Layer Alpha Preserve
====================

*Status*: proposal

*Author*: Carsten Hartenfels

*Submission data*: 25 October, 2023

*Implementations*: `Krita <https://invent.kde.org/graphics/krita/-/merge_requests/1915>`_, `Drawpile <https://github.com/drawpile/Drawpile/commit/cdf14497bafc69ce186804c5543e9f4eca3dff87>`_

Several drawing programs support alpha-preserving blend modes on layers, which
will blend the color channels as normal, but leave the alpha channel as-is.
This is sometimes also called by other names, such as alpha inheritance or
clipping groups.

The base OpenRaster specification only allows this for a single case: the
composite-op ``svg:src-over`` has an alpha-preserving variant in the form of
``svg:src-atop``.

To include this information for other blend modes, the  ``<layer/>`` element in
a layer stack definition should support the following extra attribute:

"``alpha-preserve``"
   the alpha preserve status of the layer, expressed as an `xsd:boolean
   <http://www.w3.org/TR/xmlschema-2/#boolean>`__. True values ("``true``",
   canonically), mean that the layer should be blended without modifying the
   alpha channel. False values ("``false``", canonically) mean that it should
   be blended regularly. The default is false.

This attribute must not be used together with the ``svg:src-over`` or
``svg:src-atop`` composite-ops.

Example
-------

::

    <stack>
        <layer
            name="shading" src="data/layer3.png"
            composite-op="svg:multiply"
            alpha-preserve="true"/>
        <layer
            name="recolors" src="data/layer2.png"
            composite-op="svg:src-atop"/>
        <layer
            name="colors" src="data/layer1.png"
            composite-op="svg:src-over"/>
    </stack>

Impact on Applications
----------------------

Applications that support or require alpha-preserving blend modes should store
and load this information accordingly, blending the image as intended.
Otherwise, the application should ignore the attribute.

Applications may recognize the incorrect combination of ``svg:src-over`` with
alpha preserve as equivalent to ``svg:src-atop``.

Applications should disregard the alpha-preserve attribute on ``svg:src-atop``
layers.

The alpha-preserve attribute must be omitted when saving for the archival
intent.

Support
-------

Implemented in Krita and Drawpile as of 2023-10-21.