diff --git a/src/AVDemuxThread.cpp b/src/AVDemuxThread.cpp
index 0f4bb77de..61b98e07c 100644
--- a/src/AVDemuxThread.cpp
+++ b/src/AVDemuxThread.cpp
@@ -128,6 +128,7 @@ void AVDemuxThread::seek(qint64 pos)
     }
     if (video_thread) {
         video_thread->packetQueue()->clear();
+        // TODO: the first frame (key frame) will not be decoded correctly if flush() is called.
         video_thread->packetQueue()->put(Packet());
     }
     //if (subtitle_thread) {
diff --git a/src/AVDemuxer.cpp b/src/AVDemuxer.cpp
index 958e22bc6..4b900c841 100644
--- a/src/AVDemuxer.cpp
+++ b/src/AVDemuxer.cpp
@@ -424,7 +424,6 @@ bool AVDemuxer::seek(qint64 pos)
         return false;
     }
     //replay
-    qDebug("startTime: %lld", startTime());
     if (upos <= startTime()) {
         qDebug("************seek to beginning. started = false");
         started_ = false;
diff --git a/src/QtAV/VideoFrameExtractor.h b/src/QtAV/VideoFrameExtractor.h
index 1e91b416c..642d59189 100644
--- a/src/QtAV/VideoFrameExtractor.h
+++ b/src/QtAV/VideoFrameExtractor.h
@@ -41,14 +41,18 @@ class Q_AV_EXPORT VideoFrameExtractor : public QObject
     explicit VideoFrameExtractor(QObject *parent = 0);
     void setSource(const QString value);
     QString source() const;
+    /*!
+     * \brief setAsync
+     * Extract video frames in another thread. Default is true.
+     */
     void setAsync(bool value);
     bool async() const;
     void setAutoExtract(bool value);
     bool autoExtract() const;
     /*!
      * \brief setPrecision
-     * if the difference of the next requested position is less than the value, the
-     * last one is used and not positionChanged() signal to emit.
+     * if the difference between the next requested position is less than the value, previous
+     * one is used and no positionChanged() and frameExtracted() signals to emit.
      * Default is 500ms.
      */
     void setPrecision(int value);
@@ -76,8 +80,8 @@ public slots:
     /*!
      * \brief extract
      * If last extracted frame can be use, use it.
-     * If there is a key frame in [position-precision, position+precision], then the nearest key frame will be extracted.
-     * Otherwise, the given position frame will be extracted.
+     * If there is a key frame in [position-precision, position+precision], the nearest key frame
+     * before position+precision will be extracted. Otherwise, the given position frame will be extracted.
      */
     void extract();
 private slots:
diff --git a/src/VideoFrameExtractor.cpp b/src/VideoFrameExtractor.cpp
index 027472a47..13a121355 100644
--- a/src/VideoFrameExtractor.cpp
+++ b/src/VideoFrameExtractor.cpp
@@ -133,7 +133,7 @@ class VideoFrameExtractorPrivate : public DPtrPrivate<VideoFrameExtractor>
                 continue;
             }
             QVariantHash opt, va;
-            va["Display"] = "X11"; // to support swscale
+            va["display"] = "X11"; // to support swscale
             opt["vaapi"] = va;
             decoder->setOptions(opt);
             break;
@@ -158,7 +158,7 @@ class VideoFrameExtractorPrivate : public DPtrPrivate<VideoFrameExtractor>
             if (demuxer.packet()->hasKeyFrame)
                 break;
         }
-        decoder->flush();
+        // decoder->flush(); // TODO: why key frame can't be decoded? also in VideoThread
         const qint64 t_key = qint64(demuxer.packet()->pts * 1000.0);
         //qDebug("delta t = %d, data size: %d", int(value - t_key), demuxer.packet()->data.size());
         // must decode key frame
@@ -183,7 +183,7 @@ class VideoFrameExtractorPrivate : public DPtrPrivate<VideoFrameExtractor>
         qreal t0 = qreal(value/1000LL);
         while (!demuxer.atEnd()) {
             if (!demuxer.readFrame()) {
-                qDebug("!!!!!!----read frame error!!!!!!");
+                //qDebug("!!!!!!----read frame error!!!!!!");
                 continue;
             }
             if (demuxer.stream() != vstream) {