Fix: Poor contrast in cargo dest flow legend window cargo labels.

Select foreground colour depending on the brightness of the background.

Previously all cargo labels were rendered using black text, even
the background cargo colour was dark/black.
As an example: FIRS coal was black text on a black background.

3 files changed, 6 insertions, 5 deletions

diff --git a/src/gfx.cpp b/src/gfx.cpp
index 187d197a3..c9c36019c 100644
--- a/src/gfx.cpp
+++ b/src/gfx.cpp
@@ -1111,16 +1111,17 @@ void DoPaletteAnimations()
 /**
  * Determine a contrasty text colour for a coloured background.
  * @param background Background colour.
+ * @param threshold Background colour brightness threshold below which the background is considered dark and TC_WHITE is returned, range: 0 - 255, default 128.
  * @return TC_BLACK or TC_WHITE depending on what gives a better contrast.
  */
-TextColour GetContrastColour(uint8 background)
+TextColour GetContrastColour(uint8 background, uint8 threshold)
 {
 	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;
+	/* Compare with threshold brightness which defaults to 128 (50%) */
+	return sq1000_brightness < ((uint) threshold) * ((uint) threshold) * 1000 ? TC_WHITE : TC_BLACK;
 }
 
 /**
diff --git a/src/gfx_func.h b/src/gfx_func.h
index 99461f907..9f7cb9153 100644
--- a/src/gfx_func.h
+++ b/src/gfx_func.h
@@ -187,7 +187,7 @@ int GetCharacterHeight(FontSize size);
 
 extern DrawPixelInfo *_cur_dpi;
 
-TextColour GetContrastColour(uint8 background);
+TextColour GetContrastColour(uint8 background, uint8 threshold = 128);
 
 /**
  * All 16 colour gradients
diff --git a/src/linkgraph/linkgraph_gui.cpp b/src/linkgraph/linkgraph_gui.cpp
index 015dc52ff..7cb9e5091 100644
--- a/src/linkgraph/linkgraph_gui.cpp
+++ b/src/linkgraph/linkgraph_gui.cpp
@@ -496,7 +496,7 @@ void LinkGraphLegendWindow::DrawWidget(const Rect &r, int widget) const
 		if (this->IsWidgetDisabled(widget)) return;
 		CargoSpec *cargo = CargoSpec::Get(widget - WID_LGL_CARGO_FIRST);
 		GfxFillRect(r.left + 2, r.top + 2, r.right - 2, r.bottom - 2, cargo->legend_colour);
-		DrawString(r.left, r.right, (r.top + r.bottom + 1 - FONT_HEIGHT_SMALL) / 2, cargo->abbrev, TC_BLACK, SA_HOR_CENTER);
+		DrawString(r.left, r.right, (r.top + r.bottom + 1 - FONT_HEIGHT_SMALL) / 2, cargo->abbrev, GetContrastColour(cargo->legend_colour, 73), SA_HOR_CENTER);
 	}
 }