VideoTexture: reactivate VideoTexture for scons/cmake/makefile compilation systems, fix video streaming, fix camera support in Linux, add multi-thread cache service, fix crash when a VideoFFmpeg object could not be created.

The multi-thread cache service is activated only on multi-core processors. It consists in loading, decoding and caching the video frames in a separate thread. The cache size is 5 decoded frames and 30 raw frames. Note that the opening of video file/stream/camera is not multi-thread: you will still experience a delay at the VideoFFmpeg object creation. Processing of the video frame (resize, loading to texture) is still done in the main thread. Caching is automatically enabled for video file, video streaming and video camera. Video streaming now works correctly: the videos frames are loaded at the correct rate. Network delays and frequency drifts are automatically compensated. Note: an http video source is always treated as a streaming source, even though the http protocol allows seeking. For the user it means that he cannot define start/stop range and cannot restart the video except by reopening the source. Pause/play is however possible. Video camera is now correctly handled on Linux: it will not slow down the BGE. A video camera is treated as a streaming source.
2009-03-05 15:16:43 +00:00
parent 7b197fcfdd
commit ab8e9ba3dd
8 changed files with 508 additions and 99 deletions
--- a/source/gameengine/VideoTexture/VideoFFmpeg.cpp
+++ b/source/gameengine/VideoTexture/VideoFFmpeg.cpp
@@ -55,7 +55,8 @@ m_codec(NULL), m_formatCtx(NULL), m_codecCtx(NULL),
 m_frame(NULL), m_frameDeinterlaced(NULL), m_frameRGB(NULL), m_imgConvertCtx(NULL),
 m_deinterlace(false), m_preseek(0),	m_videoStream(-1), m_baseFrameRate(25.0),
 m_lastFrame(-1),  m_eof(false), m_curPosition(-1), m_startTime(0), 
-m_captWidth(0), m_captHeight(0), m_captRate(0.f), m_isImage(false)
+m_captWidth(0), m_captHeight(0), m_captRate(0.f), m_isImage(false),
+m_isThreaded(false), m_stopThread(false), m_cacheStarted(false)
 {
 	// set video format
 	m_format = RGB24;
@@ -63,6 +64,12 @@ m_captWidth(0), m_captHeight(0), m_captRate(0.f), m_isImage(false)
 	setFlip(true);
 	// construction is OK
 	*hRslt = S_OK;
+	m_thread.first = m_thread.last = NULL;
+	pthread_mutex_init(&m_cacheMutex, NULL);
+	m_frameCacheFree.first = m_frameCacheFree.last = NULL;
+	m_frameCacheBase.first = m_frameCacheBase.last = NULL;
+	m_packetCacheFree.first = m_packetCacheFree.last = NULL;
+	m_packetCacheBase.first = m_packetCacheBase.last = NULL;
 }

 // destructor
@@ -75,6 +82,7 @@ VideoFFmpeg::~VideoFFmpeg ()
 bool VideoFFmpeg::release()
 {
 	// release
+	stopCache();
 	if (m_codecCtx)
 	{
 		avcodec_close(m_codecCtx);
@@ -112,6 +120,29 @@ bool VideoFFmpeg::release()
 	return true;
 }

+AVFrame	*VideoFFmpeg::allocFrameRGB()
+{
+	AVFrame *frame;
+	frame = avcodec_alloc_frame();
+	if (m_format == RGBA32)
+	{
+		avpicture_fill((AVPicture*)frame, 
+			(uint8_t*)MEM_callocN(avpicture_get_size(
+				PIX_FMT_RGBA,
+				m_codecCtx->width, m_codecCtx->height),
+				"ffmpeg rgba"),
+			PIX_FMT_RGBA, m_codecCtx->width, m_codecCtx->height);
+	} else 
+	{
+		avpicture_fill((AVPicture*)frame, 
+			(uint8_t*)MEM_callocN(avpicture_get_size(
+				PIX_FMT_RGB24,
+				m_codecCtx->width, m_codecCtx->height),
+				"ffmpeg rgb"),
+			PIX_FMT_RGB24, m_codecCtx->width, m_codecCtx->height);
+	}
+	return frame;
+}

