GIMP Version: 2.8.10
Operating System: Windows
GIMP Experience: Beginner Level

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Hi all gimp-gurus,
I'm interested in better understanding how the following layer modes operate:
- "erase"
- "replace"
- "anti-erase"
they are the last 3 available as usable modes when writing a script, they are NOT available AFAIK when gimp is used interactively
I made some (simply) tests and I found out that:
a) they are in any case ALL replaced in the Layers'Panel by "normal"
b) if the base layer (the one to which other layers are then added using one of the mentioned modes) has no alpha channel, then the effect is actually the same as "normal"
c) if the base layer has alpha channel, then:
c1 - "replace" acts apparently like "normal" (but I suspect there are differences)
c2 - "erase" removes the (equivalent color?) pixels from the base layer
c3 - "anti-erase" maybe does the opposite of "erase" but I'm not sure
I couldn't make too many experiments because -as they are not available interactively- I had always to change the source in the script then refresh

If someone knows in details the full bevaviour of those modes, I would be grateful.
(BTW: why are they not available interactively?)

I believe these are available under the FADE option in the edit menu. Run a filter then run Edit>fade effect.

thanks Rod!
so they are a sort of "After-the-Filter" modification
now I can understand a little better (just a little...) their use interactively
but how can be used in a script? after having run (called, executed) a filter?
do you know any example of that?
thanks

No i don't think i ever saw this option used in an effect filter.
Nothing in PDB either.

'Replace' mode uses the alpha channel of the top layer to produce the resulting alpha regardless of the value (or even presence) of the alpha below it. Note that the resulting alpha then gets blended with the alpha of the lower layer based upon the Opacity setting.


For both 'erase' and 'anti-erase' modes, the RGB channels of the top layer are ignored.

'Erase' mode subtracts the alpha channel of the top layer from the alpha channel of the bottom to produce the resulting alpha. Again, the resulting alpha then gets blended with the alpha of the lower layer based upon the Opacity setting.

'Anti-erase' mode adds the alpha channel of the top layer to the alpha channel of the bottom to produce the resulting alpha. And then the resulting alpha gets blended with the alpha of the lower layer based upon the Opacity setting.

Found it in the PDB also.