MeshTools: make generateIndices() less restrictive.

Sometimes it's desirable to just make a mesh indexed unconditionally,
which wasn't really possible with this thing, and having to special-case
that at the call site is very annoying. Now it makes a mesh
trivially indexed if it has any other primitive than the ones listed,
and if it's already indexed, it's just passed through.

doc/changelog.dox

@@ -731,6 +731,9 @@ See also:
     @relativeref{MeshTools,generateIndices()} now allow empty input, producing
     an empty index buffer as a result. Disallowing empty input was an
     unnecessary restriction that was inconsistent with other APIs.
+-   @ref MeshTools::generateIndices() now generates a trivial index buffer for
+    non-loop, non-strip and non-fan primitives, and if such a mesh is already
+    indexed it simply passes it through
 -   Added overloads to @ref MeshTools::generateLineStripIndices(),
     @relativeref{MeshTools,generateLineLoopIndices()},
     @relativeref{MeshTools,generateTriangleStripIndices()} and

src/Magnum/MeshTools/GenerateIndices.cpp

@@ -602,8 +602,9 @@ Trade::MeshData generateIndices(Trade::MeshData&& mesh) {
     } else if(mesh.primitive() == MeshPrimitive::TriangleStrip ||
               mesh.primitive() == MeshPrimitive::TriangleFan) {
         minVertexCount = 3;
-    } else CORRADE_ASSERT_UNREACHABLE("MeshTools::generateIndices(): invalid primitive" << mesh.primitive(),
-        (Trade::MeshData{MeshPrimitive::Triangles, 0}));
+    } else {
+        minVertexCount = 1;
+    }
     CORRADE_ASSERT(vertexCount == 0 || vertexCount >= minVertexCount,
         "MeshTools::generateIndices(): expected either zero or at least" << minVertexCount << "vertices for" << mesh.primitive() << Debug::nospace << ", got" << vertexCount,
         (Trade::MeshData{MeshPrimitive::Triangles, 0}));
@@ -658,7 +659,22 @@ Trade::MeshData generateIndices(Trade::MeshData&& mesh) {
             generateTriangleFanIndicesInto(mesh.indices(), Containers::arrayCast<UnsignedInt>(indexData));
         else
             generateTriangleFanIndicesInto(vertexCount, Containers::arrayCast<UnsignedInt>(indexData));
-    } else CORRADE_INTERNAL_ASSERT_UNREACHABLE(); /* LCOV_EXCL_LINE */
+    } else {
+        primitive = mesh.primitive();
+        /* If the mesh is indexed, transfer the index data as-is if possible
+           and it's the right type already */
+        if(mesh.isIndexed()) {
+            if(mesh.indexDataFlags() >= Trade::DataFlag::Owned && mesh.indexType() == MeshIndexType::UnsignedInt) {
+                indexData = mesh.releaseIndexData();
+            } else {
+                indexData = Containers::Array<char>{NoInit, mesh.indexCount()*sizeof(UnsignedInt)};
+                mesh.indicesInto(Containers::arrayCast<UnsignedInt>(indexData));
+            }
+        } else {
+            indexData = Containers::Array<char>{NoInit, vertexCount*sizeof(UnsignedInt)};
+            generateTrivialIndicesInto(Containers::arrayCast<UnsignedInt>(indexData));
+        }
+    }
 
     Trade::MeshIndexData indices{MeshIndexType::UnsignedInt, indexData};
     return Trade::MeshData{primitive, Utility::move(indexData), indices,

src/Magnum/MeshTools/GenerateIndices.h

@@ -508,15 +508,21 @@ MAGNUM_MESHTOOLS_EXPORT void generateQuadIndicesInto(const Containers::StridedAr
 @brief Convert a mesh to plain indexed lines or triangles
 @m_since{2020,06}
 
