There are many JavaScript libraries that let a user draw using their finger on a CANVAS
object and convert that to a PNG or JSON. We recommend fabric.js and you can see how we used it in this sample project:
https://bitbucket.org/vorenusventures/kazaamax-attendance
Inspect resources/assets/js/components/Edit-Attendee.vue
to see how we made it work in that presentation. Fabric serializes the CANVAS object is serialized to a base-64 encoded PNG. When you view the event in the portal, the image of the signature is displayed: