Enrolling

Enrolling is represented within libfprint as a multi-stage process. This slightly complicates things for application developers, but is required for a smooth process.

Some devices require the user to scan their finger multiple times in order to complete the enrollment process. libfprint must return control to your application in-between each scan in order for your application to instruct the user to swipe their finger again. Each scan is referred to as a stage, so a device that requires 3 scans for enrollment corresponds to you running 3 enrollment stages using libfprint.

The fp_dev_get_nr_enroll_stages() function can be used to find out how many enroll stages are needed.

In order to complete an enroll stage, you call an enroll function such as fp_enroll_finger(). The return of this function does not necessarily indicate that a stage has completed though, as the user may not have produced a good enough scan. Each stage may have to be retried several times.

The exact semantics of the enroll functions are described in the fp_enroll_finger() documentation. You should pay careful attention to the details.

Imaging

libfprint provides you with some ways to retrieve images of scanned fingers, such as the fp_dev_img_capture() function, or some enroll/verify function variants which provide images. You may wish to do something with such images in your application.

However, you must be aware that not all hardware supported by libfprint operates like this. Most hardware does operate simply by sending fingerprint images to the host computer for further processing, but some devices do all fingerprint processing in hardware and do not present images to the host computer.

You can use fp_dev_supports_imaging() to see if image capture is possible on a particular device. Your application must be able to cope with the fact that libfprint does support regular operations (e.g. enrolling and verification) on some devices which do not provide images.

fp_dev_get_driver ()

struct fp_driver *
fp_dev_get_driver (struct fp_dev *dev);

Get the fp_driver for a fingerprint device.

fp_dev_get_nr_enroll_stages ()

int
fp_dev_get_nr_enroll_stages (struct fp_dev *dev);

Gets the number of enroll stages required to enroll a fingerprint with the device.

fp_dev_get_devtype ()

uint32_t
fp_dev_get_devtype (struct fp_dev *dev);

Gets the devtype for a device.

fp_dev_supports_print_data ()

int
fp_dev_supports_print_data (struct fp_dev *dev,
                            struct fp_print_data *data);

Determines if a stored print is compatible with a certain device.

fp_dev_supports_imaging ()

int
fp_dev_supports_imaging (struct fp_dev *dev);

Determines if a device has imaging capabilities. If a device has imaging capabilities you are able to perform imaging operations such as retrieving scan images using fp_dev_img_capture(). However, not all devices are imaging devices – some do all processing in hardware. This function will indicate which class a device in question falls into.

fp_dev_supports_identification ()

int
fp_dev_supports_identification (struct fp_dev *dev);

Determines if a device is capable of identification through fp_identify_finger() and similar. Not all devices support this functionality.

fp_dev_supports_dscv_print ()

int
fp_dev_supports_dscv_print (struct fp_dev *dev,
                            struct fp_dscv_print *print);

Determines if a fp_dscv_print discovered print appears to be compatible with a certain device.

fp_dev_get_img_width ()

int
fp_dev_get_img_width (struct fp_dev *dev);

Gets the expected width of images that will be captured from the device. This function will return -1 for devices that are not imaging devices. If the width of images from this device can vary, 0 will be returned.

fp_dev_get_img_height ()

int
fp_dev_get_img_height (struct fp_dev *dev);

Gets the expected height of images that will be captured from the device. This function will return -1 for devices that are not imaging devices. If the height of images from this device can vary, 0 will be returned.

fp_operation_stop_cb ()

void
(*fp_operation_stop_cb) (struct fp_dev *dev,
                         void *user_data);

Type definition for a function that will be called when fp_async_dev_close(), fp_async_verify_stop(), fp_async_identify_stop() or fp_async_capture_stop() finishes.

fp_img_operation_cb ()

void
(*fp_img_operation_cb) (struct fp_dev *dev,
                        int result,
                        struct fp_img *img,
                        void *user_data);

Type definition for a function that will be called when fp_async_verify_start() or fp_async_capture_start() finished.

fp_dev_open_cb ()

void
(*fp_dev_open_cb) (struct fp_dev *dev,
                   int status,
                   void *user_data);

Type definition for a function that will be called when fp_async_dev_open finishes.

fp_enroll_stage_cb ()

void
(*fp_enroll_stage_cb) (struct fp_dev *dev,
                       int result,
                       struct fp_print_data *print,
                       struct fp_img *img,
                       void *user_data);

