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signature-sdk-js
Wacom’s Signature SDK library for JavaScript provides software components to capture handwritten signatures from a Web Browser.
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
To provide some context, I generate the signature image on the frontend using a sign pad (Wacom STU430) through the JavaScript library signature-sdk-js, also from Wacom. This library takes the PDF document from my Ruby backend as input and produces two output files: the PNG image of the signature and a specific ISO file. This ISO file, from what I understand, utilizing the PDF hash, ensures the secure linkage between the signature and the PDF.