Formalize object id encoding contract

tests/Rendering/unit/test_object_id_encoding.cpp

@@ -0,0 +1,33 @@
+#include <gtest/gtest.h>
+
+#include <XCEngine/Rendering/ObjectIdEncoding.h>
+
+using namespace XCEngine::Rendering;
+
+TEST(ObjectIdEncoding_Test, EncodesOnlyLow32BitsIntoPickingId) {
+    constexpr uint64_t objectId = 0x123456789ABCDEF0ull;
+
+    EXPECT_FALSE(CanEncodeObjectIdWithoutLoss(objectId));
+    EXPECT_EQ(EncodeObjectIdToEncodedId(objectId), 0x9ABCDEF0u);
+    EXPECT_EQ(EncodeObjectIdToUInt32(objectId), 0x9ABCDEF0u);
+}
+
+TEST(ObjectIdEncoding_Test, DecodesColorBackToWidenedRuntimeId) {
+    constexpr uint8_t r = 0x12u;
+    constexpr uint8_t g = 0x34u;
+    constexpr uint8_t b = 0x56u;
+    constexpr uint8_t a = 0x78u;
+
+    EXPECT_EQ(DecodeEncodedObjectIdFromColor(r, g, b, a), 0x78563412u);
+    EXPECT_EQ(DecodeObjectIdFromColor(r, g, b, a), 0x78563412ull);
+}
+
+TEST(ObjectIdEncoding_Test, EncodesColorChannelsFromLow32Bits) {
+    const auto encodedColor = EncodeObjectIdToColor(0x78563412ull);
+    constexpr float kInv255 = 1.0f / 255.0f;
+
+    EXPECT_FLOAT_EQ(encodedColor.x, 0x12u * kInv255);
+    EXPECT_FLOAT_EQ(encodedColor.y, 0x34u * kInv255);
+    EXPECT_FLOAT_EQ(encodedColor.z, 0x56u * kInv255);
+    EXPECT_FLOAT_EQ(encodedColor.w, 0x78u * kInv255);
+}