Platform: add MouseMoveEvent::relativePosition() to Glfw and Emscripten.

These toolkits don't support it, but I got really annoyed and so the
apps are emulating it.

doc/changelog.dox

@@ -375,6 +375,11 @@ See also:
     @ref Platform::GlfwApplication::exit() now have an optional parameter to
     specify the actual application return code (see
     [mosra/magnum#332](https://github.com/mosra/magnum/pull/332))
+-   @ref Platform::GlfwApplication and @ref Platform::EmscriptenApplication now
+    implement @ref Platform::GlfwApplication::MouseMoveEvent::relativePosition()
+    as well for better compatibility with @ref Platform::Sdl2Application. The
+    relative position is not supplied by the underlying toolkits, so it's
+    emulated on the application side.
 -   Extended @ref Platform::BasicScreen with
     @ref Platform::BasicScreen::mouseScrollEvent() "mouseScrollEvent()",
     @ref Platform::BasicScreen::textInputEvent() "textInputEvent()" and

src/Magnum/Platform/EmscriptenApplication.cpp

@@ -470,7 +470,14 @@ void EmscriptenApplication::setupCallbacks(bool resizable) {
 
     emscripten_set_mousemove_callback("#canvas", this, false,
         ([](int, const EmscriptenMouseEvent* event, void* userData) -> Int {
-            MouseMoveEvent e{*event};
+            auto& app = *static_cast<EmscriptenApplication*>(userData);
+            /* Avoid bogus offset at first -- report 0 when the event is called
+               for the first time */
+            Vector2i position{Int(event->canvasX), Int(event->canvasY)};
+            MouseMoveEvent e{*event,
+                app._previousMouseMovePosition == Vector2i{-1} ? Vector2i{} :
+                position - app._previousMouseMovePosition};
+            app._previousMouseMovePosition = position;
             static_cast<EmscriptenApplication*>(userData)->mouseMoveEvent(e);
             return e.isAccepted();
         }));

src/Magnum/Platform/EmscriptenApplication.h

@@ -639,7 +639,7 @@ class EmscriptenApplication {
         void setupAnimationFrame(bool ForceAnimationFrame);
 
         Vector2 _devicePixelRatio, _dpiScaling;
-        Vector2i _lastKnownCanvasSize;
+        Vector2i _lastKnownCanvasSize, _previousMouseMovePosition{-1};
 
         Flags _flags;
 
@@ -1251,6 +1251,16 @@ class EmscriptenApplication::MouseMoveEvent: public EmscriptenApplication::Input
         /** @brief Position */
         Vector2i position() const;
 
+        /**
+         * @brief Relative position
+         *
+         * Position relative to previous move event. Unlike
+         * @ref Sdl2Application, HTML APIs don't provide relative position
+         * directly, so this is calculated explicitly as a delta from previous
+         * move event position.
+         */
+        Vector2i relativePosition() const { return _relativePosition; }
+
         /** @brief Mouse buttons */
         Buttons buttons() const;
 
@@ -1263,9 +1273,10 @@ class EmscriptenApplication::MouseMoveEvent: public EmscriptenApplication::Input
     private:
         friend EmscriptenApplication;
 
-        explicit MouseMoveEvent(const EmscriptenMouseEvent& event): _event(event) {}
+        explicit MouseMoveEvent(const EmscriptenMouseEvent& event, const Vector2i& relativePosition): _event(event), _relativePosition{relativePosition} {}
 
         const EmscriptenMouseEvent& _event;
+        const Vector2i _relativePosition;
 };
 
 CORRADE_ENUMSET_OPERATORS(EmscriptenApplication::MouseMoveEvent::Buttons)

src/Magnum/Platform/GlfwApplication.cpp

@@ -517,8 +517,15 @@ void GlfwApplication::setupCallbacks() {
             app.mouseReleaseEvent(e);
     });
     glfwSetCursorPosCallback(_window, [](GLFWwindow* const window, const double x, const double y) {
-        MouseMoveEvent e{window, Vector2i{Int(x), Int(y)}};
-        static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window))->mouseMoveEvent(e);
+        auto& app = *static_cast<GlfwApplication*>(glfwGetWindowUserPointer(window));
+        /* Avoid bogus offset at first -- report 0 when the event is called for
+           the first time */
+        Vector2i position{Int(x), Int(y)};
+        MouseMoveEvent e{window, position,
+            app._previousMouseMovePosition == Vector2i{-1} ? Vector2i{} :
+            position - app._previousMouseMovePosition};
+        app._previousMouseMovePosition = position;
+        app.mouseMoveEvent(e);
     });
     glfwSetScrollCallback(_window, [](GLFWwindow* window, double xoffset, double yoffset) {
         MouseScrollEvent e(window, Vector2{Float(xoffset), Float(yoffset)});

