Unpremultiply the selected Photoshop layer #Nuke

premult-exr-psd.nk

set cut_paste_input [stack 0]
version 11.0 v1
Read {
 inputs 0
 file Untitled-1.psd
 format "3881 1081 0 0 3881 1081 1 "
 origset true
 name Read6
 selected true
 xpos 1335
 ypos -161
}
Read {
 inputs 0
 file Untitled-1.exr
 format "3881 1081 0 0 3881 1081 1 "
 origset true
 name Read5
 selected true
 xpos 1137
 ypos -169
}
add_layer {Layer-01 Layer-01.red Layer-01.green Layer-01.blue Layer-01.alpha}
Shuffle {
 in Layer-01
 name Shuffle3
 selected true
 xpos 1137
 ypos -62
}
Switch {
 inputs 2
 which 1
 name Switch1
 selected true
 xpos 1234
 ypos 30
}
set Nb6252700 [stack 0]
Constant {
 inputs 0
 channels rgb
 color {1 0 0 0}
 name Constant1
 selected true
 xpos 988
 ypos 80
}
set Nbe2ff880 [stack 0]
AddMix {
 inputs 2
 name AddMix1
 selected true
 xpos 1234
 ypos 103
}
push $Nb6252700
Dot {
 name Dot2
 selected true
 xpos 1490
 ypos 34
}
Premult {
 name Premult1
 selected true
 xpos 1456
 ypos 240
}
push $Nbe2ff880
Dot {
 name Dot1
 selected true
 xpos 1022
 ypos 285
}
Merge2 {
 inputs 2
 name Merge1
 selected true
 xpos 1456
 ypos 281
}

unpremult-layer-photoshop.jsx

/*
    Will unpremultiply the currently selected Photoshop layer
    in order to export with: http://www.exr-io.com/
*/

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 "color dodge", preferably "divide"
aboveLayer.blendMode = BlendMode.COLORDODGE;

// 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();