From 112e6f8f00612ad2c99b907d071f9dbcce10f5f7 Mon Sep 17 00:00:00 2001
From: bjorn <tie.372@gmail.com>
Date: Sun, 23 Jul 2017 22:37:52 -0700
Subject: [PATCH] lovr.math.lookAt;

Returns the angle/axis rotation for looking at a point.
---
 src/api/math.c  | 16 ++++++++++++
 src/math/mat4.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++++
 src/math/mat4.h |  1 +
 src/math/quat.c |  9 -------
 4 files changed, 85 insertions(+), 9 deletions(-)

diff --git a/src/api/math.c b/src/api/math.c
index e37fbc20..e07f2ada 100644
--- a/src/api/math.c
+++ b/src/api/math.c
@@ -18,8 +18,24 @@ int l_lovrMathNewTransform(lua_State* L) {
   return 1;
 }
 
+int l_lovrMathLookAt(lua_State* L) {
+  float from[3] = { luaL_checknumber(L, 1), luaL_checknumber(L, 2), luaL_checknumber(L, 3) };
+  float to[3] = { luaL_checknumber(L, 4), luaL_checknumber(L, 5), luaL_checknumber(L, 6) };
+  float up[3] = { luaL_optnumber(L, 7, 0), luaL_optnumber(L, 8, 1), luaL_optnumber(L, 9, 0) };
+  float m[16], q[4], angle, ax, ay, az;
+  mat4_lookAt(m, from, to, up);
+  quat_fromMat4(q, m);
+  quat_getAngleAxis(q, &angle, &ax, &ay, &az);
+  lua_pushnumber(L, -angle);
+  lua_pushnumber(L, ax);
+  lua_pushnumber(L, ay);
+  lua_pushnumber(L, az);
+  return 4;
+}
+
 const luaL_Reg lovrMath[] = {
   { "newTransform", l_lovrMathNewTransform },
+  { "lookAt", l_lovrMathLookAt },
   { NULL, NULL }
 };
 
diff --git a/src/math/mat4.c b/src/math/mat4.c
index c5ec4633..5cef4c90 100644
--- a/src/math/mat4.c
+++ b/src/math/mat4.c
@@ -246,6 +246,74 @@ mat4 mat4_perspective(mat4 m, float near, float far, float fovy, float aspect) {
   return m;
 }
 
+// Modified from gl-matrix.c
+mat4 mat4_lookAt(mat4 m, vec3 from, vec3 to, vec3 up) {
+  float x0, x1, x2, y0, y1, y2, z0, z1, z2, len;
+
+  if (from[0] == to[0] && from[1] == to[1] && from[2] == to[2]) {
+    return mat4_identity(m);
+  }
+
+  z0 = from[0] - to[0];
+  z1 = from[1] - to[1];
+  z2 = from[2] - to[2];
+
+  len = 1 / sqrt(z0 * z0 + z1 * z1 + z2 * z2);
+  z0 *= len;
+  z1 *= len;
+  z2 *= len;
+
+  x0 = up[1] * z2 - up[2] * z1;
+  x1 = up[2] * z0 - up[0] * z2;
+  x2 = up[0] * z1 - up[1] * z0;
+  len = sqrt(x0 * x0 + x1 * x1 + x2 * x2);
+  if (!len) {
+    x0 = 0;
+    x1 = 0;
+    x2 = 0;
+  } else {
+    len = 1 / len;
+    x0 *= len;
+    x1 *= len;
+    x2 *= len;
+  }
+
+  y0 = z1 * x2 - z2 * x1;
+  y1 = z2 * x0 - z0 * x2;
+  y2 = z0 * x1 - z1 * x0;
+
+  len = sqrt(y0 * y0 + y1 * y1 + y2 * y2);
+  if (!len) {
+    y0 = 0;
+    y1 = 0;
+    y2 = 0;
+  } else {
+    len = 1 / len;
+    y0 *= len;
+    y1 *= len;
+    y2 *= len;
+  }
+
+  m[0] = x0;
+  m[1] = y0;
+  m[2] = z0;
+  m[3] = 0;
+  m[4] = x1;
+  m[5] = y1;
+  m[6] = z1;
+  m[7] = 0;
+  m[8] = x2;
+  m[9] = y2;
+  m[10] = z2;
+  m[11] = 0;
+  m[12] = -(x0 * from[0] + x1 * from[1] + x2 * from[2]);
+  m[13] = -(y0 * from[0] + y1 * from[1] + y2 * from[2]);
+  m[14] = -(z0 * from[0] + z1 * from[1] + z2 * from[2]);
+  m[15] = 1;
+
+  return m;
+}
+
 void mat4_transform(mat4 m, vec3 v) {
   vec3_set(v,
     v[0] * m[0] + v[1] * m[4] + v[2] * m[8] + m[12],
diff --git a/src/math/mat4.h b/src/math/mat4.h
index 22f1d579..f04b75eb 100644
--- a/src/math/mat4.h
+++ b/src/math/mat4.h
@@ -17,5 +17,6 @@ mat4 mat4_scale(mat4 m, float x, float y, float z);
 mat4 mat4_setTransform(mat4 m, float x, float y, float z, float sx, float sy, float sz, float angle, float ax, float ay, float az);
 mat4 mat4_orthographic(mat4 m, float left, float right, float top, float bottom, float near, float far);
 mat4 mat4_perspective(mat4 m, float near, float far, float fov, float aspect);
+mat4 mat4_lookAt(mat4 m, vec3 from, vec3 to, vec3 up);
 void mat4_transform(mat4 m, vec3 v);
 void mat4_transformDirection(mat4 m, vec3 v);
diff --git a/src/math/quat.c b/src/math/quat.c
index 37fe01ac..85a58f3b 100644
--- a/src/math/quat.c
+++ b/src/math/quat.c
@@ -26,15 +26,6 @@ quat quat_fromAngleAxis(quat q, float angle, vec3 axis) {
   return q;
 }
 
-quat quat_fromDirection(quat q, vec3 forward, vec3 up) {
-  vec3 qq = (vec3) q;
-  vec3_init(qq, forward);
-  vec3_normalize(qq);
-  q[3] = 1 + vec3_dot(qq, up);
-  vec3_cross(qq, up);
-  return q;
-}
-
 quat quat_fromMat4(quat q, mat4 m) {
   float x = sqrt(MAX(0, 1 + m[0] - m[5] - m[10])) / 2;
   float y = sqrt(MAX(0, 1 - m[0] + m[5] - m[10])) / 2;