Restrict the input from the gyro to the same range as desktop version.

Also ensures that starting position when using gyro is "in the middle", and not the position corresponding to the top left corner when using mouse.

js/plax.js

@@ -31,6 +31,7 @@
       layers             = [],
       plaxActivityTarget = $(window),
       motionMax          = 1,
+      motionMin          = -1,
       motionStartX       = null,
       motionStartY       = null,
       ignoreMoveable     = false
@@ -168,6 +169,12 @@
       // Admittedly fuzzy measurements
       x = values.x / 30
       y = values.y / 30
+      // Ensure not outside of expected range, -1 to 1
+      x = x < motionMin ? motionMin : (x > motionMax ? motionMax : x)
+      y = y < motionMin ? motionMin : (y > motionMax ? motionMax : y)
+      // Normalize from -1 to 1 => 0 to 1
+      x = (x + 1) / 2
+      y = (y + 1) / 2
     }
 
     var hRatio = x/((moveable() == true) ? motionMax : plaxActivityTarget.width()),