step back message (fixes #93)

src/components/stepback.js

@@ -0,0 +1,31 @@
+AFRAME.registerComponent('stepback', {
+  init: function () {
+    this.message = document.getElementById('stepback');
+    this.camera = document.getElementById('camera');
+    this.lastTime = 0;
+    this.limit =  - (this.el.getAttribute('geometry').height / 2 - 0.6);
+    this.throttling = 300;
+  },
+
+  tick: function (time, delta) {
+    if (time - this.lastTime < this.throttling) { return; }
+
+    var camPos = this.camera.object3D.position;
+    var msgPos = this.message.object3D.position;
+
+    if (camPos.z < this.limit) {
+      this.throttling = 20;
+      this.message.object3D.visible = true;
+      this.message.getObject3D('mesh').material.opacity = 1 - Math.abs(camPos.z - msgPos.z);
+      msgPos.x = camPos.x;
+      msgPos.y = camPos.y;
+    }
+    else {
+      this.message.object3D.visible = false;
+      this.throttling = 300;
+    }
+
+    this.lastTime = time;
+  }
+
+});