### Re: [Gimp-developer] Finding out what happens when compositing layers

```On 09/16/2011 09:31 PM, 丰 wrote:
I came across this quite some time ago:
http://www.simpelfilter.de/en/basics/mixmods.html

That's wonderful!  Even mentions GIMP a bit.

Patrick
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### Re: [Gimp-developer] Finding out what happens when compositing layers

```
On 09/16/2011 02:20 AM, David Gowers (kampu) wrote:
In GEGL, the layer mode code is autogenerated from formula strings --
see operations/generated/math.rb

The opacity behaviour is not documented there, but my understanding is
that the alpha of the underlying composite is preserved, and opacity
only effects how much of the layer's COLOR channels are applied (per
the standard blending formula).

On Fri, Sep 16, 2011 at 7:09 AM, Ofnuts ofn...@laposte.net
mailto:ofn...@laposte.net wrote:

I'll trying to find out what happens when a layer is in subtract mode
with a global opacity not one or zero (i.e. what computations are done
with the pixels values of the composite image of the layers below, the
pixel values of the layer, and the general opacity). I have a git
extract of the Gimp source code (and of gegl  babl) but got
lost... Can
someone help?

Well, I know that in Normal mode, I have in each RGB channel:

Composite=Over*alpha+Under(1-alpha)

(assuming Under is fully opaque), but in Subtract mode, I would have
several possibilities

Composite=Under(1-alpha)-Over*alpha
or
Composite=Under(1-alpha)+(1-Over)*alpha
or
Composite=Under(1-alpha)+(1-Over*alpha)

and \$deity knows what else.

math.rb isn't taking the opacity in account (but is an interesting read...)

Thx.

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### Re: [Gimp-developer] Finding out what happens when compositing layers

```On Sat, Sep 17, 2011 at 8:36 AM, Ofnuts ofn...@laposte.net wrote:

**
On 09/16/2011 02:20 AM, David Gowers (kampu) wrote:

In GEGL, the layer mode code is autogenerated from formula strings -- see
operations/generated/math.rb

The opacity behaviour is not documented there, but my understanding is that
the alpha of the underlying composite is preserved, and opacity only effects
how much of the layer's COLOR channels are applied (per the standard
blending formula).

On Fri, Sep 16, 2011 at 7:09 AM, Ofnuts ofn...@laposte.net wrote:

I'll trying to find out what happens when a layer is in subtract mode
with a global opacity not one or zero (i.e. what computations are done
with the pixels values of the composite image of the layers below, the
pixel values of the layer, and the general opacity). I have a git
extract of the Gimp source code (and of gegl  babl) but got lost... Can
someone help?

Well, I know that in Normal mode, I have in each RGB channel:

Composite=Over*alpha+Under(1-alpha)

(assuming Under is fully opaque), but in Subtract mode, I would have
several possibilities

No, you just subtract Over from Under with no multiplication, then you
perform a RGB (not RGBA) Composite of the result and Under, to produce the
final composite.
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### Re: [Gimp-developer] Finding out what happens when compositing layers

```I came across this quite some time ago:
http://www.simpelfilter.de/en/basics/mixmods.html

--
2.6.32.26-175.fc12.x86_64
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### Re: [Gimp-developer] Finding out what happens when compositing layers

```I came across this quite some time ago:
http://www.simpelfilter.de/en/basics/mixmods.html

PS: I thought I had type in my word before I send the mail. I did not
know what had happened. Sorry to everybody for the spam and wish this
mail is all right.
--
2.6.32.26-175.fc12.x86_64
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### [Gimp-developer] Finding out what happens when compositing layers

```I'll trying to find out what happens when a layer is in subtract mode
with a global opacity not one or zero (i.e. what computations are done
with the pixels values of the composite image of the layers below, the
pixel values of the layer, and the general opacity). I have a git
extract of the Gimp source code (and of gegl  babl) but got lost... Can
someone help?
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### Re: [Gimp-developer] Finding out what happens when compositing layers

```In GEGL, the layer mode code is autogenerated from formula strings -- see
operations/generated/math.rb

The opacity behaviour is not documented there, but my understanding is that
the alpha of the underlying composite is preserved, and opacity only effects
how much of the layer's COLOR channels are applied (per the standard
blending formula).

On Fri, Sep 16, 2011 at 7:09 AM, Ofnuts ofn...@laposte.net wrote:

I'll trying to find out what happens when a layer is in subtract mode
with a global opacity not one or zero (i.e. what computations are done
with the pixels values of the composite image of the layers below, the
pixel values of the layer, and the general opacity). I have a git
extract of the Gimp source code (and of gegl  babl) but got lost... Can
someone help?
___
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```