gEDA-user: Composite/negative layers in gerbers

[Chris Emerson <geda-user@xxxxxxxxxxxxxxxx>]

Hi,

My board house[1] has complained about my gerbers having "negative
plots" and "composite layers".  I understand that pcb does this in some
cases.  (I've found
http://www.geda.seul.org/mailinglist/geda-user33/msg00055.html).

My two questions are:

1) (Main question) How can I tell if my gerbers are negative and/or
composite so I can check?
2) What can I do to affect whether pcb uses negative and/or composite
outputs?  Is that what the "Invert positive/negative" tick box on the
printing page does, at least for the negative stuff?

I have looked through the manual and the archives, but haven't quite
answered these as much as I'd like.

Best regards,

Chris
[1] Olimex