Re: [Gimp-docs] Composite mode options

[Ell <ell_se yahoo com>]

On Wed, 9 May 2018 15:17:49 +0200
Julien Hardelin <jhardlin orange fr> wrote:

> Hi,
>
> I don't understand options in Composite mode in Edit Layer Attributes 
> and New Layer : Auto, Union, Clip to backdrop, Clip to layer,
> Intersection.
>
> What do they do?
>
> What are they useful for?

While this isn't 100% accurate, roughly speaking, the layer's mode
(sometimes referred to as its "blend mode": Normal, Multiply, etc.)
determines how the *color values* of the layer and its backdrop are
combined, while the layer's composite mode determines how the *alpha
values* of the layer and its backdrop are combined.  (The layer's
backdrop is the content against which the layer is composited; namely,
it's the combination of the layers below it.)

There are two regions of interest when compositing the layer against
its backdrop: the opaque region of the backdrop, and the opaque region
of the layer.  The layer's blend mode determines how to combine the
colors of the *intersection* of these two regions: i.e., the common
opaque region of both the backdrop and the layer (pixels belonging to
only one of these regions don't need to be combined with anything, and
retain their original color.)  The layer's composite mode determines
which part of these regions to keep, and which to discard:

  - "Union" keeps the opaque regions of both the layer and its backdrop,
    i.e., their union [1].

  - "Clip to Backdrop" keeps only the opaque region of the backdrop
    (this includes the common opaque region to both the backdrop and the
    layer, i.e., their intersection) [2].

  - "Clip to Layer" keeps only the opaque region of the layer (this
    includes the common opaque region to both the backdrop and the
    layer, i.e., their intersection) [3].

  - "Intersection" keeps only the common opaque region to both the
    backdrop and the layer, i.e., their intersection [4].

The images referenced above show the composition of two layers -- the
bottom layer containing a red feathered circle, and the top layer
containing a green feathered circle -- using "Addition" mode, and
different composite modes (applied to the top layer).

[1] https://i.imgur.com/HenCiQC.png
[2] https://i.imgur.com/7wzwMJa.png
[3] https://i.imgur.com/CNj1tw9.png
[4] https://i.imgur.com/rzft0bk.png

--
Ell