaced in the x-direction over the value of the estimated disparity d(x,y) for the second image;

determining an update for the one or more parameter values for reducing a value of the measure of error; and

updating the one or more parameter values, using the update.

8. A camera system which is capable of recording at least two images simultaneously, the camera system comprising a camera and calibrating means arranged to process a first pair of images for calibrating the camera system, wherein the calibrating means comprises:

rectifier means arranged to transform at least one of a first and a second image of the first pair so as to align an epipolar line in the first image with a corresponding epipolar line in the second image of the first pair, resulting in a second pair of images;

estimating means arranged to estimate respective values for a disparity, which values associate respective first pixels along a first epipolar line of a first image of the second pair with respective second pixels along a corresponding second epipolar line in a second image of the second pair;

displacement means arranged to displace respective pixels of the first epipolar line in the first image of the second image pair in accordance with a respective value of the disparity for the respective pixels;

error-determining means arranged to determine a single measure of error as the sum of squares of individual error terms, where each individual error term comprises a difference between a first window of pixels at coordinates x,y on the first epipolar line in the first image and a second window of pixels on the second epipolar line in the second image, wherein the second window of pixels are displaced in the x-direction over the value of the estimated disparity d(x,y) for the second image;

error-reduction means for determining an update for the one or more parameter values for reducing a value of the measure of error;

updating means for updating the one or more parameter values, using the update.

9. The system of claim 8, comprising fetching means for fetching a further pair of images to be processed by the calibration means.

10. The camera system of claim 8, comprising a stereo camera system.

11. Calibrating means for use in the camera system of claim 8.