Photoshop needs "Color to Alpha" button

I've created a scripted version,  building upon Ayato Fujii's Action for Photoshop:

/*
Black or White to Transparency v1-0.jsx
Stephen Marsh
v1.0, 23rd March 2025
An approximation to the "Color to Alpha" function in Krita or GIMP.
https://docs.krita.org/en/reference_manual/filters/colors.html#color-to-alpha
https://docs.gimp.org/3.0/en/gimp-filter-color-to-alpha.html
Based on the original Japanese "UnMultiply" Action for black backgrounds by Ayato Fujii:
http://www.ayatoweb.com/download/down04.html
https://community.adobe.com/t5/photoshop-ecosystem-discussions/photoshop-needs-quot-color-to-alpha-quot-button/m-p/14315420#U15230303
*/

#target photoshop;

// Check if there's an open document
if (!app.documents.length) {
    alert("A document must be open to use this script!");
    throw new Error("A document must be open...");
}

// Check for RGB mode before proceeding
if (!(app.activeDocument.mode === DocumentMode.RGB)) {
    alert("The document must be in RGB mode!");
    throw new Error("RGB mode required...");
}

// Create the UI
var dlg = new Window("dialog", "Black or White to Transparency (v1.0)", undefined);
dlg.preferredSize.width = 275;

// Panel for radio buttons
var panel = dlg.add("panel", undefined, "Select:");
panel.orientation = "column";
panel.alignChildren = "left";
panel.preferredSize.width = 275;
var blackRadioButton = panel.add("radiobutton", undefined, "Black to Transparency");
whiteRadioButton = panel.add("radiobutton", undefined, "White to Transparency");
blackRadioButton.value = true; // Default selection
var applyLayerMaskCheckbox = panel.add("checkbox", undefined, "Apply Layer Mask");
applyLayerMaskCheckbox.value = false; // Default selection

// Cancel and OK buttons
var buttonGroup = dlg.add("group");
buttonGroup.orientation = "row";
buttonGroup.alignment = "right";
var cancelBtn = buttonGroup.add("button", undefined, "Cancel", { name: "cancel" });
var okBtn = buttonGroup.add("button", undefined, "OK", { name: "ok" });

// Event handler for OK button
okBtn.onClick = function () {
    if (app.activeDocument.activeLayer.isBackgroundLayer) {
        var userConfirm = confirm("Convert the Background layer to a standard layer. Yes or No?");
        // If the user confirms, convert the Background layer to a standard layer supporting transparency
        if (userConfirm) {
            app.activeDocument.suspendHistory("Undo script", "suspendFlattened()");
            function suspendFlattened() {
                app.activeDocument.activeLayer.isBackgroundLayer = false;
                app.activeDocument.activeLayer.name = "Background";
                // Proceed with the remaining operations
                if (blackRadioButton.value) {
                    blackBackground();
                } else {
                    whiteBackground();
                }
                if (applyLayerMaskCheckbox.value) {
                    applyLayerMask(true);
                }
            }
            dlg.close();
        } else {
            // If the user cancels, do nothing
            alert("Script cancelled!");
            dlg.close();
        }
    } else {
        // If the active layer is not a Background layer, proceed without confirmation
        app.activeDocument.suspendHistory("Undo script", "suspendLayer()");
        function suspendLayer() {
            if (blackRadioButton.value) {
                blackBackground();
            } else {
                whiteBackground();
            }
            if (applyLayerMaskCheckbox.value) {
                applyLayerMask(true);
            }
        }
        dlg.close();
    }
};

// Event handler for Cancel button
cancelBtn.onClick = function () {
    dlg.close();
};

// Render the dialog window
dlg.show();


