Ability to retrieve absolute time in previous frame to Timeline.

The time is relative to moment when Timeline::start() was called.

src/Timeline.cpp

@@ -23,13 +23,15 @@ namespace Magnum {
 
 void Timeline::start() {
     running = true;
-    previousFrameTime = high_resolution_clock::now();
+    _startTime = high_resolution_clock::now();
+    _previousFrameTime = _startTime;
     _previousFrameDuration = 0;
 }
 
 void Timeline::stop() {
     running = false;
-    previousFrameTime = high_resolution_clock::time_point();
+    _startTime = high_resolution_clock::time_point();
+    _previousFrameTime = _startTime;
     _previousFrameDuration = 0;
 }
 
@@ -37,16 +39,20 @@ void Timeline::nextFrame() {
     if(!running) return;
 
     auto now = high_resolution_clock::now();
-    std::uint32_t duration = duration_cast<microseconds>(now-previousFrameTime).count();
+    std::uint32_t duration = duration_cast<microseconds>(now-_previousFrameTime).count();
     _previousFrameDuration = duration/1e6f;
 
     if(_previousFrameDuration < _minimalFrameTime) {
         Corrade::Utility::sleep(_minimalFrameTime*1000 - duration/1000);
         now = high_resolution_clock::now();
-        _previousFrameDuration = duration_cast<microseconds>(now-previousFrameTime).count()/1e6f;
+        _previousFrameDuration = duration_cast<microseconds>(now-_previousFrameTime).count()/1e6f;
     }
 
-    previousFrameTime = now;
+    _previousFrameTime = now;
+}
+
+GLfloat Timeline::previousFrameTime() const {
+    return duration_cast<microseconds>(_previousFrameTime-_startTime).count()/1e6f;
 }
 
 }

src/Timeline.h

@@ -98,7 +98,7 @@ class MAGNUM_EXPORT Timeline {
         /**
          * @brief Start timeline
          *
-         * Sets previous frame duration to `0`.
+         * Sets previous frame time and duration to `0`.
          * @see stop(), previousFrameDuration()
          */
         void start();
@@ -120,13 +120,26 @@ class MAGNUM_EXPORT Timeline {
          */
         void nextFrame();
 
-        /** @brief Duration of previous frame (in seconds) */
+        /**
+         * @brief Time at previous frame (in seconds)
+         *
+         * Returns time elapsed since start() was called. If the timeline is
+         * stopped, the function returns `0.0f`.
+         */
+        GLfloat previousFrameTime() const;
+
+        /**
+         * @brief Duration of previous frame (in seconds)
+         *
+         * If the timeline is stopped, the function returns `0.0f`.
+         */
         inline constexpr GLfloat previousFrameDuration() const {
             return _previousFrameDuration;
         }
 
     private:
-        std::chrono::high_resolution_clock::time_point previousFrameTime;
+        std::chrono::high_resolution_clock::time_point _startTime;
+        std::chrono::high_resolution_clock::time_point _previousFrameTime;
         GLfloat _minimalFrameTime;
         GLfloat _previousFrameDuration;