Quoting Rob Antonishen <rob.antonis...@gmail.com>:
> On a side note, what are tattoos good for? Are they used for
> anything, anywhere?
Layer tattoos are useful when a reference to a layer needs to be
maintained across GIMP sessions, subsequent open-ings of the same
image within the same session, or even when using "File->Revert" (all
of which result in new assignments of layer IDs within the image).
Tattoos are similarly associated with channels and paths.
Example: let's say you want to create a drop shadow plug-in which when
run on a previously generated "shadow layer" will permit you to modify
the offsets, opacity, and blur, and then regenerate the shadow layer
(whereas GIMP's Drop Shadow filter would merely create a drop shadow
of the shadow layer). You would likely approach this by obtaining from
a parasite attached to the shadow layer the last values used in its
creation (if no shadow-parasite exists then the last values of the
plug-in itself would be used to create a new, shadow layer) in
addition to the tattoo of the "base layer" from which the shadow layer
was generated. You could not use the ID number of the base layer
because this would (likely) be different the next time you open the
image; however, the layer's tattoo will always be the same (and unique
from all other tattoos).
I don't really know of any scripts/plug-ins which employ tattoos,
other than some SIOD-based scripts I'd written to implement layer
groupings (never updated for TinyScheme).
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