MeshTools: assert in compile() if non-contiguous index buffers are used.

src/Magnum/MeshTools/Compile.cpp

@@ -160,6 +160,9 @@ GL::Mesh compileInternal(const Trade::MeshData& meshData, GL::Buffer&& indices,
     }
 
     if(meshData.isIndexed()) {
+        CORRADE_ASSERT(Short(meshIndexTypeSize(meshData.indexType())) == meshData.indexStride(),
+            "MeshTools::compile():" << meshData.indexType() << "with stride of" << meshData.indexStride() << "bytes isn't supported by OpenGL", GL::Mesh{});
+
         mesh.setIndexBuffer(std::move(indices), meshData.indexOffset(), meshData.indexType())
             .setCount(meshData.indexCount());
     } else mesh.setCount(meshData.vertexCount());

src/Magnum/MeshTools/Compile.h

@@ -120,6 +120,9 @@ possibly also an index buffer, if the mesh is indexed.
 -   Implementation-specific @ref Magnum::MeshPrimitive values are passed
     as-is with @ref meshPrimitiveUnwrap(). It's the user responsibility to
     ensure an implementation-specific value is valid in this context.
+-   The index buffer is expected to be contiguous (size of the index type equal
+    to @ref Trade::MeshData::indexStride()). OpenGL doesn't support interleaved
+    index buffers.
 -   Stride of all attributes is expected to be positive. OpenGL doesn't support
     zero and negative strides.
 

src/Magnum/MeshTools/Test/CompileGLTest.cpp

@@ -104,6 +104,8 @@ struct CompileGLTest: GL::OpenGLTester {
         void multipleAttributes();
         void packedAttributes();
 
+        void unsupportedIndexStride();
+
         void customAttribute();
         void unsupportedAttribute();
         void unsupportedAttributeStride();
@@ -256,6 +258,8 @@ CompileGLTest::CompileGLTest() {
         &CompileGLTest::renderSetup,
         &CompileGLTest::renderTeardown);
 
+    addTests({&CompileGLTest::unsupportedIndexStride});
+
     addInstancedTests({&CompileGLTest::customAttribute,
                        &CompileGLTest::unsupportedAttribute},
         Containers::arraySize(CustomAttributeWarningData));
@@ -1153,6 +1157,23 @@ void CompileGLTest::packedAttributes() {
     #endif
 }
 
+void CompileGLTest::unsupportedIndexStride() {
+    #ifdef CORRADE_NO_ASSERT
+    CORRADE_SKIP("CORRADE_NO_ASSERT defined, can't test assertions");
+    #endif
+
+    UnsignedShort indices[2]{};
+    Trade::MeshData data{MeshPrimitive::Points,
+        {}, indices, Trade::MeshIndexData{Containers::stridedArrayView(indices).every(2)},
+        1};
+
+    std::ostringstream out;
+    Error redirectError{&out};
+    compile(data);
+    CORRADE_COMPARE(out.str(),
+        "MeshTools::compile(): MeshIndexType::UnsignedShort with stride of 4 bytes isn't supported by OpenGL\n");
+}
+
 void CompileGLTest::customAttribute() {
     auto&& instanceData = CustomAttributeWarningData[testCaseInstanceId()];
     setTestCaseDescription(instanceData.name);