Close touch detection and tracking

1. A method, comprising:

based on a first scan of a touch-sensing surface, detecting an initial location of a first contact and an initial location of a second contact at the touch-sensing surface;

during a second scan of the touch-sensing surface subsequent to the first scan, detecting a plurality of signal levels caused by the first contact and the second contact, wherein each signal level is associated with one of a plurality of unit cells of the touch-sensing surface;

identifying a first signal level of the plurality of signal levels as a local maximum; and

locating a lost touch based on one or more signal levels associated with one or more unit cells within a fixed distance from a first unit cell associated with the local maximum.

2. The method of claim 1 , further comprising detecting a close touch condition by determining whether a distance between the initial location of the first contact and the initial location of the second contact locations is less than a threshold distance.

3. The method of claim 1 , further comprising assigning a first touch ID corresponding to the unit cell associated with the local maximum.

4. The method of claim 1 , further comprising determining whether a signal level associated with the lost touch exceeds a touch threshold.

5. The method of claim 1 , further comprising assigning a second touch ID corresponding to a unit cell associated with the lost touch.

6. The method of claim 1 , wherein identifying the first signal level as a local maximum comprises determining that the first signal level is higher than a signal level of any unit cell adjacent to a unit cell associated with the first signal level.

7. The method of claim 1 , wherein locating the lost touch comprises:

identifying a second unit cell having a highest signal level from among a subset of the plurality of unit cells, wherein each unit cell in the subset of unit cells is two unit cells away from the first unit cell; and

assigning to the second unit cell a touch ID corresponding to the lost touch.

8. The method of claim 1 , further comprising:

smoothing a path of the first contact, wherein the path of the first contact is calculated based on locations of the first contact determined by successive scans of the touch-sensing surface; and

smoothing a path of the second contact, wherein the path of the second contact is calculated based on locations of the second contact determined by the successive scans of the touch-sensing surface.

9. The method of claim 1 , wherein identifying the first signal level as a local maximum comprises comparing the first signal level with signal levels of unit cells that are horizontally and vertically adjacent to the first unit cell.

10. The method of claim 9 , further comprising:

adding the local maximum to a list of local maxima; and

removing one or more local maxima from the list in response to determining that each of the one or more local maxima corresponds to a unit cell that is within a threshold distance from a unit cell corresponding to another local maximum.

11. A method of tracking touches at a touch-sensing surface, the method comprising:

assign one of a plurality of touch IDs to a location of each of an initial number of local maxima detected from an initial scan of the touch-sensing surface;

in response to detecting fewer than the initial number of local maxima from a subsequent scan of the touch-sensing surface, determining a location of a lost touch based on signal levels associated with a subset of unit cells surrounding a first unit cell corresponding to one of the local maxima; and

assigning one of the plurality of touch IDs to the location of the lost touch.

12. The method of claim 11 , wherein determining the location of the lost touch comprises identifying a second unit cell having a highest signal level from among the subset of unit cells, wherein each unit cell in the subset of unit cells is two unit cells away from the first unit cell.

13. The method of claim 11 , further comprising smoothing a path of each of a plurality of contacts corresponding to the plurality of touch IDs, wherein the path of each of the plurality of contacts is calculated based on successive scans of the touch-sensing surface.

14. The method of claim 11 , further comprising identifying a first signal level measured from a first unit cell as one of the local maxima by comparing the first signal level with signal levels of unit cells that are horizontally and vertically adjacent to the first unit cell.

15. The method of claim 11 , further comprising:

adding each of the local maxima to a list of local maxima; and

removing one or more of the local maxima from the list in response to determining that each of the one or more local maxima corresponds to a unit cell that is within a threshold distance from a unit cell corresponding to another local maximum.

16. A capacitance sensing system, comprising:

a processor; and

a memory coupled with the processor, wherein the memory is configured to store instructions that when executed by the processor, cause the processor to perform a method comprising:

based on a first scan of a touch-sensing surface, detecting an initial location of a first contact and an initial location of a second contact at the touch-sensing surface,

during a second scan of the touch-sensing surface subsequent to the first scan, detecting a plurality of signal levels caused by the first contact and the second contact, wherein each signal level is associated with one of a plurality of unit cells of the touch-sensing surface;

identifying a first signal level of the plurality of signal levels as a local maximum, and

locating a lost touch based on one or more signal levels associated with one or more unit cells within a fixed distance from a first unit cell associated with the local maximum.

17. The capacitance sensing system of claim 16 , wherein the method further comprises:

assigning a first touch ID corresponding to the unit cell associated with the local maximum; and

assigning a second touch ID corresponding to a unit cell associated with the lost touch.

18. The capacitance sensing system of claim 16 , wherein locating the lost touch comprises:

identifying a second unit cell having a highest signal level from among a subset of the plurality of unit cells, wherein each unit cell in the subset of unit cells is two unit cells away from the first unit cell; and

assigning to the second unit cell a touch ID corresponding to the lost touch.

19. The capacitance sensing system of claim 16 , wherein identifying the first signal level as a local maximum comprises comparing the first signal level with signal levels of unit cells that are horizontally and vertically adjacent to the first unit cell.

20. The capacitance sensing system of claim 19 , wherein the method further comprises:

adding the local maximum to a list of local maxima; and

removing one or more local maxima from the list in response to determining that each of the one or more local maxima corresponds to a unit cell that is within a threshold distance from a unit cell corresponding to another local maximum.