In current implementation of components using getUserMedia(), the camera direction is not selected.
https://github.com/zkoss/zkcml/blob/master/zkmax/src/archive/web/js/zkmax/med/Camera.js#L145
https://github.com/zkoss/zkcml/blob/master/zkmax/src/archive/web/js/zkmax/barscanner/Barcodescanner.js#L166

On this type of components, it would be good to have an attribute such as facingMode accepting a string to let developers choose camera modes, or event a userMedia attribute accepting an array of options to add to the constraints (which would cover more potential use cases).
This would also let developers implement a "switch camera" button, or a "select camera source" button on mobile and tablets

For example, the following is currently necessary to show the back camera on a mobile device:

	<script><![CDATA[
zk.afterLoad('zkmax.barscanner', function() {
	
	var xBarcodescanner = {};
	zk.override(zkmax.barscanner.Barcodescanner.prototype, xBarcodescanner ,{
		bind_: function () {
			this.$supers('bind_', arguments);
			var wgt = this;
			var front = false;
			var constraints = { video: { facingMode: (front? "user" : "environment") } };
			navigator.mediaDevices.getUserMedia(constraints).then(function (s) {
				wgt.$n('video').srcObject = s;
				wgt._setReaders();
		});
	}
	});//zk.override
});//zk.afterLoad
]]></script>