[ios, macos] Premultiply color values for mgl_color

Fixes https://github.com/mapbox/mapbox-gl-native/issues/14329

2 files changed, 23 insertions, 3 deletions

diff --git a/platform/macos/CHANGELOG.md b/platform/macos/CHANGELOG.md
index ebfb3f335a..98bdda0abc 100644
--- a/platform/macos/CHANGELOG.md
+++ b/platform/macos/CHANGELOG.md
@@ -12,6 +12,7 @@
 * Added `mgl_attributed:` expression operator, which concatenate `MGLAttributedExpression` objects for specifying rich text in the `MGLSymbolStyleLayer.text` property. ([#14094](https://github.com/mapbox/mapbox-gl-native/pull/14094))
 * Fixed an issue that caused conditional expressions to crash when passed nested conditional expressions as parameters. ([#14181](https://github.com/mapbox/mapbox-gl-native/pull/14181))
 * Added `-[MGLMapViewDelegate mapView:didFailToLoadImage:]` to load missing symbol icons in the style if they are not found. ([#14302](https://github.com/mapbox/mapbox-gl-native/pull/14302))
+* Fixed a bug where non-opaque `NSColor` values were ignored when assigned to a style layer color property. ([#14406](https://github.com/mapbox/mapbox-gl-native/pull/14406))
 
 ### Offline
 
diff --git a/platform/macos/src/NSColor+MGLAdditions.mm b/platform/macos/src/NSColor+MGLAdditions.mm
index ea7d99f66d..6df3d750ce 100644
--- a/platform/macos/src/NSColor+MGLAdditions.mm
+++ b/platform/macos/src/NSColor+MGLAdditions.mm
@@ -15,15 +15,34 @@
     }
     [srgbColor getRed:&r green:&g blue:&b alpha:&a];
 
-    return { (float)r, (float)g, (float)b, (float)a };
+    // NSColor provides non-premultiplied color components, so we have to premultiply each
+    // color component with the alpha value to transform it into a valid
+    // mbgl::Color which expects premultiplied color components.
+    return { static_cast<float>(r*a), static_cast<float>(g*a), static_cast<float>(b*a), static_cast<float>(a) };
 }
 
 + (NSColor *)mgl_colorWithColor:(mbgl::Color)color {
+    // If there is no alpha value, return original color values.
+    if (color.a == 0.0f) {
+        // macOS 10.12 Sierra and below uses calibrated RGB by default.
+        if ([NSColor redColor].colorSpaceName == NSCalibratedRGBColorSpace) {
+            return [NSColor colorWithCalibratedRed:color.r green:color.g blue:color.b alpha:color.a];
+        } else {
+            return [NSColor colorWithRed:color.r green:color.g blue:color.b alpha:color.a];
+        }
+    }
+    
+    // mbgl::Color provides premultiplied color components, so we have to convert color
+    // components to non-premultiplied values to return a valid NSColor object.
+    float red = static_cast<float>((color.r / color.a));
+    float green = static_cast<float>((color.g / color.a));
+    float blue = static_cast<float>((color.b / color.a));
+
     // macOS 10.12 Sierra and below uses calibrated RGB by default.
     if ([NSColor redColor].colorSpaceName == NSCalibratedRGBColorSpace) {
-        return [NSColor colorWithCalibratedRed:color.r green:color.g blue:color.b alpha:color.a];
+        return [NSColor colorWithCalibratedRed:red green:green blue:blue alpha:color.a];
     } else {
-        return [NSColor colorWithRed:color.r green:color.g blue:color.b alpha:color.a];
+        return [NSColor colorWithRed:red green:green blue:blue alpha:color.a];
     }
 }