add luminance() operation

cppfw · Nov 6, 2024 · cf6be68 · cf6be68
1 parent e351dd0
commit cf6be68
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diff --git a/src/rasterimage/operations.hpp b/src/rasterimage/operations.hpp
@@ -155,6 +155,30 @@ constexpr r4::vector4<value_type> unpremultiply_alpha(const r4::vector4<value_ty
 	};
 }
 
+/**
+ * @brief Calculate luminance of a pixel.
+ * @param px - pixel value to calculate luminance from.
+ */
+template <typename value_type, size_t num_channels>
+constexpr value_type luminance(const r4::vector<value_type, num_channels>& px)
+{
+	if constexpr (num_channels == 1 || num_channels == 2) {
+		return px.r();
+	} else {
+		static_assert(num_channels >= 3, "logic error");
+
+		// Luminance is calculated using formula L = 0.2126 * R + 0.7152 * G + 0.0722 * B
+
+		constexpr auto red_coeff = 0.2126;
+		constexpr auto green_coeff = 0.7152;
+		constexpr auto blue_coeff = 0.0722;
+
+		return rasterimage::multiply(px.r(), value<value_type>(float(red_coeff))) +
+			rasterimage::multiply(px.g(), value<value_type>(float(green_coeff))) +
+			rasterimage::multiply(px.b(), value<value_type>(float(blue_coeff)));
+	}
+}
+
 /**
  * @brief Convert pixel of integral color value type to floating point one.
  * @param px - pixel to convert.

diff --git a/tests/unit/src/operations.cpp b/tests/unit/src/operations.cpp
@@ -98,5 +98,29 @@ const tst::set set("operations", [](tst::suite& suite) {
 		tst::check_eq(rasterimage::get_alpha(rgb), 1.0f, SL);
 		tst::check_eq(rasterimage::get_alpha(rgba), 0.4f, SL);
 	});
+
+	suite.add("luminance__float", []() {
+		r4::vector<float, 1> a = {0.1f};
+		r4::vector2<float> ga = {0.1f, 0.2f};
+		r4::vector3<float> rgb = {0.1f, 0.2f, 0.3f};
+		r4::vector4<float> rgba = {0.1f, 0.2f, 0.3f, 0.4f};
+
+		tst::check_eq(rasterimage::luminance(a), 0.1f, SL);
+		tst::check_eq(rasterimage::luminance(ga), 0.1f, SL);
+		tst::check_eq(rasterimage::luminance(rgb), 0.18596f, SL);
+		tst::check_eq(rasterimage::luminance(rgba), 0.18596f, SL);
+	});
+
+	suite.add("luminance__uint8_t", []() {
+		r4::vector<uint8_t, 1> a = {0x13};
+		r4::vector2<uint8_t> ga = {0x13, 0x14};
+		r4::vector3<uint8_t> rgb = {0x13, 0x44, 0xfe};
+		r4::vector4<uint8_t> rgba = {0x13, 0x44, 0xfe, 0xab};
+
+		tst::check_eq(rasterimage::luminance(a), uint8_t(0x13), SL);
+		tst::check_eq(rasterimage::luminance(ga), uint8_t(0x13), SL);
+		tst::check_eq(rasterimage::luminance(rgb), uint8_t(69), SL);
+		tst::check_eq(rasterimage::luminance(rgba), uint8_t(69), SL);
+	});
 });
 } // namespace