How to save an alpha channel as jpg?

The following script will save all alpha channels to JPEG from a single open, previously saved document – to the same location as the original document.

So myFilename.psd would save out as myFilename_Alpha 1.jpg myFilename_Alpha 2.jpg etc., using the name of the document and the name of each alpha channel with an underscore separator. Any existing files with the same name will be overwritten, use at your own risk.

/*
Save Open Doc Alphas to JPG.jsx
v1.0 - Stephen Marsh, 22nd December 2021
https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-to-save-an-alpha-channel-as-jpg/td-p/12595286
How to save an alpha channel as jpg?
*/

#target photoshop

(function (){
    try {
        app.activeDocument.path;

        if (app.documents.length === 1) {
            try {
                if (app.activeDocument.mode === DocumentMode.RGB || app.activeDocument.mode === DocumentMode.LAB && app.activeDocument.channels.length > 3) {
                    processOpenDocs();
                } else if (app.activeDocument.mode === DocumentMode.CMYK && app.activeDocument.channels.length > 4) {
                    processOpenDocs();
                } else if (app.activeDocument.mode === DocumentMode.GRAYSCALE || app.activeDocument.mode === DocumentMode.DUOTONE || app.activeDocument.mode === DocumentMode.INDEXEDCOLOR && app.activeDocument.channels.length > 1) {
                    processOpenDocs();
                } else if (app.activeDocument.mode === DocumentMode.MULTICHANNEL) {
                    alert("Multichannel mode is not supported, please manually save the alpha channels!");
                } else {
                    alert("There are no alpha channels to save!");
                }
            } catch (err) {}

        } else {
            alert("This script only works with a single, previously saved doc open!");
        }

    } catch (err) {
        alert("This script only works with a single, previously saved doc open!");
    }
}());


////////// Functions //////////

function processOpenDocs() {
    var origDoc = app.activeDocument;
    origPath = app.activeDocument.path;
    var dupeName = app.activeDocument.name.replace(/\.[^\.]+$/, '');
    app.activeDocument.duplicate(dupeName, true);
    
    app.activeDocument = origDoc;
    alert("The original document will now be closed...");
    var idclose = stringIDToTypeID("close");
    executeAction(idclose, undefined, DialogModes.ALL);
    
    convertToGrayscale();
    deleteChannel();
    splitChannels();
    
    while (app.documents.length > 0) {
    saveOpenDocs();
    }
    
    app.beep();
    alert("All alpha channels saved as JPEG files to:" + "\r" + origPath.fsName);
}

function convertToGrayscale() {
    var idconvertMode = stringIDToTypeID("convertMode");
    var desc640 = new ActionDescriptor();
    var idto = stringIDToTypeID("to");
    var idgrayscaleMode = stringIDToTypeID("grayscaleMode");
    desc640.putClass(idto, idgrayscaleMode);
    executeAction(idconvertMode, desc640, DialogModes.NO);
}

function deleteChannel() {
    var iddelete = stringIDToTypeID("delete");
    var desc621 = new ActionDescriptor();
    var idnull = stringIDToTypeID("null");
    var ref104 = new ActionReference();
    var idchannel = stringIDToTypeID("channel");
    var idordinal = stringIDToTypeID("ordinal");
    var idtargetEnum = stringIDToTypeID("targetEnum");
    ref104.putEnumerated(idchannel, idordinal, idtargetEnum);
    desc621.putReference(idnull, ref104);
    executeAction(iddelete, desc621, DialogModes.NO);
}

function splitChannels() {
    var idsplitChannels = stringIDToTypeID("splitChannels");
    var desc359 = new ActionDescriptor();
    executeAction(idsplitChannels, desc359, DialogModes.NO);
}

function saveOpenDocs() {
    var docName = app.activeDocument.name;
    var saveFileJPG = new File(new File(origPath + "/" + docName + ".jpg"));
    saveJPG(saveFileJPG);
    app.activeDocument.close(SaveOptions.DONOTSAVECHANGES);

    function savePSD(saveFilePSD) {
        psdSaveOptions = new PhotoshopSaveOptions();
        psdSaveOptions.embedColorProfile = true;
        psdSaveOptions.alphaChannels = true;
        psdSaveOptions.layers = true;
        psdSaveOptions.annotations = true;
        psdSaveOptions.spotColors = true;
        app.activeDocument.saveAs(saveFilePSD, psdSaveOptions, true, Extension.LOWERCASE);
    }

    function saveJPG(saveFileJPG) {
        var jpgOptions = new JPEGSaveOptions();
        jpgOptions.formatOptions = FormatOptions.STANDARDBASELINE;
        jpgOptions.embedColorProfile = true;
        jpgOptions.matte = MatteType.NONE;
        jpgOptions.quality = 12;
        app.activeDocument.saveAs(saveFileJPG, jpgOptions, true, Extension.LOWERCASE);
    }
}

Totally doable. But some steps needed:

Make the Alpha channel, in the Layers palette, use the "Split Channles" command, save the Alpha that results from the others as you desire.

You may wish to duplicate the document first, then split and discard all the other channels, while keeping the original intact.

The other way would be to make a selection and use Save Selection, >New document, in the dialog, then save it.

AFAIK direct Export of alpha channels has never been possible. Might make for a good feature request.

What I've always done is target the channel and copy/paste into a new document.

Pay attention to your grayscale profiles! A single channel is effectively an untagged grayscale file, represented in your working gray. For consistency and some degree of predictability, you should probably set your working gray to Gray Gamma 2.2, or at any rate a gray space that has the same tone response curve as your document RGB.

How did you save a channels as an image file? I used Phoroshop 2020 here and could not find a way to export an Alpha channels as a jpeg or export a Document as a Jpeg and have the saves Jpeg have a an exported Alpha channel.

Hi, I would suggest to try Edit/Preferences/Export and select "use legacy export as"