diff --git a/ui/data.js b/ui/data.js
index d9302451..f40eb191 100755
--- a/ui/data.js
+++ b/ui/data.js
@@ -577,12 +577,14 @@ function binSearch(target, lst, check, return_position, get){
 
 // Make a sparse gid list...
 //
+// if target is given this will merge gids into target...
+//
 // NOTE: the resulting list will always be sorted...
 // NOTE: this will skip all elements not in order
-function makeSparceGIDList(gids, data){
+function populateSparceGIDList(gids, target, data){
 	data = data == null ? DATA : data
 	var order = data.order
-	var res = []
+	var res = target == null ? [] : target
 
 	gids.forEach(function(e){
 		var i = order.indexOf(e)
@@ -599,9 +601,11 @@ function makeSparceGIDList(gids, data){
 // Remove all the undefined's form a sparse list...
 //
 function compactSparceList(lst){
-	return lst.filter(function(e){
-		return e !== undefined
-	})
+	// XXX is it normal that .filter(..) skips undefined values?
+	return lst.filter(function(){ return true })
+	//return lst.filter(function(e){
+	//	return e !== undefined
+	//})
 }
 
 
@@ -636,7 +640,7 @@ function compactSparceList(lst){
 // On the down side, this has some memory overhead -- ~ N - n * ref
 //
 function fastSortGIDsByOrder(gids, data){
-	return compactSparceList(makeSparceGIDList(gids, data))
+	return compactSparceList(populateSparceGIDList(gids, data))
 }
 
 
diff --git a/ui/marks.js b/ui/marks.js
index 18dcdcba..9f62b421 100755
--- a/ui/marks.js
+++ b/ui/marks.js
@@ -139,6 +139,29 @@ var getUnmarked = makeUnmarkedLister(
 		function(){ return MARKED }, 
 		function(){ return _UNMARKED_CACHE })
 
+// these two are about 3 orders of magnitude faster than the above...
+// ...which is better I do not yet know...
+function getUnmarked_s0(){
+	var order = DATA.order
+	var res = []
+	for(var i=0; i < order.length; i++){
+		if(MARKED[i] == null){
+			res.push(order[i])
+		}
+	}
+	//return res
+	return getLoadedGIDs(res)
+}
+function getUnmarked_s1(){
+	var res = DATA.order.slice()
+	// XXX is it normal that .map() and .forEach() skip undefined values?
+	MARKED.map(function(e, i){
+		delete res[i]
+	})
+	//return compactSparceList(res)
+	return getLoadedGIDs(compactSparceList(res))
+}
+
 
 // XXX make this undefined tolerant -- sparse list compatibility...
 var getMarkedGIDBefore = makeGIDBeforeGetterFromList(
@@ -692,7 +715,7 @@ var loadFileMarks = makeFileLoader(
 		MARKED_FILE_PATTERN, 
 		[],
 		function(data){ 
-			MARKED = makeSparceGIDList(data)
+			MARKED = populateSparceGIDList(data)
 		},
 		'marksLoaded')
 
@@ -747,7 +770,7 @@ function setupMarks(viewer){
 			})
 		})
 		.on('sortedImages', function(){
-			MARKED = makeSparceGIDList(MARKED)
+			MARKED = populateSparceGIDList(MARKED)
 			marksUpdated()
 		})
 		.on('horizontalShiftedImage', function(evt, gid, direction){