SYSTEMS AND METHODS FOR LUMINANCE-BASED SCENE-CHANGE DETECTION FOR CONTINUOUS AUTOFOCUS

• US 20120188386A1
• Filed: 08/11/2011
• Published: 07/26/2012
• Est. Priority Date: 01/26/2011
• Status: Active Grant

• Current Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Original Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Inventors: Kulkarni, Prajit, Lin, Sheng, Pope, David R.
• Granted Patent: US 8,922,707 B2
• Time in Patent Office: Days
• US Class Current: 348/208.11
• CPC Class Codes: H04N 23/67 Focus control based on electronic image sensor signals

Abstract

Imaging systems with image sensors and image processing circuitry are provided. The image processing circuitry may identify motion and perform autofocus (e.g., continuous autofocus) using images captured by an image sensor. Auto exposure metrics such as average luminance values and autofocus statistics such as sharpness scores may be calculated for each image. The auto exposure metrics may be used to calculate motion scores and identify directional motion between a series of captured images. The motion scores may be used with the sharpness scores to determine when to perform autofocus functions such as when to refocus a lens for a continuous autofocus application. For example, the motion scores may be monitored to identify motion that exceeds a given magnitude and duration. After identification of motion, motion scores and sharpness scores may be used to determine when a given scene has stabilized and when the lens should be refocused.

Claims

1. A method, comprising:

focusing incident light on a pixel array with a lens;

capturing a sequence of images with the pixel array;

with image processing circuitry, calculating statistical metrics from the captured sequence of images;

calculating a motion score based on the statistical metrics; and

determining when to refocus the lens based at least partly on the motion score.

2. The method defined in claim 1, wherein determining when to refocus the lens comprises identifying that the motion score of at least one image is greater than a first threshold.

3. The method defined in claim 2, wherein determining when to refocus the lens further comprises identifying a reference image from the sequence of images.

4. The method defined in claim 3, wherein determining when to refocus the lens further comprises incrementing a motion count and wherein the motion count is indicative of how many images from the sequence of images have a motion score that is greater than the first threshold.

5. The method defined in claim 4, wherein determining when to refocus the lens further comprises identifying that the motion count is greater than a second threshold.

6. The method defined in claim 5, wherein determining when to refocus the lens further comprises, in response to identifying that the motion count is greater than a motion counter threshold, incrementing a stable count and wherein the stable count is indicative of how many images from the sequence of images have a motion score that is less than the first threshold.

7. The method defined in claim 6, wherein determining when to refocus the lens further comprises identifying that the stable count is greater than a third threshold.

8. The method defined in claim 7, wherein determining when to refocus the lens further comprises:

calculating a focus-change value based on a focus score of the reference image and a focus score of a given image from the sequence of images; and

identifying that the focus-change value is greater than a fourth threshold, the method further comprising;

in response to identifying that the focus-change value is greater than the fourth threshold, refocusing the lens.

9. A method for identifying motion from a first image and a second image using image processing circuitry, comprising:

calculating a first set of values associated with automatic exposure metrics of the first image;

calculating a second set of values associated with automatic exposure metrics of the second image; and

calculating a first set of motion scores based on differences between the values in the first set of values and the values in the second set of values, wherein each motion score of the first set of motion scores is associated with a direction of motion.

10. The method defined in claim 9 wherein calculating the first set of values comprises calculating zone luminance values.

11. The method defined in claim 9 wherein calculating the first set of motion scores comprises:

forming a first subset from the first set of values and a second subset from the second set of values; and

calculating a motion score based on a difference between a sum of differences between the first subset and the second subset.

12. The method defined in claim 9 further comprising:

identifying a highest motion score from the first set of motion scores;

identifying a second-highest motion score from the first set of motion scores; and

calculating a final motion score based on the highest motion score and the second-highest motion score.

13. The method defined in claim 12 further comprising:

identifying that the final motion score exceeds a motion threshold.

14. The method defined in claim 13 further comprising:

identifying a direction of motion associated with the final motion score.

15. The method defined in claim 9 further comprising:

calculating a third set of values associated with automatic exposure metrics of a third image; and

calculating a second set of motion scores based on differences between the values in the second set of values and the values in the third set of values, wherein each motion score of the second set of motion scores is associated with a direction of motion.

16. The method defined in claim 15 further comprising:

identifying motion in a first direction based on the first set of motion scores;

identifying motion in a second direction based on the second set of motion scores;

identifying that the first direction is equal to the second direction; and

in response to identifying that the first direction is equal to the second direction, refocusing a lens.

21. The method defined in claim 16 further comprising:

calculating a first sharpness score from the first image;

calculating a second sharpness score from the second image; and

calculating a difference between the first and second sharpness scores, wherein refocusing the lens comprises refocusing the lens in response to identifying that the difference is greater than a threshold value.

17. An imager, comprising:

a pixel array that receives incident light to form captured images;

a lens that focuses incident light on the pixel array;

a motion detection module that calculates motion scores using statistical metrics associated with the captured images; and

a processing module that monitors the motion scores and determines when to refocus the lens based on the motion scores.

18. The imager defined in claim 17 further comprising image processing circuitry operable to receive the captured images from the pixel array and operable to calculate zone luminance values based on the captured images.

19. The imager defined in claim 18, wherein the motion detection module comprises motion detection circuitry operable to receive the zone luminance values from the image processing circuitry and operable to calculate the motion scores based on the zone luminance values.

20. The imager defined in claim 19 further comprising an auto-focus module operable to re-focus the lens, wherein the processing module is operable to transmit control signals to the auto-focus module that direct the auto-focus module to re-focus the lens.