///// Functions /////
function cTID(s) {
    return app.charIDToTypeID(s);
}
function sTID(s) {
    return app.stringIDToTypeID(s);
}

function blackBackground() {

    // Rename standard name Alpha channels to include a hyphen to avoid conflicts
    var theAlphas = app.activeDocument.channels;
    for (var i = 0; i < theAlphas.length; i++) {
        var channel = theAlphas[i];
        if (channel.name.match(/^Alpha \d+$/)) {
            var newName = channel.name.replace("Alpha ", "Alpha-");
            channel.name = newName;
        }
    }

    function selectChannel(channel, isName) {
        var desc = new ActionDescriptor();
        var ref = new ActionReference();
        if (isName) {
            ref.putName(cTID('Chnl'), channel);
        } else {
            ref.putEnumerated(cTID('Chnl'), cTID('Chnl'), channel);
        }
        desc.putReference(cTID('null'), ref);
        executeAction(cTID('slct'), desc, DialogModes.NO);
    }

    function makeChannel(targetChannel, calcMode, sourceChannel) {
        var desc1 = new ActionDescriptor();
        desc1.putClass(cTID('Nw  '), cTID('Chnl'));
        var desc2 = new ActionDescriptor();
        var ref1 = new ActionReference();
        ref1.putEnumerated(cTID('Chnl'), cTID('Ordn'), cTID('Trgt'));
        desc2.putReference(cTID('T   '), ref1);
        desc2.putEnumerated(cTID('Clcl'), cTID('Clcn'), calcMode);
        var ref2 = new ActionReference();
        ref2.putEnumerated(cTID('Chnl'), cTID('Chnl'), sourceChannel);
        desc2.putReference(cTID('Src2'), ref2);
        desc1.putObject(cTID('Usng'), cTID('Clcl'), desc2);
        executeAction(cTID('Mk  '), desc1, DialogModes.NO);
    }

    function setSelection(value, isName) {
        var desc = new ActionDescriptor();
        var ref1 = new ActionReference();
        ref1.putProperty(cTID('Chnl'), sTID('selection'));
        desc.putReference(cTID('null'), ref1);
        if (isName) {
            var ref2 = new ActionReference();
            ref2.putName(cTID('Chnl'), value);
            desc.putReference(cTID('T   '), ref2);
        } else {
            desc.putEnumerated(cTID('T   '), cTID('Ordn'), value);
        }
        executeAction(cTID('setd'), desc, DialogModes.NO);
    }

    function paste() {
        var desc = new ActionDescriptor();
        desc.putEnumerated(cTID('AntA'), cTID('Annt'), cTID('Anno'));
        executeAction(cTID('past'), desc, DialogModes.NO);
    }

    // Action conversion steps
    selectChannel(cTID('Rd  '), false);                     // Select Red channel
    makeChannel(null, cTID('Lghn'), cTID('Grn '));          // Alpha 1: Red lighten Green
    makeChannel(null, cTID('Lghn'), cTID('Bl  '));          // Alpha 2: Red lighten Blue
    makeChannel(null, sTID('blendDivide'), cTID('Rd  '));   // Alpha 3: Red divide Red
    selectChannel('Alpha 2', true);                         // Select Alpha 2
    makeChannel(null, sTID('blendDivide'), cTID('Grn '));   // Alpha 4: Alpha 2 divide Green
    selectChannel('Alpha 2', true);                         // Select Alpha 2
    makeChannel(null, sTID('blendDivide'), cTID('Bl  '));   // Alpha 5: Alpha 2 divide Blue

    selectChannel('Alpha 3', true);                         // Select Alpha 3
    setSelection(cTID('Al  '), false);                      // Set selection to all
    executeAction(cTID('copy'));                            // Copy
    selectChannel(cTID('Rd  '), false);                     // Select Red channel
    paste();                                                // Paste

    selectChannel('Alpha 4', true);                         // Select Alpha 4
    setSelection(cTID('Al  '), false);                      // Set selection to all
    executeAction(cTID('copy'));                            // Copy
    selectChannel(cTID('Grn '), false);                     // Select Green channel
    paste();                                                // Paste

