Unpremultiply the selected Photoshop layer

unpremult.jsx

/*
    Will unpremultiply the currently selected Photoshop layer
    
*/

var doc = app.activeDocument;

// Duplicate this layer
var premultLayer = app.activeDocument.activeLayer.duplicate();
premultLayer.blendMode = BlendMode.NORMAL;

// Apply layer mask to the duplicate

// Create a black layer underneath the premult one
var beneathLayer = app.activeDocument.artLayers.add();
doc.selection.selectAll();
var blackColor = new SolidColor();
blackColor.rgb.red = 0;
blackColor.rgb.green = 0;
blackColor.rgb.blue = 0;
doc.selection.fill(blackColor, ColorBlendMode.NORMAL, 100, false);

beneathLayer.move(premultLayer, ElementPlacement.PLACEAFTER);



// Create a black layer above the premult one
var aboveLayer = beneathLayer.duplicate();
aboveLayer.move(premultLayer, ElementPlacement.PLACEBEFORE);

// Fill the premult opaque areas with white for the layer above
// Select transparency on the premult layer. Unfortunately we need the
// action ref abominations for that.
function SelectTransparency()
{
    var idChnl = charIDToTypeID( "Chnl" );

    var actionSelect = new ActionReference();
    actionSelect.putProperty( idChnl, charIDToTypeID( "fsel" ) );     

    var actionTransparent = new ActionReference();    
    actionTransparent.putEnumerated( idChnl, idChnl, charIDToTypeID( "Trsp" ) );

    var actionDesc = new ActionDescriptor();
    actionDesc.putReference( charIDToTypeID( "null" ), actionSelect );
    actionDesc.putReference( charIDToTypeID( "T   " ), actionTransparent );

    executeAction( charIDToTypeID( "setd" ), actionDesc, DialogModes.NO );
}

doc.activeLayer = premultLayer;
SelectTransparency();
doc.activeLayer = aboveLayer;

var whiteColor = new SolidColor();
whiteColor.rgb.red = 255;
whiteColor.rgb.green = 255;
whiteColor.rgb.blue = 255;
app.activeDocument.selection.fill(whiteColor, ColorBlendMode.NORMAL, 100, false);

// Set the layer above to "divide"
aboveLayer.blendMode = BlendMode.DIVIDE;

// Merge the cake of 3 layers together, using a layer set.
var unpremultSet = doc.layerSets.add();
beneathLayer.move(unpremultSet, ElementPlacement.INSIDE);
premultLayer.move(unpremultSet, ElementPlacement.INSIDE);
aboveLayer.move(unpremultSet, ElementPlacement.INSIDE);
unpremultSet.merge();