WPF InkCanvas - how to determine if it has been "signed"

Question

I'm using a WPF InkCanvas control to capture signatures in a Tablet PC application.

One of my requirements is to validate whether or not the application has really been "signed". Right now I'm doing this by checking the Strokes collection of the InkCanvas - if there are 0 strokes, then I know the user has not "signed".

However, if the user enters a single slash, or even a single dot, this counts as a stroke and my validation test will pass, even though the signature isn't really valid.

Any ideas about how to build a better test for this? Granted, the use case for what is and is not a valid signature is pretty fuzzy, but I want to try to eliminate obviously bad signatures.

Or is this simply unsolvable in any straightforward way?

Answer 1

I know there are algorithms to test if a signature is a valid match for an existing signature, like the one outlined here. However, finding out if something is a signature in the first place seems to be much more complex.

From Wikipedia:

On legal documents, an illiterate signatory can make a "mark" (often an "X" but occasionally a personalized symbol)
...
Several cultures whose languages use writing systems other than alphabets do not share the Western notion of signatures per se: the "signing" of one's name results in a written product no different from the result of "writing" one's name in the standard way. For these languages, to write or to sign involves the same written characters. Three such examples are Chinese, Japanese, and Korean.

While this could be approached using Intelligent Character Recognition, I also know that my signature rarely looks like it has any characters in it. It's even worse if I am using one of those UPS or FedEx package singing pads. Next time a package arrives though I will try signing with just a dash and a dot and see if it allows it (Which I think it will allow since it's already close enough to that).

Because a signature may not match any recognizable words or characters, trying to 'validate' it any further then you currently are could actually be discarding some valid signatures. If for some reason you do still need to know if there is something more then a dot, take a look at validating instead that the signature fills a certain sized rectangle. That still may invalidate results that shouldn't be, so if you do attempt to add any validation to it make sure to document fully the expectations of a valid signature.