[ios, macos] Premultiply color values for mgl_colorupstream/fix-halo-color-nb

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

5 files changed, 28 insertions, 7 deletions

diff --git a/platform/darwin/test/MGLExpressionTests.mm b/platform/darwin/test/MGLExpressionTests.mm
index 06f06d8c6e..e7d1608018 100644
--- a/platform/darwin/test/MGLExpressionTests.mm
+++ b/platform/darwin/test/MGLExpressionTests.mm
@@ -282,9 +282,14 @@ using namespace std::string_literals;
         XCTAssertEqualObjects([expression expressionValueWithObject:nil context:nil], color);
     }
     {
-        MGLColor *color = [MGLColor mgl_colorWithColor:{ 255.0/255, 239.0/255, 213.0/255, 0.5 }]; // papayawhip
+        // Transform color components to non-premultiplied values
+        float alpha = 0.5;
+        float red = (255.0 * alpha) / 255;
+        float green = (239.0 * alpha) / 255;
+        float blue = (213.0 * alpha) / 255;
+        MGLColor *color = [MGLColor mgl_colorWithColor:{ red, green, blue, alpha }]; // papayawhip
         NSExpression *expression = [NSExpression expressionForConstantValue:color];
-        NSArray *jsonExpression = @[@"rgba", @255, @239, @213, @0.5];
+        NSArray *jsonExpression = @[@"rgba", @127.5, @119.5, @106.5, @0.5];
         XCTAssertEqualObjects(expression.mgl_jsonExpressionObject, jsonExpression);
         XCTAssertEqualObjects([expression expressionValueWithObject:nil context:nil], color);
     }
diff --git a/platform/ios/CHANGELOG.md b/platform/ios/CHANGELOG.md
index 72e3e75cb2..86b501e13f 100644
--- a/platform/ios/CHANGELOG.md
+++ b/platform/ios/CHANGELOG.md
@@ -6,6 +6,7 @@ Mapbox welcomes participation and contributions from everyone. Please read [CONT
 
 * Fixed an Interface Builder crash when using an `MGLMapView` in a storyboard. ([#14379](https://github.com/mapbox/mapbox-gl-native/pull/14379))
 * Fix a bug that wrong position of attribution dialog after rotation. ([#14185](https://github.com/mapbox/mapbox-gl-native/pull/14185))
+* Fixed a bug where using `MGLSymbolStyleLayer.textHaloColor` with an alpha value would result in incorrect color values. ([#14406](https://github.com/mapbox/mapbox-gl-native/pull/14406))
 
 ## 4.10.0
 
diff --git a/platform/ios/src/UIColor+MGLAdditions.mm b/platform/ios/src/UIColor+MGLAdditions.mm
index 7c1fbddc20..5a4b5cf06e 100644
--- a/platform/ios/src/UIColor+MGLAdditions.mm
+++ b/platform/ios/src/UIColor+MGLAdditions.mm
@@ -6,7 +6,8 @@
 {
     CGFloat r, g, b, a;
     [self getRed:&r green:&g blue:&b alpha:&a];
-    return { (float)r, (float)g, (float)b, (float)a };
+    // Premultiply color components
+    return { static_cast<float>(r*a), static_cast<float>(g*a), static_cast<float>(b*a), static_cast<float>(a) };
 }
 
 - (mbgl::style::PropertyValue<mbgl::Color>)mgl_colorPropertyValue
@@ -17,7 +18,13 @@
 
 + (UIColor *)mgl_colorWithColor:(mbgl::Color)color
 {
-    return [UIColor colorWithRed:color.r green:color.g blue:color.b alpha:color.a];
+    // Since CIColor expects color values to be non-multiplied, we transform them here
+    // to non-premultiplied values
+    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));
+
+    return [UIColor colorWithRed:red green:green blue:blue alpha:color.a];
 }
 
 @end
diff --git a/platform/macos/CHANGELOG.md b/platform/macos/CHANGELOG.md
index ebfb3f335a..26f93377b2 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 using `MGLSymbolStyleLayer.textHaloColor` with an alpha value would result in incorrect color values. ([#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..0714ac1ada 100644
--- a/platform/macos/src/NSColor+MGLAdditions.mm
+++ b/platform/macos/src/NSColor+MGLAdditions.mm
@@ -15,15 +15,22 @@
     }
     [srgbColor getRed:&r green:&g blue:&b alpha:&a];
 
-    return { (float)r, (float)g, (float)b, (float)a };
+    // Premultiply 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 {
+    // Since CIColor expects color values to be non-multiplied, we transform them here
+    // to non-premultiplied values
+    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];
     }
 }