 // set initial parameters
 void VideoFFmpeg::initParams (short width, short height, float rate, bool image)
@@ -122,6 +153,7 @@ void VideoFFmpeg::initParams (short width, short height, float rate, bool image)
 	m_isImage = image;
 }

+
 int VideoFFmpeg::openStream(const char *filename, AVInputFormat *inputFormat, AVFormatParameters *formatParams)
 {
 	AVFormatContext *formatCtx;
@@ -189,7 +221,6 @@ int VideoFFmpeg::openStream(const char *filename, AVInputFormat *inputFormat, AV
 	m_videoStream = videoStream;
 	m_frame = avcodec_alloc_frame();
 	m_frameDeinterlaced = avcodec_alloc_frame();
-	m_frameRGB = avcodec_alloc_frame();

 	// allocate buffer if deinterlacing is required
 	avpicture_fill((AVPicture*)m_frameDeinterlaced, 
@@ -207,12 +238,6 @@ int VideoFFmpeg::openStream(const char *filename, AVInputFormat *inputFormat, AV
 	{
 		// allocate buffer to store final decoded frame
 		m_format = RGBA32;
-		avpicture_fill((AVPicture*)m_frameRGB, 
-			(uint8_t*)MEM_callocN(avpicture_get_size(
-			PIX_FMT_RGBA,
-			m_codecCtx->width, m_codecCtx->height),
-			"ffmpeg rgba"),
-			PIX_FMT_RGBA, m_codecCtx->width, m_codecCtx->height);
 		// allocate sws context
 		m_imgConvertCtx = sws_getContext(
 			m_codecCtx->width,
@@ -227,12 +252,6 @@ int VideoFFmpeg::openStream(const char *filename, AVInputFormat *inputFormat, AV
 	{
 		// allocate buffer to store final decoded frame
 		m_format = RGB24;
-		avpicture_fill((AVPicture*)m_frameRGB, 
-			(uint8_t*)MEM_callocN(avpicture_get_size(
-			PIX_FMT_RGB24,
-			m_codecCtx->width, m_codecCtx->height),
-			"ffmpeg rgb"),
-			PIX_FMT_RGB24, m_codecCtx->width, m_codecCtx->height);
 		// allocate sws context
 		m_imgConvertCtx = sws_getContext(
 			m_codecCtx->width,
@@ -244,19 +263,247 @@ int VideoFFmpeg::openStream(const char *filename, AVInputFormat *inputFormat, AV
 			SWS_FAST_BILINEAR,
 			NULL, NULL, NULL);
 	}
+	m_frameRGB = allocFrameRGB();
+
 	if (!m_imgConvertCtx) {
 		avcodec_close(m_codecCtx);
+		m_codecCtx = NULL;
 		av_close_input_file(m_formatCtx);
+		m_formatCtx = NULL;
 		av_free(m_frame);
+		m_frame = NULL;
 		MEM_freeN(m_frameDeinterlaced->data[0]);
 		av_free(m_frameDeinterlaced);
+		m_frameDeinterlaced = NULL;
 		MEM_freeN(m_frameRGB->data[0]);
 		av_free(m_frameRGB);
+		m_frameRGB = NULL;
 		return -1;
 	}
 	return 0;
 }

