Does anyone know what type of effect/filter/method I can use to subtract one image from another?

By "subtract" I mean having the resulting image equal to a pixel by pixel representation of what chroma and luma would have to be added to the first image to get it to equal the second image.

I've seen this type of effect used for comparing compression techniques against an original image. If there's no visual difference, the resulting image is a uniform dark grey or black. If there is a visual difference, it shows as various squiggles representing detail that has been lost and artifacts that have been added.

Thanks much for any help :).

I haven't done this myself, but I think this is a "difference key". You don't state what software you're using, but I believe this is available in Premier so it might also be in AE.

I know there are easier ways to get to this but its early and I'm still nursing my morning
coffee...

In Aftereffects....

down on the time line panel... hoover the mouse over " source name " right click
and choose columns then check " Mode "....

when the mode button shows up it will show " normal "... click the normal button
and the drop down will show you all of the layer modes... in there you will see
"difference" and " classic difference "

also... your setup may already have the modes checked, if so then just click and you will
see the drop down for all of the layer modes....

Hey guys,

I don't have AfterEffets, but I do have Avid MC, Vegas, Boris Red, Boris CC and TMPGEnc Xpress. I've looked through their effects lists from something that sounds like a difference key. I'll keep on looking.

Thanks very much for the help.

Ok.... you might ask the same question in the Vegas area... I'm sure there has to be something that will do the same thing in Vegas..

or if you have it you could bring it all into Photoshop extended also....

Ray and Tripp,

I found it! Thanks much for your help. Boris Contiuum Complete has a Compostie effect with the choice to take the difference between two tracks by setting Apply Mode to Difference.

Thanks again for steering me in the right direction ;).