Type definition for a function that will be called when fp_async_enroll_start() finishes. See fp_enroll_finger_img() for the expected behaviour of your program based on the result .

fp_identify_cb ()

void
(*fp_identify_cb) (struct fp_dev *dev,
                   int result,
                   size_t match_offset,
                   struct fp_img *img,
                   void *user_data);

Type definition for a function that will be called when fp_async_identify_start() finishes.

fp_dev_open ()

struct fp_dev *
fp_dev_open (struct fp_dscv_dev *ddev);

Opens and initialises a device. This is the function you call in order to convert a fp_dscv_dev discovered device into an actual device handle that you can perform operations with.

fp_async_dev_open ()

int
fp_async_dev_open (struct fp_dscv_dev *ddev,
                   fp_dev_open_cb callback,
                   void *user_data);

Opens and initialises a device. This is the function you call in order to convert a fp_dscv_dev discovered device into an actual device handle that you can perform operations with.

The error status of the opening will be provided as an argument to the fp_dev_open_cb callback.

fp_dev_close ()

void
fp_dev_close (struct fp_dev *dev);

Closes a device. You must call this function when you have finished using a fingerprint device.

fp_async_dev_close ()

void
fp_async_dev_close (struct fp_dev *dev,
                    fp_operation_stop_cb callback,
                    void *user_data);

Closes a device. You must call this function when you have finished using a fingerprint device.

fp_enroll_finger ()

int
fp_enroll_finger (struct fp_dev *dev,
                  struct fp_print_data **print_data);

Performs an enroll stage. See Enrolling for an explanation of enroll stages. This function is just a shortcut to calling fp_enroll_finger_img() with a NULL image parameter. Be sure to read the description of fp_enroll_finger_img() in order to understand its behaviour.

fp_enroll_finger_img ()

int
fp_enroll_finger_img (struct fp_dev *dev,
                      struct fp_print_data **print_data,
                      struct fp_img **img);

Performs an enroll stage. See Enrolling for an explanation of enroll stages.

If no enrollment is in process, this kicks of the process and runs the first stage. If an enrollment is already in progress, calling this function runs the next stage, which may well be the last.

A negative error code may be returned from any stage. When this occurs, further calls to the enroll function will start a new enrollment process, i.e. a negative error code indicates that the enrollment process has been aborted. These error codes only ever indicate unexpected internal errors or I/O problems.

The RETRY codes from fp_enroll_result may be returned from any enroll stage. These codes indicate that the scan was not successful in that the user did not position their finger correctly or similar. When a RETRY code is returned, the enrollment stage is not advanced, so the next call into this function will retry the current stage again. The current stage may need to be retried several times.

The FP_ENROLL_FAIL code may be returned from any enroll stage. This code indicates that even though the scans themselves have been acceptable, data processing applied to these scans produces incomprehensible results. In other words, the user may have been scanning a different finger for each stage or something like that. Like negative error codes, this return code indicates that the enrollment process has been aborted.

The FP_ENROLL_PASS code will only ever be returned for non-final stages. This return code indicates that the scan was acceptable and the next call into this function will advance onto the next enroll stage.

The FP_ENROLL_COMPLETE code will only ever be returned from the final enroll stage. It indicates that enrollment completed successfully, and that print_data has been assigned to point to the resultant enrollment data. The print_data parameter will not be modified during any other enrollment stages, hence it is actually legal to pass NULL as this argument for all but the final stage.

If the device is an imaging device, it can also return the image from the scan, even when the enroll fails with a RETRY or FAIL code. It is legal to call this function even on non-imaging devices, just don't expect them to provide images.

fp_async_enroll_start ()

int
fp_async_enroll_start (struct fp_dev *dev,
                       fp_enroll_stage_cb callback,
                       void *user_data);

Starts an enrollment and calls callback for each enrollment stage. See Enrolling for an explanation of enroll stages.

fp_async_enroll_stop ()

int
fp_async_enroll_stop (struct fp_dev *dev,
                      fp_operation_stop_cb callback,
                      void *user_data);

Stops an ongoing enrollment started with fp_async_enroll_start().

fp_verify_finger ()

int
fp_verify_finger (struct fp_dev *dev,
                  struct fp_print_data *enrolled_print);

Performs a new scan and verify it against a previously enrolled print. This function is just a shortcut to calling fp_verify_finger_img() with a NULL image output parameter.

