color: add luminance function

include/cglm/cglm.h

@@ -20,6 +20,7 @@
 #include "euler.h"
 #include "plane.h"
 #include "box.h"
+#include "color.h"
 #include "util.h"
 #include "io.h"
 

include/cglm/color.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c), Recep Aslantas.
+ *
+ * MIT License (MIT), http://opensource.org/licenses/MIT
+ * Full license can be found in the LICENSE file
+ */
+
+#ifndef cglm_color_h
+#define cglm_color_h
+
+#include "common.h"
+#include "vec3.h"
+
+/*!
+ * @brief averages the color channels into one value
+ *
+ * @param[in]  rgb RGB color
+ */
+CGLM_INLINE
+float
+glm_luminance(vec3 rgb) {
+  vec3 l = {0.212671f, 0.715160f, 0.072169f};
+  return glm_dot(rgb, l);
+}
+
+#endif /* cglm_color_h */