diff --git a/sjplayer/src/sjplayer/SquareElement.d b/sjplayer/src/sjplayer/SquareElement.d
index 9b001f3..0b2a517 100644
--- a/sjplayer/src/sjplayer/SquareElement.d
+++ b/sjplayer/src/sjplayer/SquareElement.d
@@ -1,10 +1,14 @@
 /// License: MIT
 module sjplayer.SquareElement;
 
+import std.algorithm,
+       std.typecons;
+
 import gl4d;
 
 import sjplayer.ElementDrawer,
-       sjplayer.ElementInterface;
+       sjplayer.ElementInterface,
+       sjplayer.util.linalg;
 
 ///
 class SquareElement : ElementInterface {
@@ -30,7 +34,33 @@ class SquareElement : ElementInterface {
   }
 
   override DamageCalculationResult CalculateDamage(vec2 p1, vec2 p2) const {
-    return DamageCalculationResult(0, 0);  // TODO:
+    if (!alive) return DamageCalculationResult(0, 0);
+
+    const m = matrix.inverse;
+    const a = (m * vec3(p1, 1)).xy;
+    const b = (m * vec3(p2, 1)).xy;
+
+    if ((-1 <= a.x && a.x <= 1 && -1 <= a.y && a.y <= 1) ||
+        (-1 <= b.x && b.x <= 1 && -1 <= b.y && b.y <= 1)) {
+      return DamageCalculationResult(damage, 0);
+    }
+
+    enum edges = [
+      tuple(vec2(-1,  1), vec2(-1, -1)),
+      tuple(vec2(-1, -1), vec2( 1, -1)),
+      tuple(vec2( 1, -1), vec2( 1,  1)),
+      tuple(vec2( 1,  1), vec2(-1,  1)),
+    ];
+    float[4] distances;
+    static foreach (i, edge; edges) {
+      distances[i] = CalculateDistance2LineSegment(edge[0], edge[1], a, b);
+    }
+    const min_distance = distances[].minElement;
+
+    if (min_distance == 0) {
+      return DamageCalculationResult(damage, 0);
+    }
+    return DamageCalculationResult(0, 1-(min_distance-1).clamp(0f, 1f));
   }
 
   ///
diff --git a/sjplayer/src/sjplayer/util/linalg.d b/sjplayer/src/sjplayer/util/linalg.d
index 83196ac..dad0398 100644
--- a/sjplayer/src/sjplayer/util/linalg.d
+++ b/sjplayer/src/sjplayer/util/linalg.d
@@ -30,3 +30,50 @@ unittest {
   assert(CalculateDistanceOriginAndLineSegment(
         vec2(-2, 1), vec2(-1, 1)).approxEqual(sqrt(2f)));
 }
+
+///
+float CalculateDistance2LineSegment(vec2 a1, vec2 b1, vec2 a2, vec2 b2) {
+  float cross2(vec2 v1, vec2 v2) {
+    return v1.x * v2.y - v1.y * v2.x;
+  }
+
+  if (a1 == b1) return CalculateDistanceOriginAndLineSegment(a2 - a1, b2 - a1);
+  if (a2 == b2) return CalculateDistanceOriginAndLineSegment(a1 - a2, b1 - a2);
+
+  const a1_to_b1 = b1 - a1;
+  const a2_to_b2 = b2 - a2;
+
+  if (cross2(a1_to_b1, a2_to_b2) == 0) {
+    return CalculateDistanceOriginAndLineSegment(a2 - a1, b2 - a1);
+  }
+
+  const a1_to_a2 = a2 - a1;
+  const a1_to_b2 = b2 - a1;
+  const a2_to_a1 = a1 - a2;
+  const a2_to_b1 = b1 - a2;
+
+  if (cross2(a1_to_a2, a1_to_b1) * cross2(a1_to_b2, a1_to_b1) < 0 &&
+      cross2(a2_to_a1, a2_to_b2) * cross2(a2_to_b1, a2_to_b2) < 0) {
+    return 0;
+  }
+
+  const b1_to_a2 = a2 - b1;
+  const b1_to_b2 = b2 - b1;
+  return min(
+      CalculateDistanceOriginAndLineSegment(a1_to_a2, a1_to_b2),
+      CalculateDistanceOriginAndLineSegment(b1_to_a2, b1_to_b2)
+    );
+}
+///
+unittest {
+  import std;
+  assert(CalculateDistance2LineSegment(
+        vec2(1, 0), vec2(1, 0), vec2(0, 1), vec2(0, -1)).approxEqual(1f));
+
+  assert(CalculateDistance2LineSegment(
+        vec2(-1, 0), vec2(1, 0), vec2(0, 1), vec2(0, -1)).approxEqual(0f));
+  assert(CalculateDistance2LineSegment(
+        vec2(-1, 0), vec2(1, 0), vec2(1, 1), vec2(1, -1)).approxEqual(0f));
+  assert(CalculateDistance2LineSegment(
+        vec2(-1, 0), vec2(1, 0), vec2(2, 1), vec2(2, -1)).approxEqual(1f));
+}