+/*
+ * This thread is used to load video frame asynchronously.
+ * It provides a frame caching service. 
+ * The main thread is responsible for positionning the frame pointer in the
+ * file correctly before calling startCache() which starts this thread.
+ * The cache is organized in two layers: 1) a cache of 20-30 undecoded packets to keep
+ * memory and CPU low 2) a cache of 5 decoded frames. 
+ * If the main thread does not find the frame in the cache (because the video has restarted
+ * or because the GE is lagging), it stops the cache with StopCache() (this is a synchronous
+ * function: it sends a signal to stop the cache thread and wait for confirmation), then
+ * change the position in the stream and restarts the cache thread.
+ */
+void *VideoFFmpeg::cacheThread(void *data)
+{
+	VideoFFmpeg* video = (VideoFFmpeg*)data;
+	// holds the frame that is being decoded
+	CacheFrame *currentFrame = NULL;
+	CachePacket *cachePacket;
+	bool endOfFile = false;
+	int frameFinished = 0;
+
+	while (!video->m_stopThread)
+	{
+		// packet cache is used solely by this thread, no need to lock
+		// In case the stream/file contains other stream than the one we are looking for,
+		// allow a bit of cycling to get rid quickly of those frames
+		frameFinished = 0;
+		while (	   !endOfFile 
+				&& (cachePacket = (CachePacket *)video->m_packetCacheFree.first) != NULL 
+				&& frameFinished < 25)
+		{
+			// free packet => packet cache is not full yet, just read more
+			if (av_read_frame(video->m_formatCtx, &cachePacket->packet)>=0) 
+			{
+				if (cachePacket->packet.stream_index == video->m_videoStream)
+				{
+					// make sure fresh memory is allocated for the packet and move it to queue
+					av_dup_packet(&cachePacket->packet);
+					BLI_remlink(&video->m_packetCacheFree, cachePacket);
+					BLI_addtail(&video->m_packetCacheBase, cachePacket);
+					break;
+				} else {
+					// this is not a good packet for us, just leave it on free queue
+					// Note: here we could handle sound packet
+					av_free_packet(&cachePacket->packet);
+					frameFinished++;
+				}
+				
+			} else {
+				if (video->m_isFile)
+					// this mark the end of the file
+					endOfFile = true;
+				// if we cannot read a packet, no need to continue
+				break;
+			}
+		}
+		// frame cache is also used by main thread, lock
+		if (currentFrame == NULL) 
+		{
+			// no current frame being decoded, take free one
+			pthread_mutex_lock(&video->m_cacheMutex);
+			if ((currentFrame = (CacheFrame *)video->m_frameCacheFree.first) != NULL)
+				BLI_remlink(&video->m_frameCacheFree, currentFrame);
+			pthread_mutex_unlock(&video->m_cacheMutex);
+		}
+		if (currentFrame != NULL)
+		{
+			// this frame is out of free and busy queue, we can manipulate it without locking
+			frameFinished = 0;
+			while (!frameFinished && (cachePacket = (CachePacket *)video->m_packetCacheBase.first) != NULL)
+			{
+				BLI_remlink(&video->m_packetCacheBase, cachePacket);
+				// use m_frame because when caching, it is not used in main thread
+				// we can't use currentFrame directly because we need to convert to RGB first
+				avcodec_decode_video(video->m_codecCtx, 
+					video->m_frame, &frameFinished, 
+					cachePacket->packet.data, cachePacket->packet.size);
+				if(frameFinished) 
+				{
+					AVFrame * input = video->m_frame;
+
+					/* This means the data wasnt read properly, this check stops crashing */
+					if (   input->data[0]!=0 || input->data[1]!=0 
+						|| input->data[2]!=0 || input->data[3]!=0)
+					{
+						if (video->m_deinterlace) 
+						{
+							if (avpicture_deinterlace(
+								(AVPicture*) video->m_frameDeinterlaced,
+								(const AVPicture*) video->m_frame,
+								video->m_codecCtx->pix_fmt,
+								video->m_codecCtx->width,
+								video->m_codecCtx->height) >= 0)
+							{
