Does 1 point of dam roll equal 1 hp of damage?

Or is it more similar to the resistance/vuln system where 1 point does not equal 1% but rather closer to some sort of exponential curve where in this case it may start that way but has diminishing returns.

If dam roll is added at the very end of the whole damage dealing process it seems like going from 20 to 40 dam roll is a lot more than just adding 20 hit points of damage to the end of a hit.

I think damroll is just added at the end.

Of course, you then have vulns, resistances, dam redux etc come into play.

Some skills are not affected by damroll (eg most assassin skills).

The thing that makes damroll nasty is that it is adding on to every hit and also that some skills have a multiplier to damage that I think comes after damroll is added.