From 2ac5321bdf0aa1da27b9396473ab9f99cfe9b672 Mon Sep 17 00:00:00 2001
From: bjorn <tie.372@gmail.com>
Date: Fri, 5 Jan 2018 22:08:11 -0800
Subject: [PATCH] mat4:reflect;

---
 modules/mat4.lua   | 28 ++++++++++++++++++++++++++++
 spec/mat4_spec.lua | 10 ++++++++++
 2 files changed, 38 insertions(+)

diff --git a/modules/mat4.lua b/modules/mat4.lua
index 98db3fd..7db4c5a 100644
--- a/modules/mat4.lua
+++ b/modules/mat4.lua
@@ -491,6 +491,34 @@ function mat4.shear(out, a, yx, zx, xy, zy, xz, yz)
 	return out:mul(tmp, a)
 end
 
+--- Reflect a matrix across a plane.
+-- @tparam mat4 Matrix to store the result
+-- @tparam a Matrix to reflect
+-- @tparam vec3 position A point on the plane
+-- @tparam vec3 normal The (normalized!) normal vector of the plane
+function mat4.reflect(out, a, position, normal)
+	local nx, ny, nz = normal:unpack()
+	local d = -position:dot(normal)
+	tmp[1] = 1 - 2 * nx ^ 2
+	tmp[2] = 2 * nx * ny
+	tmp[3] = -2 * nx * nz
+	tmp[4] = 0
+	tmp[5] = -2 * nx * ny
+	tmp[6] = 1 - 2 * ny ^ 2
+	tmp[7] = -2 * ny * nz
+	tmp[8] = 0
+	tmp[9] = -2 * nx * nz
+	tmp[10] = -2 * ny * nz
+	tmp[11] = 1 - 2 * nz ^ 2
+	tmp[12] = 0
+	tmp[13] = -2 * nx * d
+	tmp[14] = -2 * ny * d
+	tmp[15] = -2 * nz * d
+	tmp[16] = 1
+
+	return out:mul(tmp, a)
+end
+
 --- Transform matrix to look at a point.
 -- @tparam mat4 out Matrix to store result
 -- @tparam mat4 a Matrix to transform
diff --git a/spec/mat4_spec.lua b/spec/mat4_spec.lua
index 201d4b1..19b0fff 100644
--- a/spec/mat4_spec.lua
+++ b/spec/mat4_spec.lua
@@ -261,6 +261,16 @@ describe("mat4:", function()
 		assert.is.equal(-1, a[10])
 	end)
 
+	it("reflects a matrix along a plane", function()
+		local origin = vec3(5, 1, 0)
+		local normal = vec3(0, -1, 0):normalize()
+		local a = mat4():reflect(mat4(), origin, normal)
+		local p = a * vec3(-5, 2, 5)
+		assert.is.equal(p.x, -5)
+		assert.is.equal(p.y, 0)
+		assert.is.equal(p.z, 5)
+	end)
+
 	it("projects a matrix into screen space", function()
 		local v  = vec3(0, 0, 10)
 		local a  = mat4()