-Expects that @p mesh is one of @ref MeshPrimitive::LineStrip,
+If @p mesh is one of @ref MeshPrimitive::LineStrip,
 @ref MeshPrimitive::LineLoop, @ref MeshPrimitive::TriangleStrip or
-@ref MeshPrimitive::TriangleFan primitives and has either @cpp 0 @ce vertices
-or at least @cpp 2 @ce vertices for a line-based primitive and @cpp 3 @ce
-vertices for a triangle-based primitive. If it's indexed, the index type is
-expected to be non-implementation-specific. Calls one of
+@ref MeshPrimitive::TriangleFan primitives, calls one of
 @ref generateLineStripIndices(), @ref generateLineLoopIndices(),
 @ref generateTriangleStripIndices() or @ref generateTriangleFanIndices()
-functions or their indexed overloads to generate the index buffer.
+functions or their indexed overloads to generate the index buffer. In that case
+expects that the mesh has either @cpp 0 @ce vertices or at least @cpp 2 @ce
+vertices for a line-based primitive and @cpp 3 @ce vertices for a
+triangle-based primitive. If @p mesh is a different
+@relativeref{Magnum,MeshPrimitive}, it's passed through unchanged if already
+indexed, and with indices generated using @ref generateTrivialIndices()
+otherwise.
+
+If @p mesh is already indexed, the index type is expected to be
+non-implementation-specific.
 
 The resulting mesh always has @ref MeshIndexType::UnsignedInt, call
 @ref compressIndices(const Trade::MeshData&, MeshIndexType) on the result to
@@ -533,8 +539,9 @@ MAGNUM_MESHTOOLS_EXPORT Trade::MeshData generateIndices(const Trade::MeshData& m
 
 Compared to @ref generateIndices(const Trade::MeshData&) this function can
 transfer ownership of @p mesh vertex buffer (in case it is owned) to the
-returned instance instead of making a copy of it. Attribute data is copied
-always.
+returned instance instead of making a copy of it, and index buffer as well if
+it's owned, doesn't need expanding and is already with
+@ref MeshIndexType::UnsignedInt. Attribute data is copied always.
 @see @ref Trade::MeshData::vertexDataFlags()
 */
 MAGNUM_MESHTOOLS_EXPORT Trade::MeshData generateIndices(Trade::MeshData&& mesh);

src/Magnum/MeshTools/Test/GenerateIndicesTest.cpp

@@ -29,6 +29,7 @@
 #include <Corrade/Containers/StridedArrayView.h>
 #include <Corrade/TestSuite/Tester.h>
 #include <Corrade/TestSuite/Compare/Container.h>
+#include <Corrade/Utility/Algorithms.h>
 #include <Corrade/Utility/DebugStl.h>
 #include <Corrade/Utility/FormatStl.h>
 
@@ -86,7 +87,10 @@ struct GenerateIndicesTest: TestSuite::Tester {
     void generateIndicesMeshDataEmpty();
     void generateIndicesMeshDataMove();
     void generateIndicesMeshDataNoAttributes();
-    void generateIndicesMeshDataInvalidPrimitive();
+    void generateIndicesMeshDataTrivial();
+    template<class T> void generateIndicesMeshDataTrivialIndexed();
+    void generateIndicesMeshDataTrivialIndexedMove();
+    void generateIndicesMeshDataTrivialIndexedMoveDifferentIndexType();
     void generateIndicesMeshDataInvalidVertexCount();
     void generateIndicesMeshDataImplementationSpecificIndexType();
 };
@@ -283,7 +287,12 @@ GenerateIndicesTest::GenerateIndicesTest() {
 
     addTests({&GenerateIndicesTest::generateIndicesMeshDataMove,
               &GenerateIndicesTest::generateIndicesMeshDataNoAttributes,
-              &GenerateIndicesTest::generateIndicesMeshDataInvalidPrimitive});
+              &GenerateIndicesTest::generateIndicesMeshDataTrivial,
+              &GenerateIndicesTest::generateIndicesMeshDataTrivialIndexed<UnsignedInt>,
+              &GenerateIndicesTest::generateIndicesMeshDataTrivialIndexed<UnsignedShort>,
+              &GenerateIndicesTest::generateIndicesMeshDataTrivialIndexed<UnsignedByte>,
+              &GenerateIndicesTest::generateIndicesMeshDataTrivialIndexedMove,
+              &GenerateIndicesTest::generateIndicesMeshDataTrivialIndexedMoveDifferentIndexType});
 
     addInstancedTests({&GenerateIndicesTest::generateIndicesMeshDataInvalidVertexCount},
         Containers::arraySize(MeshDataInvalidVertexCountData));
@@ -1303,16 +1312,142 @@ void GenerateIndicesTest::generateIndicesMeshDataNoAttributes() {
     CORRADE_COMPARE(out.attributeCount(), 0);
 }
 
