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Moving off generic conceptual ideas and onto libfprint-specific implementation details, here are some introductory notes regarding how libfprint copes with compatibility of fingerprints.
libfprint deals with a whole variety of different fingerprint readers and the design includes considerations of compatibility and interoperability between multiple devices. Your application should also be prepared to work with more than one type of fingerprint reader and should consider that enrolled fingerprint X may not be compatible with the device the user has plugged in today.
libfprint implements the principle that fingerprints from different devices are not necessarily compatible. For example, different devices may see significantly different areas of fingerprint surface, and comparing images between the devices would be unreliable. Also, devices can stretch and distort images in different ways.
libfprint also implements the principle that in some cases, fingerprints are compatible between different devices. If you go and buy two identical fingerprint readers, it seems logical that you should be able to enroll on one and verify on another without problems.
libfprint takes a fairly simplistic approach to these issues. Internally, fingerprint hardware is driven by individual drivers. libfprint enforces that a fingerprint that came from a device backed by driver X is never compared to a fingerprint that came from a device backed by driver Y.
Additionally, libfprint is designed for the situation where a single driver may support a range of devices which differ in imaging or scanning properties. For example, a driver may support two ranges of devices which even though are programmed over the same interface, one device sees substantially less of the finger flesh, therefore images from the two device types should be incompatible despite being from the same driver. To implement this, each driver assigns a device type to each device that it detects based on its imaging characteristics. libfprint ensures that two prints being compared have the same device type.
In summary, libfprint represents fingerprints in several internal structures and each representation will offer you a way of determining the driver and device ID of the print in question. Prints are only compatible if the driver ID and devtypes match. libfprint does offer you some "is this print compatible?" helper functions, so you don't have to worry about these details too much.
Each driver is assigned a unique string identifier by the project maintainer.
The only reason you may be interested in retrieving the driver ID for a driver is for the purpose of checking if some print data is compatible with a device. libfprint uses the driver ID as one way of checking that the print you are trying to verify is compatible with the device in question - it ensures that enrollment data from one driver is never fed to another. Note that libfprint does provide you with helper functions to determine whether a print is compatible with a device, so under most circumstances, you don't have to worry about driver IDs at all.
Internally, the behind a device assigns a string identifier to the device This cannot be used as a unique ID for a specific device as many devices under the same range may share the same devtype. The device ID may even be the same string in all cases. It is guaranteed to have a non-zero length and be a valid file name. It defaults to "0".
The only reason you may be interested in retrieving the device ID for a device is for the purpose of checking if some print data is compatible with a device. libfprint uses the device ID as one way of checking that the print you are verifying is compatible with the device in question - the device ID must be equal. This effectively allows drivers to support more than one type of device where the data from each one is not compatible with the other. Note that libfprint does provide you with helper functions to determine whether a print is compatible with a device, so under most circumstances, you don't have to worry about devtypes at all.