move contrasting_text_color() into ArdourCanvas

2 files changed, 34 insertions, 0 deletions

diff --git a/libs/canvas/canvas/utils.h b/libs/canvas/canvas/utils.h
index cd9d884475..e269ca215c 100644
--- a/libs/canvas/canvas/utils.h
+++ b/libs/canvas/canvas/utils.h
@@ -31,5 +31,7 @@ namespace ArdourCanvas {
 	extern LIBCANVAS_API void set_source_rgba (Cairo::RefPtr<Cairo::Context>, Color);
 
 	Distance LIBCANVAS_API distance_to_segment_squared (Duple const & p, Duple const & p1, Duple const & p2, double& t, Duple& at);
+
+        uint32_t LIBCANVAS_API contrasting_text_color (uint32_t c);
 }
 
diff --git a/libs/canvas/utils.cc b/libs/canvas/utils.cc
index bdc8fad039..99516c849b 100644
--- a/libs/canvas/utils.cc
+++ b/libs/canvas/utils.cc
@@ -220,3 +220,35 @@ ArdourCanvas::distance_to_segment_squared (Duple const & p, Duple const & p1, Du
 	return ((dpqx * dpqx) + (dpqy * dpqy));
 }
 
+uint32_t
+ArdourCanvas::contrasting_text_color (uint32_t c)
+{
+	double r, g, b, a;
+	ArdourCanvas::color_to_rgba (c, r, g, b, a);
+
+	const double black_r = 0.0;
+	const double black_g = 0.0;
+	const double black_b = 0.0;
+
+	const double white_r = 1.0;
+	const double white_g = 1.0;
+	const double white_b = 1.0;
+
+	/* Use W3C contrast guideline calculation */
+
+	double white_contrast = (max (r, white_r) - min (r, white_r)) +
+		(max (g, white_g) - min (g, white_g)) + 
+		(max (b, white_b) - min (b, white_b));
+
+	double black_contrast = (max (r, black_r) - min (r, black_r)) +
+		(max (g, black_g) - min (g, black_g)) + 
+		(max (b, black_b) - min (b, black_b));
+
+	if (white_contrast > black_contrast) {		
+		/* use white */
+		return ArdourCanvas::rgba_to_color (1.0, 1.0, 1.0, 1.0);
+	} else {
+		/* use black */
+		return ArdourCanvas::rgba_to_color (0.0, 0.0, 0.0, 1.0);
+	}
+}