-void GenerateIndicesTest::generateIndicesMeshDataInvalidPrimitive() {
-    CORRADE_SKIP_IF_NO_ASSERT();
+void GenerateIndicesTest::generateIndicesMeshDataTrivial() {
+    const Vector2 positions[]{
+        {1.5f, 0.3f},
+        {2.5f, 1.3f},
+        {3.5f, 2.3f},
+        {4.5f, 3.3f},
+        {5.5f, 4.3f},
+    };
 
-    Trade::MeshData mesh{MeshPrimitive::Triangles, 2};
+    /* Should work with just any primitive, not just Lines etc */
+    Trade::MeshData mesh{MeshPrimitive::Meshlets, {}, positions, {
+        Trade::MeshAttributeData{Trade::MeshAttribute::Position,
+            Containers::arrayView(positions)},
+    }};
 
-    std::ostringstream out;
-    Error redirectError{&out};
-    generateIndices(mesh);
-    CORRADE_COMPARE(out.str(),
-        "MeshTools::generateIndices(): invalid primitive MeshPrimitive::Triangles\n");
+    Trade::MeshData out = generateIndices(mesh);
+    CORRADE_COMPARE(out.primitive(), MeshPrimitive::Meshlets);
+    CORRADE_VERIFY(out.isIndexed());
+    CORRADE_COMPARE(out.indexType(), MeshIndexType::UnsignedInt);
+    CORRADE_COMPARE_AS(out.indices<UnsignedInt>(), Containers::arrayView<UnsignedInt>({
+        0, 1, 2, 3, 4
+    }), TestSuite::Compare::Container);
+
+    CORRADE_COMPARE(out.attributeCount(), 1);
+    CORRADE_COMPARE_AS(out.attribute<Vector2>(Trade::MeshAttribute::Position),
+        Containers::arrayView<Vector2>({
+            {1.5f, 0.3f}, {2.5f, 1.3f}, {3.5f, 2.3f}, {4.5f, 3.3f}, {5.5f, 4.3f}
+        }), TestSuite::Compare::Container);
+}
+
+template<class T> void GenerateIndicesTest::generateIndicesMeshDataTrivialIndexed() {
+    setTestCaseTemplateName(Math::TypeTraits<T>::name());
+
+    const Vector2 positions[]{
+        {1.5f, 0.3f},
+        {2.5f, 1.3f},
+        {3.5f, 2.3f},
+        {4.5f, 3.3f},
+        {5.5f, 4.3f},
+    };
+
+    const T indices[]{60, 21, 72, 93};
+
+    /* Should work with just any primitive, not just Lines etc */
+    Trade::MeshData mesh{MeshPrimitive::Meshlets,
+        {}, indices, Trade::MeshIndexData{indices},
+        {}, positions, {
+            Trade::MeshAttributeData{Trade::MeshAttribute::Position,
+                Containers::arrayView(positions)},
+        }};
+
+    Trade::MeshData out = generateIndices(mesh);
+    CORRADE_COMPARE(out.primitive(), MeshPrimitive::Meshlets);
+    CORRADE_VERIFY(out.isIndexed());
+    CORRADE_COMPARE(out.indexType(), MeshIndexType::UnsignedInt);
+    CORRADE_COMPARE_AS(out.indices<UnsignedInt>(), Containers::arrayView<UnsignedInt>({
+        60, 21, 72, 93
+    }), TestSuite::Compare::Container);
+
+    CORRADE_COMPARE(out.attributeCount(), 1);
+    CORRADE_COMPARE_AS(out.attribute<Vector2>(Trade::MeshAttribute::Position),
+        Containers::arrayView<Vector2>({
+            {1.5f, 0.3f}, {2.5f, 1.3f}, {3.5f, 2.3f}, {4.5f, 3.3f}, {5.5f, 4.3f}
+        }), TestSuite::Compare::Container);
+}
+
+void GenerateIndicesTest::generateIndicesMeshDataTrivialIndexedMove() {
+    const Vector2 positions[]{
+        {1.5f, 0.3f},
