Accurate frame period reporting in oat-framserve wcam

- Controlling frame rate of webcams is not worth it.
- Instead, I have choosen to send the measured frame period along with
  each frame

src/frameserver/FileReader.cpp

@@ -46,8 +46,6 @@ FileReader::FileReader(const std::string &image_sink_address,
 
 void FileReader::connectToNode() {
 
-    // TODO: bind without using example frame from video stream. See PGGigECam.cpp
-    // for example
     cv::Mat example_frame;
     file_reader_ >> example_frame;
 

src/frameserver/WebCam.cpp

@@ -53,9 +53,6 @@ void WebCam::connectToNode() {
 
     shared_frame_ = frame_sink_.retrieve(
             example_frame.rows, example_frame.cols, example_frame.type());
-
-    // TODO: this does not appear to be very accurate/work
-    shared_frame_.sample().set_rate_hz(cv_camera_->get(CV_CAP_PROP_FPS));
 }
 
 bool WebCam::serveFrame() {
@@ -81,7 +78,12 @@ bool WebCam::serveFrame() {
     }
 
     // Increment sample count
-    shared_frame_.sample().incrementCount();
+    if (shared_frame_.sample().count() == 0)
+        start_ = clock_.now();
+
+    auto period = 
+        std::chrono::duration_cast<Sample::Microseconds>(clock_.now() - start_);
+    shared_frame_.sample().incrementCount(period);
 
     // Tell sources there is new data
     frame_sink_.post();

src/frameserver/WebCam.h

@@ -51,6 +51,10 @@ class WebCam : public FrameServer {
     // The webcam object
     int64_t index_;
     std::unique_ptr<cv::VideoCapture> cv_camera_;
+
+    // frame generation clock
+    std::chrono::steady_clock clock_;
+    std::chrono::steady_clock::time_point start_;
 };
 
 }      /* namespace oat */