Wacom’s Signature Library provides software components to capture handwritten signatures from a pen tablet. The library simplifies the interaction with Wacom pen tablets and provides API to manage and display signatures. While this introduction is based on the Windows version of the Signature Library, the principles also apply to the mobile versions.
Figure 1: Overview of the Signature SDK components.
Sign pro PDF uses the Signature Library to capture and render signatures. To avoid a common misconception, it should be noted that the Signature Library does not include PDF related API and sign pro PDF uses a separate SDK to display and modify PDF documents.
Figure 2: Screenshot of Sign pro PDF application.
An application using the Signature Library will provide a way of starting signature capture, for example by displaying a ‘sign’ button. The resulting call to the signature capture function will display a dialog:
Figure 3: Screenshot of signature capturing control.
The signature capture function isolates the application from the type of pen input device. The Signature Library detects the type of connected device and automatically runs the code needed to communicate with it. Usually the pen input device will be an STU tablet such as the STU-540 or a Wintab device such as the DTU-1141. From an interface perspective these are very different devices but the Signature Library allows an application to use them in a common way.
The dialog shows the username, reason and timestamp which will be saved with the signature. While signing, the user can see the pen trace and clear it to sign again if needed before accepting the signature with the OK button.
A dialog is used so that the signing process can be independent of the underlying document to which the signature will be applied. For example, if the document view has been zoomed the signature can still be captured in the normal way and then adjusted to fit into the document display area.
To provide a high quality pen and ink experience the signature components use Wacom’s Ink Layer Language (WILL) technology for signature capture and image creation.
The Signature Library installation includes common language translations. The Windows installation language is detected and the relevant language is selected automatically.
The Signature Library creates a Signature Object to hold a captured signature. A signature can be handled by an application in its native binary format (Forensic Signature Store - FSS) or in the Base64 text encoded format such as the following example:
The Signature Object contains the following data:
- Date timestamp
- Application specific data
- System information
- Pen data
API is provided to create an image from a Signature Object. Options include:
- Image format (png/bmp etc.)
- Image size
The EncodeData option produces an image of the signature which contains the complete SignatureObject concealed in pixel attributes (using steganography), for example:
While the data is not immediately visible in the image, API allows the Signature Object to be extracted. The object can then be used for further processing, such as extracting signature capture details. Although copy and paste can be used in the normal way, care must be taken to maintain the original image contents. For example the image cannot be modified in Paint.
Data is stored in the Signature Object in an undisclosed proprietary format and API is provided to extract parts of it:
- Date timestamp
- Application specific data
One type of application specific data is the hash of a signed document. Before starting signature capture the application can calculate a hash value for the document and include it in the signature data. At a later time the application can recalculate the hash and use the API to compare the new and saved values to determine whether any changes have been made since signing. The results can then be used to indicate the validity of a signature.
API is not provided to extract the pen data because this is personal information which could be used fraudulently. When legitimate access to the full data is needed, such as when the authenticity of a signature is assessed by a qualified Forensic Document Examiner, this is possible using the SignatureScope application:
A demonstration version of SignatureScope is freely available to Signature Library users and is fully functional with signatures captured in the application. The full version, only available to qualified document examiners, is able to import signatures for analysis.
Signature Libraries are available for:
Regardless of the operating system, signatures are created in the same Signature Object format for full compatibility. This means that signatures can be readily exchanged and used in related applications:
- Dynamic Signature Verification (DSV)
- Sign pro PDF
Pen Tablet Devices
The Signature Library detects the type of Wacom hardware connected and uses the appropriate pen interface.
There are two distinct types of pen device supported:
- Signature tablet – range of STU tablets
- Pen tablets and displays – range of Intuos, DTU and Cintiq tablets
The STU tablets have a dedicated USB interface for pen input. The pen displays and pen tablets are installed with a driver. The driver provides a Wintab interface which is used for signature capture.
The installation requires COM registration with a number of supporting entries in the Windows registry and is achieved by running the installer supplied in the 'Wacom Ink SDK for signature' download package.
Third party device compatibility & support is offered as part of the SDK's Enterprise version, but a valid license is required. For more information about pen input support and to confirm the compatibility of your device, contact Wacom support.