+        {2.5f, 1.3f},
+        {3.5f, 2.3f},
+        {4.5f, 3.3f},
+        {5.5f, 4.3f},
+    };
+
+    Containers::Array<char> indexData{4*sizeof(UnsignedInt)};
+    Containers::ArrayView<UnsignedInt> indices = Containers::arrayCast<UnsignedInt>(indexData);
+    Utility::copy({60, 21, 72, 93}, indices);
+
+    Trade::MeshData out = generateIndices(Trade::MeshData{meshPrimitiveWrap(0xcaca),
+        Utility::move(indexData), Trade::MeshIndexData{indices},
+        {}, positions, {
+            Trade::MeshAttributeData{Trade::MeshAttribute::Position,
+                Containers::arrayView(positions)},
+        }});
+    CORRADE_COMPARE(out.primitive(), meshPrimitiveWrap(0xcaca));
+    CORRADE_VERIFY(out.isIndexed());
+    CORRADE_COMPARE(out.indexType(), MeshIndexType::UnsignedInt);
+    CORRADE_COMPARE_AS(out.indices<UnsignedInt>(), Containers::arrayView<UnsignedInt>({
+        60, 21, 72, 93
+    }), TestSuite::Compare::Container);
+
+    /* The index data should be moved, not copied */
+    CORRADE_COMPARE(out.indexData().data(), static_cast<void*>(indices.data()));
+
+    CORRADE_COMPARE(out.attributeCount(), 1);
+    CORRADE_COMPARE_AS(out.attribute<Vector2>(Trade::MeshAttribute::Position),
+        Containers::arrayView<Vector2>({
+            {1.5f, 0.3f}, {2.5f, 1.3f}, {3.5f, 2.3f}, {4.5f, 3.3f}, {5.5f, 4.3f}
+        }), TestSuite::Compare::Container);
+}
+
+void GenerateIndicesTest::generateIndicesMeshDataTrivialIndexedMoveDifferentIndexType() {
+    const Vector2 positions[]{
+        {1.5f, 0.3f},
+        {2.5f, 1.3f},
+        {3.5f, 2.3f},
+        {4.5f, 3.3f},
+        {5.5f, 4.3f},
+    };
+
+    Containers::Array<char> indexData{4*sizeof(UnsignedShort)};
+    Containers::ArrayView<UnsignedShort> indices = Containers::arrayCast<UnsignedShort>(indexData);
+    Utility::copy({60, 21, 72, 93}, indices);
+
+    Trade::MeshData out = generateIndices(Trade::MeshData{MeshPrimitive::Points,
+        Utility::move(indexData), Trade::MeshIndexData{indices},
+        {}, positions, {
+            Trade::MeshAttributeData{Trade::MeshAttribute::Position,
+                Containers::arrayView(positions)},
+        }});
+    CORRADE_COMPARE(out.primitive(), MeshPrimitive::Points);
+    CORRADE_VERIFY(out.isIndexed());
+    CORRADE_COMPARE(out.indexType(), MeshIndexType::UnsignedInt);
+    CORRADE_COMPARE_AS(out.indices<UnsignedInt>(), Containers::arrayView<UnsignedInt>({
+        60, 21, 72, 93
+    }), TestSuite::Compare::Container);
+
+    /* In this case it has to be copied because the type is different */
+    CORRADE_VERIFY(out.indexData().data() != static_cast<void*>(indices.data()));
+
+    CORRADE_COMPARE(out.attributeCount(), 1);
+    CORRADE_COMPARE_AS(out.attribute<Vector2>(Trade::MeshAttribute::Position),
+        Containers::arrayView<Vector2>({
+            {1.5f, 0.3f}, {2.5f, 1.3f}, {3.5f, 2.3f}, {4.5f, 3.3f}, {5.5f, 4.3f}
+        }), TestSuite::Compare::Container);
 }
 
 void GenerateIndicesTest::generateIndicesMeshDataInvalidVertexCount() {