feat(ugc): the fast hidden-face method back, next to LU Toolbox's

UgcHsr::Options::method picks how hidden faces are found: toolbox (LU Toolbox's
paths, the default, unchanged) or fast, the test the UGC server used before
"hidden faces removed as LU Toolbox's Remove Hidden Faces decides them",
restored unchanged as UgcRender::VisibleFromAround: the opaque mesh rendered
from 42 directions around the model (Options::fastResolution pixels square,
1024 as before) and the triangles that show in none removed. It is much
faster, but it also removes faces seen only by bounced light (interiors,
recesses), which LU Toolbox keeps. Nothing sets it yet.

Check: the UGC tests (the fast method removes a box seen only through a
chimney, which the paths keep; its files are the same every time).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/dUgcTests/UgcTests.cpp

@@ -2002,6 +2002,67 @@ namespace {
 	}
 }
 
+TEST(UgcHsr, FastMethodRendersFromAround) {
+	EXPECT_EQ(UgcHsr::Parse("fast"), UgcHsr::eMethod::FAST);
+	EXPECT_EQ(UgcHsr::Parse("toolbox"), UgcHsr::eMethod::TOOLBOX);
+	EXPECT_FALSE(UgcHsr::Parse("slow"));
+	EXPECT_EQ(UgcRender::SphereDirections().size(), 42u);
+
+	// A small box inside the big one: its faces can't be seen
+	UgcBricks::BrickLibrary library(MakeRes(), 0);
+	std::string error;
+	const auto parts = UgcModel::ParseLxfml(R"(<LXFML versionMajor="5"><Bricks>
+		<Brick><Part designID="3002" materials="21"><Bone transformation="1,0,0,0,1,0,0,0,1,0,0,0"/></Part></Brick>
+		<Brick><Part designID="3001" materials="21"><Bone transformation="1,0,0,0,1,0,0,0,1,0,0,0"/></Part></Brick>
+		</Bricks></LXFML>)", error);
+	auto model = UgcModel::Build(parts, library);
+	UgcHsr::Options options;
+	options.method = UgcHsr::eMethod::FAST;
+	options.fastResolution = 256;
+	const auto result = UgcHsr::RemoveHiddenFaces(model, options);
+	EXPECT_EQ(result.trianglesRemoved, 12u);
+	EXPECT_EQ(model.opaque.TriangleCount(), 12u);
+	EXPECT_EQ(result.paths, 0u);
+
+	// A small box in a closed chamber whose only opening is a narrow chimney at the other end: nothing outside sees it
+	// straight, so the fast method removes it; light bounced in through the chimney reaches some of it, so the toolbox
+	// method keeps that
+	UgcModel::Model chamber;
+	auto& mesh = chamber.opaque;
+	AddBox(mesh, glm::vec3(-0.9f, 0.0f, -0.15f), glm::vec3(-0.6f, 0.2f, 0.15f)); // the small box
+	AddBox(mesh, glm::vec3(-1.2f, -0.2f, -1.2f), glm::vec3(1.2f, 0.0f, 1.2f));   // floor
+	AddBox(mesh, glm::vec3(-1.2f, 0.0f, -1.2f), glm::vec3(-1.0f, 0.5f, 1.2f));   // walls
+	AddBox(mesh, glm::vec3(1.0f, 0.0f, -1.2f), glm::vec3(1.2f, 0.5f, 1.2f));
+	AddBox(mesh, glm::vec3(-1.0f, 0.0f, -1.2f), glm::vec3(1.0f, 0.5f, -1.0f));
+	AddBox(mesh, glm::vec3(-1.0f, 0.0f, 1.0f), glm::vec3(1.0f, 0.5f, 1.2f));
+	AddBox(mesh, glm::vec3(-1.2f, 0.5f, -1.2f), glm::vec3(0.6f, 0.7f, 1.2f));    // roof around the chimney's hole
+	AddBox(mesh, glm::vec3(0.9f, 0.5f, -1.2f), glm::vec3(1.2f, 0.7f, 1.2f));
+	AddBox(mesh, glm::vec3(0.6f, 0.5f, -1.2f), glm::vec3(0.9f, 0.7f, -0.15f));
+	AddBox(mesh, glm::vec3(0.6f, 0.5f, 0.15f), glm::vec3(0.9f, 0.7f, 1.2f));
+	AddBox(mesh, glm::vec3(0.5f, 0.7f, -0.25f), glm::vec3(0.6f, 2.2f, 0.25f));   // the chimney
+	AddBox(mesh, glm::vec3(0.9f, 0.7f, -0.25f), glm::vec3(1.0f, 2.2f, 0.25f));
+	AddBox(mesh, glm::vec3(0.6f, 0.7f, -0.25f), glm::vec3(0.9f, 2.2f, -0.15f));
+	AddBox(mesh, glm::vec3(0.6f, 0.7f, 0.15f), glm::vec3(0.9f, 2.2f, 0.25f));
+	const auto seen = UgcRender::VisibleFromAround(chamber, 512, false);
+	ASSERT_EQ(seen.size(), mesh.TriangleCount());
+	for (size_t t = 0; t < 12; t++) EXPECT_FALSE(seen[t]) << t;
+	UgcHsr::Options toolbox;
+	toolbox.samples = 32;
+	const auto traced = UgcHsr::Visible(mesh, toolbox);
+	EXPECT_TRUE(std::any_of(traced.begin(), traced.begin() + 12, [](bool kept) { return kept; }));
+
+	// Its files are the same every time
+	auto settings = SmallSettings();
+	settings.hsr.method = UgcHsr::eMethod::FAST;
+	settings.hsr.fastResolution = 128;
+	const auto first = UgcJobs::ProcessModel(LXFML5, library, settings, 7);
+	ASSERT_TRUE(first.ok) << first.error;
+	EXPECT_EQ(first.files.at("model.nif.checksum"), UgcJobs::ProcessModel(LXFML5, library, settings, 7).files.at("model.nif.checksum"));
+#if defined(__linux__) && defined(__x86_64__) && defined(__GNUC__) && !defined(__clang__)
+	EXPECT_EQ(UgcFormats::Md5Hex(*ZCompression::Gunzip(first.files.at("model.nif.gz"))), "4bd664412c88333431842eb82a0abfac");
+#endif
+}
+
 TEST(UgcRays, NamesAndFallback) {
 	for (const auto backend : { UgcRays::eBackend::BUILTIN, UgcRays::eBackend::EMBREE, UgcRays::eBackend::HIPRT }) {
 		EXPECT_EQ(UgcRays::Parse(UgcRays::Name(backend)), backend);