+								input = video->m_frameDeinterlaced;
+							}
+						}
+						// convert to RGB24
+						sws_scale(video->m_imgConvertCtx,
+							input->data,
+							input->linesize,
+							0,
+							video->m_codecCtx->height,
+							currentFrame->frame->data,
+							currentFrame->frame->linesize);
+						// move frame to queue, this frame is necessarily the next one
+						currentFrame->framePosition = ++video->m_curPosition;
+						pthread_mutex_lock(&video->m_cacheMutex);
+						BLI_addtail(&video->m_frameCacheBase, currentFrame);
+						pthread_mutex_unlock(&video->m_cacheMutex);
+						currentFrame = NULL;
+					}
+				}
+				av_free_packet(&cachePacket->packet);
+				BLI_addtail(&video->m_packetCacheFree, cachePacket);
+			} 
+			if (currentFrame && endOfFile) 
+			{
+				// no more packet and end of file => put a special frame that indicates that
+				currentFrame->framePosition = -1;
+				pthread_mutex_lock(&video->m_cacheMutex);
+				BLI_addtail(&video->m_frameCacheBase, currentFrame);
+				pthread_mutex_unlock(&video->m_cacheMutex);
+				currentFrame = NULL;
+				// no need to stay any longer in this thread
+				break;
+			}
+		}
+		// small sleep to avoid unnecessary looping
+		PIL_sleep_ms(10);
+	}
+	// before quitting, put back the current frame to queue to allow freeing
+	if (currentFrame)
+	{
+		pthread_mutex_lock(&video->m_cacheMutex);
+		BLI_addtail(&video->m_frameCacheFree, currentFrame);
+		pthread_mutex_unlock(&video->m_cacheMutex);
+	}
+	return 0;
+}
+
+// start thread to cache video frame from file/capture/stream
+// this function should be called only when the position in the stream is set for the
+// first frame to cache
+bool VideoFFmpeg::startCache()
+{
+	if (!m_cacheStarted && m_isThreaded)
+	{
+		m_stopThread = false;
+		for (int i=0; i<CACHE_FRAME_SIZE; i++)
+		{
+			CacheFrame *frame = new CacheFrame();
+			frame->frame = allocFrameRGB();
+			BLI_addtail(&m_frameCacheFree, frame);
+		}
+		for (int i=0; i<CACHE_PACKET_SIZE; i++) 
+		{
+			CachePacket *packet = new CachePacket();
+			BLI_addtail(&m_packetCacheFree, packet);
+		}
+		BLI_init_threads(&m_thread, cacheThread, 1);
+		BLI_insert_thread(&m_thread, this);
+		m_cacheStarted = true;
+	}
+	return m_cacheStarted;
+}
+
+void VideoFFmpeg::stopCache()
+{
+	if (m_cacheStarted)
+	{
+		m_stopThread = true;
+		BLI_end_threads(&m_thread);
+		// now delete the cache
+		CacheFrame *frame;
+		CachePacket *packet;
+		while ((frame = (CacheFrame *)m_frameCacheBase.first) != NULL)
+		{
+			BLI_remlink(&m_frameCacheBase, frame);
+			MEM_freeN(frame->frame->data[0]);
+			av_free(frame->frame);
+			delete frame;
+		}
+		while ((frame = (CacheFrame *)m_frameCacheFree.first) != NULL)
+		{
+			BLI_remlink(&m_frameCacheFree, frame);
+			MEM_freeN(frame->frame->data[0]);
+			av_free(frame->frame);
+			delete frame;
+		}
+		while((packet = (CachePacket *)m_packetCacheBase.first) != NULL)
+		{
+			BLI_remlink(&m_packetCacheBase, packet);
+			av_free_packet(&packet->packet);
+			delete packet;
+		}
+		while((packet = (CachePacket *)m_packetCacheFree.first) != NULL)
+		{
+			BLI_remlink(&m_packetCacheFree, packet);
+			delete packet;
+		}
+		m_cacheStarted = false;
+	}
+}
+
+void VideoFFmpeg::releaseFrame(AVFrame* frame)
+{
+	if (frame == m_frameRGB)
+	{
+		// this is not a frame from the cache, ignore
+		return;
+	}
+	// this frame MUST be the first one of the queue
+	pthread_mutex_lock(&m_cacheMutex);
+	CacheFrame *cacheFrame = (CacheFrame *)m_frameCacheBase.first;
+	assert (cacheFrame != NULL && cacheFrame->frame == frame);
+	BLI_remlink(&m_frameCacheBase, cacheFrame);
+	BLI_addtail(&m_frameCacheFree, cacheFrame);
+	pthread_mutex_unlock(&m_cacheMutex);
+}
+
 // open video file
 void VideoFFmpeg::openFile (char * filename)
 {
@@ -280,8 +527,12 @@ void VideoFFmpeg::openFile (char * filename)
 	VideoBase::openFile(filename);

