Darkfield tracking

1. A method for darkfield tracking at a sensor, the method comprising:

determining whether an object is interacting with the sensor;

provided an object is not interacting with the sensor, making a determination that a darkfield candidate image can be captured at the sensor;

determining whether to capture a darkfield candidate image at the sensor based at least in part on the determination that a darkfield candidate image can be captured at the sensor;

provided an object is interacting with the sensor, making the determination that a darkfield candidate image cannot be captured at the sensor;

responsive to making a determination to capture the darkfield candidate image, capturing the darkfield candidate image at the sensor, wherein the darkfield candidate image is an image absent an object interacting with the sensor; and

updating a darkfield estimate with the darkfield candidate image.

2. The method of claim 1 , wherein the determination to capture the darkfield candidate image is also based at least in part on making a determination that a minimum amount of time has passed since a most recent darkfield candidate image capture.

3. The method of claim 1 , wherein the determination to capture the darkfield candidate image is also based at least in part on making a determination that a temperature change since a most recent darkfield candidate image capture has exceeded a temperature threshold.

4. The method of claim 1 , further comprising:

provided an object is interacting with the sensor, generating an object lifting signal when it is determined the object is no longer interacting with the sensor.

5. The method of claim 1 , wherein the updating the darkfield estimate with the darkfield candidate image comprises:

merging the darkfield candidate image with the darkfield estimate.

6. The method of claim 1 , wherein the updating the darkfield estimate with the darkfield candidate image comprises:

provided the darkfield estimate is not stored, storing the darkfield candidate image as the darkfield estimate.

7. The method of claim 1 , wherein the sensor is an ultrasonic sensor, wherein the determining whether an object is interacting with the sensor comprises:

transmitting signals at ultrasonic transducers of the ultrasonic sensor;

receiving reflected signals at ultrasonic transducers of the ultrasonic sensor; and

provided the signals are not indicative of an object interacting with the ultrasonic sensor, determining that an object is not interacting with the ultrasonic sensor.

8. The method of claim 7 , further comprising:

comparing the reflected signals to a void flags threshold around a moving average; and

provided the reflected signals are within the void flags threshold, determining that the signals are not indicative of an object interacting with the ultrasonic sensor.

9. The method of claim 8 , further comprising:

updating the moving average provided the signals are within a moving average threshold, wherein the moving average threshold is larger than the void flags threshold.

10. The method of claim 8 , further comprising:

provided the reflected signals are within the void flags threshold, making the determination that a darkfield candidate image can be captured at the sensor.

11. The method of claim 1 , further comprising:

responsive to detecting an object interacting with the sensor, capturing an image comprising the object; and

correcting the image using the darkfield estimate.

12. The method of claim 1 , further comprising:

responsive to making the determination that a darkfield candidate image cannot be captured at the sensor, activating a timer; and

responsive to a predetermined period of the timer lapsing, repeating the determining whether an object is interacting with the sensor.

13. The method of claim 1 , further comprising:

responsive to making the determination that a darkfield candidate image cannot be captured at the sensor, comparing a signal received at the sensor to a sensor temperature correlation scheme; and

modifying signal thresholds according to the sensor temperature correlation scheme during the determining whether an object is interacting with the sensor.

14. The method of claim 1 , further comprising:

provided an object is not interacting with the sensor:

determining a temperature of the sensor;

associating the temperature with a corresponding pixel value; and

storing the temperature and pixel value pair.

15. The method of claim 1 , wherein determining whether an object is interacting with the sensor comprises:

measuring a temperature of the sensor;

receiving a pixel value for a pixel of the sensor;

comparing the pixel value to an expected pixel value based on the temperature of the sensor; and

determining whether an object is interacting with the sensor based on the comparing the pixel value to an expected pixel value based on the temperature of the sensor.

16. The method of claim 15 , wherein the pixel value is received for a plurality of pixels.

17. An electronic device comprising:

a fingerprint sensor;

a memory; and

a processor configured to:

determine whether an object is interacting with the fingerprint sensor;

provided an object is not interacting with the fingerprint sensor, make a determination that a darkfield candidate image can be captured at the fingerprint sensor;

determine whether to capture a darkfield candidate image at the fingerprint sensor based at least in part on the determination that a darkfield candidate image can be captured at the fingerprint sensor;

responsive to making a determination to capture the darkfield candidate image, capture the darkfield candidate image at the fingerprint sensor, wherein the darkfield candidate image is an image absent an object interacting with the fingerprint sensor;

update a darkfield estimate with the darkfield candidate image

responsive to detecting an object interacting with the fingerprint sensor, capture an image comprising the object; and

correct the image using the darkfield estimate.

18. The electronic device of claim 17 , wherein the processor is further configured to:

merge the darkfield candidate image with the darkfield estimate.

19. The electronic device of claim 17 , wherein the fingerprint sensor is an ultrasonic fingerprint sensor, wherein the processor is further configured to:

transmit signals at ultrasonic transducers of the ultrasonic fingerprint sensor;

receive reflected signals at ultrasonic transducers of the ultrasonic fingerprint sensor; and

provided the signals are not indicative of an object interacting with the ultrasonic fingerprint sensor, determine that an object is not interacting with the ultrasonic fingerprint sensor.

20. The electronic device of claim 17 , wherein the processor is further configured to:

forward the image to a matcher for authentication.

21. A non-transitory computer readable storage medium having computer readable program code stored thereon for causing a computer system to perform a method for darkfield tracking at a sensor, the method comprising:

determining whether an object is interacting with the sensor;

provided an object is not interacting with the sensor, making a determination that a darkfield candidate image can be captured at the sensor;

determining whether to capture a darkfield candidate image at the sensor based at least in part on the determination that a darkfield candidate image can be captured at the sensor;

responsive to making a determination to capture the darkfield candidate image, capturing the darkfield candidate image at the sensor, wherein the darkfield candidate image is an image absent an object interacting with the sensor;

updating a darkfield estimate with the darkfield candidate image, wherein the updating the darkfield estimate with the darkfield candidate image comprises:

merging the darkfield candidate image with the darkfield estimate;

responsive to detecting an object interacting with the sensor, capturing an image comprising the object; and

correcting the image using the darkfield estimate.

22. The non-transitory computer readable storage medium of claim 21 , wherein the sensor is an ultrasonic sensor, wherein the determining whether an object is interacting with the sensor comprises:

transmitting signals at ultrasonic transducers of the ultrasonic sensor;

receiving reflected signals at ultrasonic transducers of the ultrasonic sensor; and

provided the signals are not indicative of an object interacting with the ultrasonic sensor, determining that an object is not interacting with the ultrasonic sensor.