[EMAIL PROTECTED] (2002-11-29 at 1833.03 +0100):
> - Simple linking of layers so that some operations such as toggling
>   their visibility or moving the whole group of layers can easily be
>   applied to them (bug #86337, bug #86277).  These operations would 
>   not modify the pixels in the layers.  This could probably also be
>   used for implementing the clipping groups (bug #51112).

Clipping sounds more like ATOP operation in other docs, not like
simple move or visibility, so I would put it in the other group, or
make it some kind of blend mode (hidden) or pre process in blend
operations. It "modifies" pixels, by looking at the bg alpha.

The traditional document about digital composition is from Siggraph
84, by Porter & Duff, just in case people want to read more this ATOP
thing. Scanned copy at http://www.keithp.com/~keithp/porterduff/.

> - Grouping of layers in such a way that a merged image of the layers
>   is stored in a virtual layer and some operations can be applied to
>   (merged view + results) and to apply the PDB function only to the
>   regions that have been modified, but this is only an optimization.

You say they are the same, which from the one point of view is
correct. But from other they have some differences that would make
nicer to consider them a bit special. Layer effects would allow
external ops, and use two drawables only, "in" and "out", with "in"
one being updated rarelly (and thus "out" too). They are more artistic
oriented, to say it quickly.

Adjustment layers would allow core ops and N inputs, with those N
changing a lot, and thus lot of recomputation, like blend modes. What
is more, I would see them as blend modes that have some vars to
control the formulas, and the RGB channels working as selection
does. This also means that, probably, layer effects have a region of
interest of multiple pixels for each output pixel, while adjustment
layers only get a pixel, operate and put back (use LUTs?).

If the system can be done so fast that there is no big differences, I
see no problem with making all the same. Otherwise, maybe a forced
separation would be better.

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