 	if (
+		// ffmpeg reports that http source are actually non stream
+		// but it is really not desirable to seek on http file, so force streaming.
+		// It would be good to find this information from the context but there are no simple indication
+		!strncmp(filename, "http://", 7) ||
 #ifdef FFMPEG_PB_IS_POINTER
-        m_formatCtx->pb && m_formatCtx->pb->is_streamed
+        (m_formatCtx->pb && m_formatCtx->pb->is_streamed)
 #else
        m_formatCtx->pb.is_streamed
 #endif
@@ -304,7 +555,13 @@ void VideoFFmpeg::openFile (char * filename)
 		m_avail = false;
 		play();
 	}
-
+	// check if we should do multi-threading?
+	if (!m_isImage && BLI_system_thread_count() > 1)
+	{
+		// never thread image: there are no frame to read ahead
+		// no need to thread if the system has a single core
+		m_isThreaded =  true;
+	}
 }


@@ -385,6 +642,12 @@ void VideoFFmpeg::openCam (char * file, short camIdx)
 	m_formatCtx->flags |= AVFMT_FLAG_NONBLOCK;
 	// open base class
 	VideoBase::openCam(file, camIdx);
+	// check if we should do multi-threading?
+	if (BLI_system_thread_count() > 1)
+	{
+		// no need to thread if the system has a single core
+		m_isThreaded =  true;
+	}
 }

 // play video
@@ -427,9 +690,12 @@ void VideoFFmpeg::setRange (double start, double stop)
 	try
 	{
 		// set range
-		VideoBase::setRange(start, stop);
-		// set range for video
-		setPositions();
+		if (m_isFile)
+		{
+			VideoBase::setRange(start, stop);
+			// set range for video
+			setPositions();
+		}
 	}
 	CATCH_EXCP;
 }
@@ -451,43 +717,61 @@ void VideoFFmpeg::calcImage (unsigned int texId)
 // load frame from video
 void VideoFFmpeg::loadFrame (void)
 {
-	// get actual time
-	double actTime = PIL_check_seconds_timer() - m_startTime;
-	// if video has ended
-	if (m_isFile && actTime * m_frameRate >= m_range[1])
-	{
-		// if repeats are set, decrease them
-		if (m_repeat > 0) 
-			--m_repeat;
-		// if video has to be replayed
-		if (m_repeat != 0)
-		{
-			// reset its position
-			actTime -= (m_range[1] - m_range[0]) / m_frameRate;
-			m_startTime += (m_range[1] - m_range[0]) / m_frameRate;
-		}
-		// if video has to be stopped, stop it
-		else 
-			m_status = SourceStopped;
-	}
-	// if video is playing
 	if (m_status == SourcePlaying)
 	{
+		// get actual time
+		double startTime = PIL_check_seconds_timer();
+		double actTime = startTime - m_startTime;
+		// if video has ended
+		if (m_isFile && actTime * m_frameRate >= m_range[1])
+		{
+			// in any case, this resets the cache
+			stopCache();
+			// if repeats are set, decrease them
+			if (m_repeat > 0) 
+				--m_repeat;
+			// if video has to be replayed
+			if (m_repeat != 0)
+			{
+				// reset its position
+				actTime -= (m_range[1] - m_range[0]) / m_frameRate;
+				m_startTime += (m_range[1] - m_range[0]) / m_frameRate;
+			}
+			// if video has to be stopped, stop it
+			else 
+			{
+				m_status = SourceStopped;
+				return;
+			}
+		}
 		// actual frame
-		long actFrame = m_isFile ? long(actTime * actFrameRate()) : m_lastFrame + 1;
+		long actFrame = (m_isImage) ? m_lastFrame+1 : long(actTime * actFrameRate());
 		// if actual frame differs from last frame
 		if (actFrame != m_lastFrame)
 		{
+			AVFrame* frame;
 			// get image
-			if(grabFrame(actFrame))
+			if((frame = grabFrame(actFrame)) != NULL)
 			{
-				AVFrame* frame = getFrame();
+				if (!m_isFile && !m_cacheStarted) 
+				{
+					// streaming without cache: detect synchronization problem
+					double execTime = PIL_check_seconds_timer() - startTime;
+					if (execTime > 0.005) 
+					{
+						// exec time is too long, it means that the function was blocking
+						// resynchronize the stream from this time
+						m_startTime += execTime;
+					}
+				}
 				// save actual frame
 				m_lastFrame = actFrame;
 				// init image, if needed
 				init(short(m_codecCtx->width), short(m_codecCtx->height));
 				// process image
 				process((BYTE*)(frame->data[0]));
+				// finished with the frame, release it so that cache can reuse it
+				releaseFrame(frame);
 				// in case it is an image, automatically stop reading it
 				if (m_isImage)
 				{
@@ -495,6 +779,12 @@ void VideoFFmpeg::loadFrame (void)
 					// close the file as we don't need it anymore
 					release();
 				}
+			} else if (!m_isFile)
+			{
+				// we didn't get a frame and we are streaming, this may be due to
+				// a delay in the network or because we are getting the frame too fast.
+				// In the later case, shift time by a small amount to compensate for a drift
+				m_startTime += 0.01;
 			}
 		}
 	}
@@ -507,77 +797,135 @@ void VideoFFmpeg::setPositions (void)
 	// set video start time
 	m_startTime = PIL_check_seconds_timer();
 	// if file is played and actual position is before end position
-	if (m_isFile && !m_eof && m_lastFrame >= 0 && m_lastFrame < m_range[1] * actFrameRate())
+	if (!m_eof && m_lastFrame >= 0 && (!m_isFile || m_lastFrame < m_range[1] * actFrameRate()))
 		// continue from actual position
 		m_startTime -= double(m_lastFrame) / actFrameRate();
-	else
+	else {
 		m_startTime -= m_range[0];
+		// start from begining, stop cache just in case
+		stopCache();
+	}
 }

