(svn r24668) -Feature(ette) [FS#5311]: Draw cargo labels in the station list black or white depending on the background colour. (sbr)

src/gfx.cpp

@@ -1546,6 +1546,20 @@ void DoPaletteAnimations()
 	}
 }
 
+/**
+ * Determine a contrasty text colour for a coloured background.
+ * @param background Background colour.
+ * @return TC_BLACK or TC_WHITE depending on what gives a better contrast.
+ */
+TextColour GetContrastColour(uint8 background)
+{
+	Colour c = _cur_palette.palette[background];
+	/* Compute brightness according to http://www.w3.org/TR/AERT#color-contrast.
+	 * The following formula computes 1000 * brightness^2, with brightness being in range 0 to 255. */
+	uint sq1000_brightness = c.r * c.r * 299 + c.g * c.g * 587 + c.b * c.b * 114;
+	/* Compare with threshold brightness 128 (50%) */
+	return sq1000_brightness < 128 * 128 * 1000 ? TC_WHITE : TC_BLACK;
+}
 
 /**
  * Initialize _stringwidth_table cache

src/gfx_func.h

@@ -177,6 +177,8 @@ static inline byte GetCharacterHeight(FontSize size)
 
 extern DrawPixelInfo *_cur_dpi;
 
+TextColour GetContrastColour(uint8 background);
+
 /**
  * All 16 colour gradients
  * 8 colours per gradient from darkest (0) to lightest (7)

src/station_gui.cpp

@@ -109,6 +109,7 @@ static void StationsWndShowStationRating(int left, int right, int y, CargoID typ
 	if (!cs->IsValid()) return;
 
 	int colour = cs->rating_colour;
+	TextColour tc = GetContrastColour(colour);
 	uint w = (minu(amount, units_full) + 5) / 36;
 
 	int height = GetCharacterHeight(FS_SMALL);
@@ -126,7 +127,7 @@ static void StationsWndShowStationRating(int left, int right, int y, CargoID typ
 		}
 	}
 
-	DrawString(left + 1, right, y, cs->abbrev, TC_BLACK);
+	DrawString(left + 1, right, y, cs->abbrev, tc);
 
 	/* Draw green/red ratings bar (fits into 14 pixels) */
 	y += height + 2;
@@ -455,7 +456,8 @@ public:
 					const CargoSpec *cs = _sorted_cargo_specs[widget - WID_STL_CARGOSTART];
 					int cg_ofst = HasBit(this->cargo_filter, cs->Index()) ? 2 : 1;
 					GfxFillRect(r.left + cg_ofst, r.top + cg_ofst, r.right - 2 + cg_ofst, r.bottom - 2 + cg_ofst, cs->rating_colour);
-					DrawString(r.left + cg_ofst, r.right + cg_ofst, r.top + cg_ofst, cs->abbrev, TC_BLACK, SA_HOR_CENTER);
+					TextColour tc = GetContrastColour(cs->rating_colour);
+					DrawString(r.left + cg_ofst, r.right + cg_ofst, r.top + cg_ofst, cs->abbrev, tc, SA_HOR_CENTER);
 				}
 				break;
 		}