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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