    selectChannel('Alpha 5', true);                         // Select Alpha 5
    setSelection(cTID('Al  '), false);                      // Set selection to all
    executeAction(cTID('copy'));                            // Copy
    selectChannel(cTID('Bl  '), false);                     // Select Blue channel
    paste();                                                // Paste

    setSelection(cTID('None'), false);                      // Deselect
    setSelection('Alpha 2', true);                          // Load Alpha 2 as selection
    selectChannel(sTID('RGB'), false);                      // Select RGB channel
    executeAction(cTID('Invs'));                            // Inverse selection
    maskSelection("hideSelection");                         // Add a layer mask

    setSelection(cTID('None'), false);                      // Deselect
    selectChannel('Alpha 1', true);                         // Select Alpha 1
    selectChannel('Alpha 2', true);                         // Extend selection to Alpha 2
    selectChannel('Alpha 3', true);                         // Extend selection to Alpha 3
    selectChannel('Alpha 4', true);                         // Extend selection to Alpha 4
    selectChannel('Alpha 5', true);                         // Extend selection to Alpha 5

    // Delete all working Alpha channels
    var desc = new ActionDescriptor();
    var list = new ActionList();
    var alphaChannels = ['Alpha 5', 'Alpha 4', 'Alpha 3', 'Alpha 2', 'Alpha 1'];
    for (var i = 0; i < alphaChannels.length; i++) {
        var ref = new ActionReference();
        ref.putName(cTID('Chnl'), alphaChannels[i]);
        list.putReference(ref);
    }
    desc.putList(cTID('null'), list);
    executeAction(cTID('Dlt '), desc, DialogModes.NO);

    // Rename hyphenated Alpha channels back to standard names
    var theAlphas = app.activeDocument.channels;
    for (var i = 0; i < theAlphas.length; i++) {
        var channel = theAlphas[i];
        if (channel.name.match(/^Alpha-\d+$/)) {
            var newName = channel.name.replace("Alpha-", "Alpha ");
            channel.name = newName;
        }
    }
}

function whiteBackground() {
    // Invert the white to black
    app.activeDocument.activeLayer.invert();
    // Convert the black background to transparency
    blackBackground();
    // Invert the black back to white
    app.activeDocument.activeLayer.invert();
}

function maskSelection(maskParameter) {
    // Parameter = "revealSelection" or "hideSelection"
    var s2t = function (s) {
        return app.stringIDToTypeID(s);
    };
    var descriptor = new ActionDescriptor();
    var reference = new ActionReference();
    descriptor.putClass(s2t("new"), s2t("channel"));
    reference.putEnumerated(s2t("channel"), s2t("channel"), s2t("mask"));
    descriptor.putReference(s2t("at"), reference);
    descriptor.putEnumerated(s2t("using"), s2t("userMaskEnabled"), s2t(maskParameter));
    executeAction(s2t("make"), descriptor, DialogModes.NO);
}

function applyLayerMask(apply) {
    var s2t = function (s) {
        return app.stringIDToTypeID(s);
    };
    var descriptor = new ActionDescriptor();
    var reference = new ActionReference();
    reference.putEnumerated(s2t("channel"), s2t("channel"), s2t("mask"));
    descriptor.putReference(s2t("null"), reference);
    descriptor.putBoolean(s2t("apply"), apply);
    if (apply) {
        executeAction(s2t("delete"), descriptor, DialogModes.NO);
    }
}

1. Copy the code text to the clipboard
2. Open a new blank file in a plain-text editor (not in a word processor)
3. Paste the code in
4. Save as a plain text format file – .txt
5. Rename the saved file extension from .txt to .jsx
6. Install or browse to the .jsx file to run (see below)

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html

Whoooa, what?  I didnt know this tool existed!  This is exactly what I needed!  This is SO much easier than fiddling with channels and layer masks.  Thank you!