tkokof · October 29, 2018 08:56 · Oct 29, 2018
diff --git a/NormalCompression.cs b/NormalCompression.cs
@@ -0,0 +1,77 @@
+public static class Sample
+{
+    static int gsN = 127;  // N(N+1)/2 = 8128 < 2^{13}
+    static float gsB = 2 * gsN + 1;
+    static float gsB2 = gsB * gsB;
+    static float gsFactor = (gsN - 1) * Mathf.Sqrt(0.5f);
+    static float gsInvFactor = 1.0f / gsFactor;
+
+    public static ushort CompressNormal(Vector3 normal)
+    {
+        var x = normal.x;
+        var y = normal.y;
+        var z = normal.z;
+        Debug.Assert(MathUtil.IsSame(x * x + y * y + z * z, 1));
+
+        // Determine octant.
+        ushort index = 0;
+
+        if (x < 0.0)
+        {
+            index |= 0x8000;
+            x = -x;
+        }
+        if (y < 0.0)
+        {
+            index |= 0x4000;
+            y = -y;
+        }
+        if (z < 0.0)
+        {
+            index |= 0x2000;
+            z = -z;
+        }
+
+        // Determine mantissa.
+        ushort usX = (ushort)Mathf.Floor(gsFactor * x);
+        ushort usY = (ushort)Mathf.Floor(gsFactor * y);
+        ushort mantissa = (ushort)(usX + ((usY * (255 - usY)) >> 1));
+        index |= mantissa;
+
+        return index;
+    }
+
+    public static Vector3 DecompressNormal(ushort index)
+    {
+        ushort mantissa = (ushort)(index & 0x1FFF);
+
+        // Extract triangular indices.
+        float temp = gsB2 - 8 * mantissa;
+        ushort usY = (ushort)Mathf.Floor(0.5f * (gsB - Mathf.Sqrt(Mathf.Abs(temp))));
+        ushort usX = (ushort)(mantissa - ((usY * (255 - usY)) >> 1));
+
+        var normal = Vector3.zero;
+
+        // Build approximate normal.
+        normal.x = usX * gsInvFactor;
+        normal.y = usY * gsInvFactor;
+        temp = 1.0f - normal.x * normal.x - normal.y * normal.y;
+        normal.z = Mathf.Sqrt(Mathf.Abs(temp));
+
+        // Determine octant.
+        if ((index & 0x8000) != 0)
+        {
+            normal.x = -normal.x;
+        }
+        if ((index & 0x4000) != 0)
+        {
+            normal.y = -normal.y;
+        }
+        if ((index & 0x2000) != 0)
+        {
+            normal.z = -normal.z;
+        }
+
+        return normal;
+    }
+}