Determine if layer is black and white (or desaturated)

Question

Is there a way to programmatically determine if a layer of a PSD is black and white?

Answer 1

What mode are the images you want to check in anyway? CMYK, RGB …?
I suppose there are several options to achieve it.
• Iterating through the pixels with a ColorSampler would take too long for larger images, I guess.
• The Histogram would be unreliable because the numbers for each value in each Channel could accidentally be identical.
• But hiding other Layers, duplicating the image (mergeLayersOnly), changing to L*a*b, reading the a- and b-Histograms, closing the copy, un-hiding other Layers should work.
If the a- and b-Histograms are all bunched up around 127 (some tolerance could be set) it should be a desaturated Layer.
// check layer for 0 saturation;
// 2011, use it at your own risk;
#target photoshop;
if (app.documents.length > 0) {
var myDocument = app.activeDocument;
hideShowOthers ();
// duplicate;
var theCopy = myDocument.duplicate("copy", true);
theCopy.changeMode(ChangeMode.LAB);
// get histograms;
var theA = theCopy.channels[1].histogram;
var theB = theCopy.channels[2].histogram;
// check histrograms;
var check = true;
check = checkHistogram (theA, check, 0);
check = checkHistogram (theB, check, 0);
// close;
theCopy.close(SaveOptions.DONOTSAVECHANGES);
// re-show;
hideShowOthers ();
if (check == true) {alert ("layer is desaturated")}
else{alert ("layer contains color")}
////// check histogram for cumulation around 227 //////
function checkHistogram (histogram, check, tolerance) {
     for (var m = 0; m < 126 - tolerance; m++) {
          if (histogram[m] != 0) {check =  false}
     for (var n = 255; m > 127 + tolerance; m--) {
          if (histogram[m] != 0) {check =  false}
     return check
////// hide/show all others //////
function hideShowOthers () {
// =======================================================
var idShw = charIDToTypeID( "Shw " );
    var desc2 = new ActionDescriptor();
    var idnull = charIDToTypeID( "null" );
        var list1 = new ActionList();
            var ref1 = new ActionReference();
            var idLyr = charIDToTypeID( "Lyr " );
            var idOrdn = charIDToTypeID( "Ordn" );
            var idTrgt = charIDToTypeID( "Trgt" );
            ref1.putEnumerated( idLyr, idOrdn, idTrgt );
        list1.putReference( ref1 );
    desc2.putList( idnull, list1 );
    var idTglO = charIDToTypeID( "TglO" );
    desc2.putBoolean( idTglO, true );
executeAction( idShw, desc2, DialogModes.NO );
But someone else may have a better solution ready.