Merge #6706

6706: Fix `color_cast` between `ColorHSLA` and `ColorHSVA` losing alpha, add test r=def- a=Robyt3



## Checklist

- [X] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [X] Written a unit test (especially base/) or added coverage to integration test
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: Robert Müller <robytemueller@gmail.com>

src/base/color.h

@@ -228,14 +228,14 @@ template<>
 inline ColorHSLA color_cast(const ColorHSVA &hsv)
 {
 	float l = hsv.v * (1 - hsv.s * 0.5f);
-	return ColorHSLA(hsv.h, (l == 0.0f || l == 1.0f) ? 0 : (hsv.v - l) / minimum(l, 1 - l), l);
+	return ColorHSLA(hsv.h, (l == 0.0f || l == 1.0f) ? 0 : (hsv.v - l) / minimum(l, 1 - l), l, hsv.a);
 }
 
 template<>
 inline ColorHSVA color_cast(const ColorHSLA &hsl)
 {
 	float v = hsl.l + hsl.s * minimum(hsl.l, 1 - hsl.l);
-	return ColorHSVA(hsl.h, v == 0.0f ? 0 : 2 - (2 * hsl.l / v), v);
+	return ColorHSVA(hsl.h, v == 0.0f ? 0 : 2 - (2 * hsl.l / v), v, hsl.a);
 }
 
 template<>

src/test/color.cpp

@@ -23,3 +23,19 @@ TEST(Color, HRHConv)
 		}
 	}
 }
+
+// Any color_cast should keep the same alpha value
+TEST(Color, ConvKeepsAlpha)
+{
+	const int Max = 10;
+	for(int i = 0; i <= Max; i++)
+	{
+		const float Alpha = i / (float)Max;
+		EXPECT_FLOAT_EQ(color_cast<ColorRGBA>(ColorHSLA(0.1f, 0.2f, 0.3f, Alpha)).a, Alpha);
+		EXPECT_FLOAT_EQ(color_cast<ColorRGBA>(ColorHSVA(0.1f, 0.2f, 0.3f, Alpha)).a, Alpha);
+		EXPECT_FLOAT_EQ(color_cast<ColorHSLA>(ColorRGBA(0.1f, 0.2f, 0.3f, Alpha)).a, Alpha);
+		EXPECT_FLOAT_EQ(color_cast<ColorHSLA>(ColorHSVA(0.1f, 0.2f, 0.3f, Alpha)).a, Alpha);
+		EXPECT_FLOAT_EQ(color_cast<ColorHSVA>(ColorRGBA(0.1f, 0.2f, 0.3f, Alpha)).a, Alpha);
+		EXPECT_FLOAT_EQ(color_cast<ColorHSVA>(ColorHSLA(0.1f, 0.2f, 0.3f, Alpha)).a, Alpha);
+	}
+}