See also fp_verify_finger_img().

fp_verify_finger_img ()

int
fp_verify_finger_img (struct fp_dev *dev,
                      struct fp_print_data *enrolled_print,
                      struct fp_img **img);

Performs a new scan and verifies it against a previously enrolled print. If the device is an imaging device, it can also return the image from the scan, even when the verify fails with a RETRY code. It is legal to call this function even on non-imaging devices, just don't expect them to provide images.

fp_async_verify_start ()

int
fp_async_verify_start (struct fp_dev *dev,
                       struct fp_print_data *data,
                       fp_img_operation_cb callback,
                       void *user_data);

Starts a verification and calls callback when the verification has finished. See fp_verify_finger_img() for the synchronous API. When the callback has been called, you must call fp_async_verify_stop().

fp_async_verify_stop ()

int
fp_async_verify_stop (struct fp_dev *dev,
                      fp_operation_stop_cb callback,
                      void *user_data);

Finishes an ongoing verification started with fp_async_verify_start().

fp_identify_finger ()

int
fp_identify_finger (struct fp_dev *dev,
                    struct fp_print_data **print_gallery,
                    size_t *match_offset);

Performs a new scan and attempts to identify the scanned finger against a collection of previously enrolled fingerprints. This function is just a shortcut to calling fp_identify_finger_img() with a NULL image output parameter.

See also fp_identify_finger_img().

fp_identify_finger_img ()

int
fp_identify_finger_img (struct fp_dev *dev,
                        struct fp_print_data **print_gallery,
                        size_t *match_offset,
                        struct fp_img **img);

Performs a new scan and attempts to identify the scanned finger against a collection of previously enrolled fingerprints. If the device is an imaging device, it can also return the image from the scan, even when identification fails with a RETRY code. It is legal to call this function even on non-imaging devices, just don't expect them to provide images.

This function returns codes from fp_verify_result. The return code FP_VERIFY_MATCH indicates that the scanned fingerprint does appear in the print gallery, and the match_offset output parameter will indicate the index into the print gallery array of the matched print.

This function will not necessarily examine the whole print gallery, it will return as soon as it finds a matching print.

Not all devices support identification. -ENOTSUP will be returned when this is the case.

fp_async_identify_start ()

int
fp_async_identify_start (struct fp_dev *dev,
                         struct fp_print_data **gallery,
                         fp_identify_cb callback,
                         void *user_data);

Performs a new scan and verifies it against a previously enrolled print. See also: fp_verify_finger_img()

fp_async_identify_stop ()

int
fp_async_identify_stop (struct fp_dev *dev,
                        fp_operation_stop_cb callback,
                        void *user_data);

Stops an ongoing identification started with fp_async_identify_start().

fp_dev_img_capture ()

int
fp_dev_img_capture (struct fp_dev *dev,
                    int unconditional,
                    struct fp_img **img);

Captures a fp_img from a device. The returned image is the raw image provided by the device, you may wish to standardize it.

If set, the unconditional flag indicates that the device should capture an image unconditionally, regardless of whether a finger is there or not. If unset, this function will block until a finger is detected on the sensor.

See fp_dev_supports_imaging().

fp_async_capture_start ()

int
fp_async_capture_start (struct fp_dev *dev,
                        int unconditional,
                        fp_img_operation_cb callback,
                        void *user_data);

Start the capture of an fp_img from a device. When the callback has been called, you must call fp_async_capture_stop().

fp_async_capture_stop ()

int
fp_async_capture_stop (struct fp_dev *dev,
                       fp_operation_stop_cb callback,
                       void *user_data);

Stops an ongoing verification started with fp_async_capture_start().

struct fp_dev

struct fp_dev;

fp_dev is an opaque structure type. You must access it using the functions in this section.

enum fp_scan_type

Numeric codes used to refer to the scan type of the device. Devices require either swiping or pressing the finger on the device. This is useful for front-ends.

enum fp_capture_result

Whether a capture failed or completed.

enum fp_enroll_result

Enrollment result codes returned from fp_enroll_finger(). Result codes with RETRY in the name suggest that the scan failed due to user error. Applications will generally want to inform the user of the problem and then retry the enrollment stage. For more info on the semantics of interpreting these result codes and tracking enrollment process, see Enrolling

enum fp_verify_result

Verification result codes returned from fp_verify_finger(). Return codes are also shared with fp_identify_finger(). Result codes with RETRY in the name suggest that the scan failed due to user error. Applications will generally want to inform the user of the problem and then retry the verify operation.