How signature Validation works??

is it works on Pixels ? or Size ?

As we applying a small test with the following cases:

1- Amending any annotation (sticky note ,, reduction ,, adding stamp ,, add highlight ) don't effect on the signature validation.

2-Amending any small edit on the document  (like adding dots or one character in the original document using paint application ) the original document size is increase and the Signature is not valid.

3-swapping the document pages together and test signature validation --> result is not valid.

so my conclusion is : signature validation works by check pixels and any edit on pixels effects on the size. 

So, if any one have an accurate answer--> Is the signature Validation affected by pixels or sizing or another else ??

thanks,