Taking the difference between images in Photoshop

Black_Dog

Does anybody know of a tool or method to take the difference between two images/layers in Photoshop? Of course there is the "difference" blend mode, but that returns the magnitude or absolute value of difference. What I'm after is "signed" delta values. Images aren't a signed data format but storing delta as the difference from 128,128,128 or putting a sign in the alpha channel or mask should work.

Anything like that out there?