SceneGraph: added Object::setParentKeepTransformation().

src/SceneGraph/Object.h

@@ -205,9 +205,20 @@ template<class Transformation> class MAGNUM_SCENEGRAPH_EXPORT Object: public Abs
         /**
          * @brief Set parent object
          * @return Pointer to self (for method chaining)
+         *
+         * @see setParentKeepTransformation()
          */
         Object<Transformation>* setParent(Object<Transformation>* parent);
 
+        /**
+         * @brief Set parent object and keep absolute transformation
+         * @return Pointer to self (for method chaining)
+         *
+         * While setParent() preserves only relative transformation of the
+         * object, this funcition preserves absolute transformation.
+         */
+        Object<Transformation>* setParentKeepTransformation(Object<Transformation>* parent);
+
         /*@}*/
 
         /** @{ @name Object transformation */

src/SceneGraph/Object.hpp

@@ -87,6 +87,17 @@ template<class Transformation> Object<Transformation>* Object<Transformation>::s
     return this;
 }
 
+template<class Transformation> Object<Transformation>* Object<Transformation>::setParentKeepTransformation(Object<Transformation>* parent) {
+    CORRADE_ASSERT(scene() == parent->scene(), "SceneGraph::Object::setParentKeepTransformation(): both parents must be in the same scene", this);
+
+    const auto transformation = Transformation::compose(
+        Transformation::inverted(parent->absoluteTransformation()), absoluteTransformation());
+    setParent(parent);
+    this->setTransformation(transformation);
+
+    return this;
+}
+
 template<class Transformation> typename Transformation::DataType Object<Transformation>::absoluteTransformation() const {
     if(!parent()) return Transformation::transformation();
     return Transformation::compose(parent()->absoluteTransformation(), Transformation::transformation());

src/SceneGraph/Test/ObjectTest.cpp

@@ -36,6 +36,7 @@ class ObjectTest: public Corrade::TestSuite::Tester {
 
         void parenting();
         void scene();
+        void setParentKeepTransformation();
         void absoluteTransformation();
         void transformations();
         void transformationsRelative();
@@ -66,6 +67,7 @@ class CachingObject: public Object3D, AbstractFeature<3> {
 ObjectTest::ObjectTest() {
     addTests({&ObjectTest::parenting,
               &ObjectTest::scene,
+              &ObjectTest::setParentKeepTransformation,
               &ObjectTest::absoluteTransformation,
               &ObjectTest::transformations,
               &ObjectTest::transformationsRelative,
@@ -120,6 +122,31 @@ void ObjectTest::scene() {
     CORRADE_VERIFY(childOfOrphan->scene() == nullptr);
 }
 
+void ObjectTest::setParentKeepTransformation() {
+    Object3D root;
+    root.rotateZ(Deg(35.0f));
+
+    Object3D* childOne = new Object3D(&root);
+    Object3D* childTwo = new Object3D(&root);
+
+    childOne->translate(Vector3::xAxis(2.0f));
+    childTwo->rotateY(Deg(90.0f));
+
+    /* Old parent and new parent must share the same scene */
+    std::ostringstream o;
+    Error::setOutput(&o);
+    Scene3D scene;
+    childOne->setParentKeepTransformation(&scene);
+    CORRADE_COMPARE(o.str(), "SceneGraph::Object::setParentKeepTransformation(): both parents must be in the same scene\n");
+    CORRADE_COMPARE(childOne->parent(), &root);
+
+    /* Reparent to another and keep absolute transformation */
+    auto transformation = childOne->absoluteTransformation();
+    childOne->setParentKeepTransformation(childTwo);
+    CORRADE_VERIFY(childOne->parent() == childTwo);
+    CORRADE_COMPARE(childOne->absoluteTransformation(), transformation);
+}
+
 void ObjectTest::absoluteTransformation() {
     Scene3D s;