On Fri, 18 May 2018 18:45:46 +0200
Julien Hardelin <jhard...@orange.fr> wrote:

> Hi,
> I have difficulty to create an example for Pass Through layer mode.
> I found several Photoshop tutorials on the Web, but they all use 
> adjustment layers that don't exist in GIMP.
> I thought that I can use the Addition layer mode on a layer inside
> the layer group, as in the attached example ( layer#2), where the
> layer group mode is either Normal or Pass Through. But the pixels
> values I get in the image after Merge Visible Layers or Flatten are
> not demonstrative.
> Could you supply me with a demonstrative example for this Pass
> Through layer mode?

Normally, the layers inside a layer group are isolated from the rest of
the image -- the layer group is essentially a separate sub-image, living
inside the bigger image; you can merge the group into a single layer,
replace the original group with it, and the result would be the same.
In [1], the group uses Normal mode, and note that the green and blue
layers don't affect the red layer: the green layer's color isn't added
to the the red layer's color, and the blue layer only erases the green

Layer groups using Pass-through mode are different: the layers inside
them "see" the layers below the group, and interact with them according
to their layer mode.  In [2], the group uses Pass-through mode, and
note that the green layer's color *is* added to the red layer's color,
and the blue layer erases both the green and the red layers.

In simple cases, pass-through groups behave as though there is no group
involved at all.  In [3], the green and blue layers are not inside a
group, and the result is the same as pass-through-example-2.png.  In
these cases, the group is primarily an organizational tool: it allows
you to group together several layers, achieving some desired effect,
and handle them as a unit.

However, in general, pass-through groups are not equivalent to having
no group at all.  For example, when the group's opacity is less than
100%, pass-through groups still behave as a single unit, applying the
opacity to the group as a whole (like a normal group would) rather than
to the individual layers, while still letting the group layers interact
with the background layers.  Compare [4], [5], and [6], which
demonstrate the same compositions as above, with the group (or the
individual layers, in the last example) having an opacity of 50%.  When
using pass-through groups to group together several layers achieving a
collective effect, the group's opacity essentially lets you control the
"strength" of the effect, which can't be achieved using either normal
groups, or individual layers.

The names of the relevant layers in the linked images specify the layer
mode, with the composite mode in parentheses where applicable, and the
layer's opacity.

[1] https://i.imgur.com/UHvDUgh.png
[2] https://i.imgur.com/DrVveyh.png
[3] https://i.imgur.com/nOnZisL.png
[4] https://i.imgur.com/jdzY5M4.png
[5] https://i.imgur.com/oTPZzD3.png
[6] https://i.imgur.com/KyjbomU.png

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