From b8b1c33d080c757f849ebdd3a864e8c72879fb63 Mon Sep 17 00:00:00 2001
From: "C. J. Howard" <cjhoward@cjhoward.org>
Date: Thu, 6 May 2021 19:53:39 +0800
Subject: [PATCH] Add utility function to convert between visual magnitude and
 lux

---
 CMakeLists.txt            |  1 -
 src/astro/astro.hpp       |  1 +
 src/astro/blackbody.hpp   |  2 +-
 src/astro/color-index.hpp |  4 ++--
 src/astro/illuminance.cpp | 36 +++++++++++++++++++++++++++++
 src/astro/illuminance.hpp | 48 +++++++++++++++++++++++++++++++++++++++
 6 files changed, 88 insertions(+), 4 deletions(-)
 create mode 100644 src/astro/illuminance.cpp
 create mode 100644 src/astro/illuminance.hpp

diff --git a/CMakeLists.txt b/CMakeLists.txt
index bc40e61..d4aa351 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -14,7 +14,6 @@ find_package(SDL2 REQUIRED COMPONENTS SDL2::SDL2-static SDL2::SDL2main CONFIG)
 find_package(OpenAL REQUIRED CONFIG)
 find_library(physfs REQUIRED NAMES physfs-static PATHS "${CMAKE_PREFIX_PATH}/lib")
 
-
 # Determine dependencies
 set(STATIC_LIBS
 	dr_wav
diff --git a/src/astro/astro.hpp b/src/astro/astro.hpp
index f818809..f8ce409 100644
--- a/src/astro/astro.hpp
+++ b/src/astro/astro.hpp
@@ -29,6 +29,7 @@ namespace astro {}
 #include "color-index.hpp"
 #include "constants.hpp"
 #include "coordinates.hpp"
+#include "illuminance.hpp"
 #include "orbit.hpp"
 
 #endif // ANTKEEPER_ASTRO_HPP
diff --git a/src/astro/blackbody.hpp b/src/astro/blackbody.hpp
index 8731bdc..2f5d254 100644
--- a/src/astro/blackbody.hpp
+++ b/src/astro/blackbody.hpp
@@ -29,7 +29,7 @@ namespace astro
  * Calculates the color of an ideal black-body radiator, given its temperature in Kelvin.
  *
  * @param t Temperature, in Kelvin.
- * @return Correlated color, in linear RGB space.
+ * @return Correlated color, in linear sRGB.
  *
  * @see https://en.wikipedia.org/wiki/Planckian_locus
  * @see https://en.wikipedia.org/wiki/CIE_1960_color_space
diff --git a/src/astro/color-index.hpp b/src/astro/color-index.hpp
index 682fae7..0d3dd95 100644
--- a/src/astro/color-index.hpp
+++ b/src/astro/color-index.hpp
@@ -26,10 +26,10 @@ namespace astro
 /**
  * Approximates the temperature of a star, given its B-V index.
  *
- * @see Ballesteros, F. J. (2012). "New insights into black bodies". EPL 97 (2012) 34008.
- *
  * @param bv B-V index.
  * @return Temperature, in Kelvin.
+ *
+ * @see Ballesteros, F. J. (2012). "New insights into black bodies". EPL 97 (2012) 34008.
  */
 double bv_to_k(double bv);
 
diff --git a/src/astro/illuminance.cpp b/src/astro/illuminance.cpp
new file mode 100644
index 0000000..3d39452
--- /dev/null
+++ b/src/astro/illuminance.cpp
@@ -0,0 +1,36 @@
+/*
+ * Copyright (C) 2021  Christopher J. Howard
+ *
+ * This file is part of Antkeeper source code.
+ *
+ * Antkeeper source code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Antkeeper source code is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Antkeeper source code.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "illuminance.hpp"
+#include <cmath>
+
+namespace astro
+{
+
+double vmag_to_lux(double mv)
+{
+	return std::pow(10.0, (-14.18 - mv) * 0.4);
+}
+
+double lux_to_vmag(double ev)
+{
+	return -14.18 - 2.5 * std::log(ev);
+}
+
+} // namespace astro
diff --git a/src/astro/illuminance.hpp b/src/astro/illuminance.hpp
new file mode 100644
index 0000000..0c57f94
--- /dev/null
+++ b/src/astro/illuminance.hpp
@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2021  Christopher J. Howard
+ *
+ * This file is part of Antkeeper source code.
+ *
+ * Antkeeper source code is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Antkeeper source code is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Antkeeper source code.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef ANTKEEPER_ASTRO_ILLUMINANCE_HPP
+#define ANTKEEPER_ASTRO_ILLUMINANCE_HPP
+
+namespace astro
+{
+
+/**
+ * Converts apparent (visual) magnitude to lux.
+ *
+ * @param mv Illuminance in apparent magnitude.
+ * @return Illuminance in lux.
+ *
+ * @see https://en.wikipedia.org/wiki/Illuminance
+ */
+double vmag_to_lux(double mv);
+
+/**
+ * Converts lux to apparent (visual) magnitude.
+ *
+ * @param ev Illuminance in lux.
+ * @return Illuminance in apparent magnitude.
+ *
+ * @see https://en.wikipedia.org/wiki/Illuminance
+ */
+double lux_to_vmag(double ev);
+
+} // namespace astro
+
+#endif // ANTKEEPER_ASTRO_ILLUMINANCE_HPP