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diff --git a/src/main/scala/sims/geometry/Projection.scala b/src/main/scala/sims/geometry/Projection.scala
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+++ b/src/main/scala/sims/geometry/Projection.scala
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+/*
+ * Simple Mechanics Simulator (SiMS)
+ * copyright (c) 2009 Jakob Odersky
+ * made available under the MIT License
+*/
+
+package sims.geometry
+
+import sims.math._
+
+/**Projection on an axis.
+ * <p>
+ * Projections are commonly used in SiMS for collision detection.
+ * @param axis directional vector of the axis of the projection
+ * @param lower lower value of the projection
+ * @param upper upper value of the projection*/
+case class Projection(axis: Vector2D,
+                      lower: Double,
+                      upper: Double) {
+  require(axis != Vector2D.Null, "A projection's axis cannot be given by a null vector!")
+  
+  /**Checks this projection for overlap with another projection <code>other</code>.
+   * @throws IllegalArgumentExcepion if both projections axes aren't the same*/
+  def overlaps(other: Projection): Boolean = {
+    require(axis == other.axis, "Cannot compare two projections on different axes!")
+      !((other.lower - this.upper) > 0 || (this.lower - other.upper) > 0)
+  }
+  
+  /**Returns the overlap between this projection and another projection <code>other</code>.
+  * @throws IllegalArgumentExcepion if both projections axes aren't the same*/
+  def overlap(other: Projection): Double = {
+    require(axis == other.axis, "Cannot compare two projections on different axes!")
+        (math.max(lower, other.lower) - math.min(upper, other.upper)).abs
+  }
+}