MeshTools: add a primitiveCount() utility.

I need it in magnum-player.

doc/changelog.dox

@@ -151,6 +151,8 @@ See also:
     into per-vertex
 -   New @ref MeshTools::concatenate() and @ref MeshTools::concatenateInto()
     tool for batching multiple generic meshes together
+-   @ref MeshTools::primitiveCount() for calculating number of generated
+    primitives for a particular primitive type
 -   New family of @ref MeshTools::generateLineStripIndices(),
     @ref MeshTools::generateLineLoopIndices(),
     @ref MeshTools::generateTriangleStripIndices() and

src/Magnum/MeshTools/GenerateIndices.cpp

@@ -33,6 +33,29 @@
 
 namespace Magnum { namespace MeshTools {
 
+UnsignedInt primitiveCount(const MeshPrimitive primitive, const UnsignedInt elementCount) {
+    if(primitive == MeshPrimitive::Points ||
+       primitive == MeshPrimitive::Edges ||
+       primitive == MeshPrimitive::Faces ||
+       primitive == MeshPrimitive::Instances)
+        return elementCount;
+    if(primitive == MeshPrimitive::Lines)
+        return elementCount/2;
+    if(primitive == MeshPrimitive::LineStrip)
+        return elementCount < 1 ? 0 : elementCount - 1;
+    if(primitive == MeshPrimitive::LineLoop)
+        /* For a single element it'll define a degenerate line, which
+           technically still *is* a primitive */
+        return elementCount;
+    if(primitive == MeshPrimitive::Triangles)
+        return elementCount/3;
+    if(primitive == MeshPrimitive::TriangleStrip ||
+       primitive == MeshPrimitive::TriangleFan)
+        return elementCount < 2 ? 0 : elementCount - 2;
+    CORRADE_ASSERT(false,
+        "MeshTools::primitiveCount(): invalid primitive" << primitive, {});
+}
+
 void generateLineStripIndicesInto(const UnsignedInt vertexCount, const Containers::StridedArrayView1D<UnsignedInt>& indices) {
     CORRADE_ASSERT(vertexCount >= 2,
         "MeshTools::generateLineStripIndicesInto(): expected at least two vertices, got" << vertexCount, );

src/Magnum/MeshTools/GenerateIndices.h

@@ -26,7 +26,7 @@
 */
 
 /** @file
- * @brief Function @ref Magnum::MeshTools::generateLineStripIndices(), @ref Magnum::MeshTools::generateLineStripIndicesInto(), @ref Magnum::MeshTools::generateLineLoopIndices(), @ref Magnum::MeshTools::generateLineLoopIndicesInto(), @ref Magnum::MeshTools::generateTriangleStripIndices(), @ref Magnum::MeshTools::generateTriangleStripIndicesInto(), @ref Magnum::MeshTools::generateTriangleFanIndices(), @ref Magnum::MeshTools::generateTriangleFanIndicesInto(), @ref Magnum::MeshTools::generateIndices()
+ * @brief Function @ref Magnum::MeshTools::primitiveCount(), @ref Magnum::MeshTools::generateLineStripIndices(), @ref Magnum::MeshTools::generateLineStripIndicesInto(), @ref Magnum::MeshTools::generateLineLoopIndices(), @ref Magnum::MeshTools::generateLineLoopIndicesInto(), @ref Magnum::MeshTools::generateTriangleStripIndices(), @ref Magnum::MeshTools::generateTriangleStripIndicesInto(), @ref Magnum::MeshTools::generateTriangleFanIndices(), @ref Magnum::MeshTools::generateTriangleFanIndicesInto(), @ref Magnum::MeshTools::generateIndices()
  * @m_since_latest
  */
 
@@ -36,6 +36,18 @@
 
 namespace Magnum { namespace MeshTools {
 
+/**
+@brief Actual primitive count for given primitive type and element count
+@m_since_latest
+
+Returns how many primitives is generated for given @p primitive and
+@p elementCount, for example for @ref MeshPrimitive::Triangles returns
+@cpp elementCount/3 @ce. Expects that @p primitive is valid, return value is
+rounded down if there's not enough elements for given primitive type (so for
+14 vertices you get just 4 triangles, for example).
+*/
+MAGNUM_MESHTOOLS_EXPORT UnsignedInt primitiveCount(MeshPrimitive primitive, UnsignedInt elementCount);
+
 /**
 @brief Create index buffer for a line strip primitive
 @m_since_latest

src/Magnum/MeshTools/Test/GenerateIndicesTest.cpp

@@ -39,6 +39,9 @@ namespace Magnum { namespace MeshTools { namespace Test { namespace {
 struct GenerateIndicesTest: TestSuite::Tester {
     explicit GenerateIndicesTest();
 
+    void primitiveCount();
+    void primitiveCountInvalidPrimitive();
+
     void generateLineStripIndices();
     void generateLineStripIndicesWrongVertexCount();
     void generateLineStripIndicesIntoWrongSize();
@@ -91,7 +94,10 @@ const struct {
 };
 
 GenerateIndicesTest::GenerateIndicesTest() {
-    addTests({&GenerateIndicesTest::generateLineStripIndices,
+    addTests({&GenerateIndicesTest::primitiveCount,
+              &GenerateIndicesTest::primitiveCountInvalidPrimitive,
+
+              &GenerateIndicesTest::generateLineStripIndices,
               &GenerateIndicesTest::generateLineStripIndicesWrongVertexCount,
               &GenerateIndicesTest::generateLineStripIndicesIntoWrongSize,
 
@@ -115,6 +121,41 @@ GenerateIndicesTest::GenerateIndicesTest() {
               &GenerateIndicesTest::generateIndicesMeshDataInvalidPrimitive});
 }
 
+void GenerateIndicesTest::primitiveCount() {
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::Points, 42), 42);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::Instances, 13), 13);
+
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::Lines, 4), 2);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::Lines, 5), 2);
+
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::LineStrip, 1), 0);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::LineStrip, 2), 1);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::LineStrip, 4), 3);
+
+    /* This is a degenerate line, which technically still is a primitive */
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::LineLoop, 1), 1);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::LineLoop, 2), 2);
+
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::Triangles, 2), 0);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::Triangles, 3), 1);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::Triangles, 6), 2);
+
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::TriangleStrip, 2), 0);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::TriangleFan, 2), 0);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::TriangleStrip, 3), 1);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::TriangleFan, 3), 1);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::TriangleStrip, 7), 5);
+    CORRADE_COMPARE(MeshTools::primitiveCount(MeshPrimitive::TriangleFan, 7), 5);
+}
+
+void GenerateIndicesTest::primitiveCountInvalidPrimitive() {
+    std::ostringstream out;
+    Error redirectError{&out};
+    MeshTools::primitiveCount(MeshPrimitive(0xdead), 2);
+    CORRADE_COMPARE(out.str(),
+        "MeshTools::primitiveCount(): invalid primitive MeshPrimitive(0xdead)\n");
+}
+
 void GenerateIndicesTest::generateLineStripIndices() {
     /* Minimal input */
     CORRADE_COMPARE_AS(MeshTools::generateLineStripIndices(2),