src/Magnum/Platform/GlfwApplication.h

@@ -590,8 +590,8 @@ class GlfwApplication {
         #endif
         int _exitCode = 0;
 
-        Vector2i _minWindowSize;
-        Vector2i _maxWindowSize;
+        Vector2i _minWindowSize, _maxWindowSize;
+        Vector2i _previousMouseMovePosition{-1};
 };
 
 #ifdef MAGNUM_TARGET_GL
@@ -1590,6 +1590,16 @@ class GlfwApplication::MouseMoveEvent: public GlfwApplication::InputEvent {
         /** @brief Position */
         Vector2i position() const { return _position; }
 
+        /**
+         * @brief Relative position
+         *
+         * Position relative to previous move event. Unlike
+         * @ref Sdl2Application, GLFW doesn't provide relative position
+         * directly, so this is calculated explicitly as a delta from previous
+         * move event position.
+         */
+        Vector2i relativePosition() const { return _relativePosition; }
+
         /**
          * @brief Modifiers
          *
@@ -1600,10 +1610,10 @@ class GlfwApplication::MouseMoveEvent: public GlfwApplication::InputEvent {
     private:
         friend GlfwApplication;
 
-        explicit MouseMoveEvent(GLFWwindow* window, const Vector2i& position): _window{window}, _position{position} {}
+        explicit MouseMoveEvent(GLFWwindow* window, const Vector2i& position, const Vector2i& relativePosition): _window{window}, _position{position}, _relativePosition{relativePosition} {}
 
         GLFWwindow* const _window;
-        const Vector2i _position;
+        const Vector2i _position, _relativePosition;
         Containers::Optional<Buttons> _buttons;
         Containers::Optional<Modifiers> _modifiers;
 };

src/Magnum/Platform/Sdl2Application.h

@@ -2191,7 +2191,7 @@ class Sdl2Application::MouseMoveEvent: public Sdl2Application::InputEvent {
         /**
          * @brief Relative position
          *
-         * Position relative to previous event.
+         * Position relative to previous move event.
          */
         Vector2i relativePosition() const { return _relativePosition; }
 

src/Magnum/Platform/Test/EmscriptenApplicationTest.cpp

@@ -80,7 +80,7 @@ struct EmscriptenApplicationTest: Platform::Application {
     }
 
     void mouseMoveEvent(MouseMoveEvent& event) override {
-        Debug{} << "mouse move event:" << event.position() << Int(event.buttons());
+        Debug{} << "mouse move event:" << event.position() << event.relativePosition() << Int(event.buttons());
     }
 
     void mouseScrollEvent(MouseScrollEvent& event) override {

src/Magnum/Platform/Test/GlfwApplicationTest.cpp

@@ -60,6 +60,10 @@ struct GlfwApplicationTest: Platform::Application {
         }
     }
 
+    void mouseMoveEvent(MouseMoveEvent& event) override {
+        Debug{} << "mouse move event:" << event.position() << event.relativePosition() << UnsignedInt(event.buttons());
+    }
+
     void textInputEvent(TextInputEvent& event) override {
         Debug{} << "text input event:" << std::string{event.text(), event.text().size()};
     }

src/Magnum/Platform/Test/Sdl2ApplicationTest.cpp

@@ -63,7 +63,7 @@ struct Sdl2ApplicationTest: Platform::Application {
     }
 
     void mouseMoveEvent(MouseMoveEvent& event) override {
-        Debug{} << "mouse move event:" << event.position() << Uint32(event.buttons());
+        Debug{} << "mouse move event:" << event.position() << event.relativePosition() << Uint32(event.buttons());
     }
 
     void mouseScrollEvent(MouseScrollEvent& event) override {