The "registration color" is a feature that has recently been added, I
and I don't know anything about it.
Concerning Recompose, its function is to reverse Decompose. If you
decompose a layer and then want to reassemble the original layer using
Compose, you have to do a lot of work -- you have to set up all the
component layers properly using menus. Recompose does all of this
silently, and replaces the original layer (the one that was decomposed)
with the result. If you don't alter any of the decomposed layers, Recompose
will give you exactly the result you started with (except for numerical
so you won't see anything change. If you alter one of the decomposed layers
and then choose Recompose, you should see the original layer change.
The reason for setting this up was to make it easier to edit individual
color channels of a layer. For example, if you want to edit the luminosity
of a layer without affecting hues, you can decompose it as LAB, do
whatever you want to the L channel, then do recompose, and your original
layer will automatically show the result.
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