geom: Add game_color_to_gray()

Returns the gray value of a color. Alpha is ignored.

2 files changed, 11 insertions, 0 deletions

diff --git a/libgame/game-geom-basics.c b/libgame/game-geom-basics.c
index 880084f..4448521 100644
--- a/libgame/game-geom-basics.c
+++ b/libgame/game-geom-basics.c
@@ -325,6 +325,16 @@ hls_to_rgb (GameColor *dest, const GameHLS *src)
   }
 }
 
+double
+game_color_to_gray (const GameColor *color)
+{
+  g_return_val_if_fail (color != NULL, 0.5);
+
+  return color->r * 0.298828125 
+       + color->g * 0.587890625
+       + color->b * 0.11328125;
+}
+
 /**
  * game_color_shade:
  * @dest: result of shading
diff --git a/libgame/game-geom-basics.h b/libgame/game-geom-basics.h
index 57d8751..49a2d1a 100644
--- a/libgame/game-geom-basics.h
+++ b/libgame/game-geom-basics.h
@@ -102,6 +102,7 @@ GType		game_color_get_type		(void) G_GNUC_CONST;
 char *		game_color_to_string		(const GameColor *    	color);
 gboolean      	game_color_from_string		(GameColor *    	color,
 						 const char *		string);
+double          game_color_to_gray              (const GameColor *      color);
 void		game_color_shade		(GameColor *		dest, 
 						 const GameColor *	src, 
 						 double			factor);