Layer Alpha Preserve

Status: proposal

Author: Carsten Hartenfels

Submission data: 25 October, 2023

Implementations: Krita, Drawpile

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. 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.