Do small changes

src/device/device.cc

@@ -26,6 +26,7 @@
 #include "internal/motions.h"
 #include "internal/streams.h"
 #include "internal/strings.h"
+#include "internal/times.h"
 #include "internal/types.h"
 #include "uvc/uvc.h"
 
@@ -419,13 +420,18 @@ void Device::StartVideoStreaming() {
           static std::uint8_t drop_count = 1;
           if (drop_count > 0) {
             --drop_count;
+            continuation();
             return;
           }
+          // auto &&time_beg = times::now();
           if (streams_->PushStream(Capabilities::STEREO, data)) {
             CallbackPushedStreamData(Stream::LEFT);
             CallbackPushedStreamData(Stream::RIGHT);
           }
           continuation();
+          // VLOG(2) << "Stereo video callback cost "
+          //         << times::count<times::milliseconds>(times::now() - time_beg)
+          //         << " ms";
         });
   } else {
     LOG(FATAL) << "Not any stream capabilities are supported by this device";

src/internal/streams.cc

@@ -259,20 +259,26 @@ void Streams::AllocStreamData(
 void Streams::AllocStreamData(
     const Stream &stream, const StreamRequest &request, const Format &format) {
   stream_data_t data;
-  if (stream == Stream::LEFT || stream == Stream::RIGHT) {
-    data.img = std::make_shared<ImgData>();
-  } else {
-    data.img = nullptr;
-  }
+
   if (HasStreamDatas(stream)) {
     // If cached equal to limits_max, drop the oldest one.
     if (stream_datas_map_.at(stream).size() == GetStreamDataMaxSize(stream)) {
       auto &&datas = stream_datas_map_[stream];
-      data.frame = datas.front().frame;  // reuse this frame
+      // reuse the dropped data
+      data.img = datas.front().img;
+      data.frame = datas.front().frame;
       datas.erase(datas.begin());
       VLOG(2) << "Stream data of " << stream << " is dropped as out of limits";
     }
   }
+
+  if (stream == Stream::LEFT || stream == Stream::RIGHT) {
+    if(!data.img) {
+      data.img = std::make_shared<ImgData>();
+    }
+  } else {
+    data.img = nullptr;
+  }
   if (!data.frame) {
     data.frame = std::make_shared<frame_t>(
         request.width, request.height, format, nullptr);