 // position pointer in file, position in second
-bool VideoFFmpeg::grabFrame(long position)
+AVFrame *VideoFFmpeg::grabFrame(long position)
 {
 	AVPacket packet;
 	int frameFinished;
 	int posFound = 1;
 	bool frameLoaded = false;
 	long long targetTs = 0;
+	CacheFrame *frame;

-	// first check if the position that we are looking for is in the preseek range
-	// if so, just read the frame until we get there
-	if (position > m_curPosition + 1 
-		&& m_preseek 
-		&& position - (m_curPosition + 1) < m_preseek) 
+	if (m_cacheStarted)
 	{
-		while(av_read_frame(m_formatCtx, &packet)>=0) 
-		{
-			if (packet.stream_index == m_videoStream) 
+		// when cache is active, we must not read the file directly
+		do {
+			pthread_mutex_lock(&m_cacheMutex);
+			frame = (CacheFrame *)m_frameCacheBase.first;
+			pthread_mutex_unlock(&m_cacheMutex);
+			// no need to remove the frame from the queue: the cache thread does not touch the head, only the tail
+			if (frame == NULL)
 			{
-				avcodec_decode_video(
-					m_codecCtx, 
-					m_frame, &frameFinished, 
-					packet.data, packet.size);
-				if (frameFinished)
-					m_curPosition++;
+				// no frame in cache, in case of file it is an abnormal situation
+				if (m_isFile)
+				{
+					// go back to no threaded reading
+					stopCache();
+					break;
+				}
+				return NULL;
 			}
-			av_free_packet(&packet);
-			if (position == m_curPosition+1)
-				break;
-		}
-	}
-	// if the position is not in preseek, do a direct jump
-	if (position != m_curPosition + 1) 
-	{ 
-		double timeBase = av_q2d(m_formatCtx->streams[m_videoStream]->time_base);
-		long long pos = (long long)
-			((long long) (position - m_preseek) * AV_TIME_BASE / m_baseFrameRate);
-		long long startTs = m_formatCtx->streams[m_videoStream]->start_time;
-
-		if (pos < 0)
-			pos = 0;
-
-		if (startTs != AV_NOPTS_VALUE)
-			pos += (long long)(startTs * AV_TIME_BASE * timeBase);
-
-		if (position <= m_curPosition || !m_eof)
-		{
-			// no need to seek past the end of the file
-			if (av_seek_frame(m_formatCtx, -1, pos, AVSEEK_FLAG_BACKWARD) >= 0)
+			if (frame->framePosition == -1) 
 			{
-				// current position is now lost, guess a value. 
-				// It's not important because it will be set at this end of this function
-				m_curPosition = position - m_preseek - 1;
+				// this frame mark the end of the file (only used for file)
+				// leave in cache to make sure we don't miss it
+				m_eof = true;
+				return NULL;
+			}
+			// for streaming, always return the next frame, 
+			// that's what grabFrame does in non cache mode anyway.
+			if (!m_isFile || frame->framePosition == position)
+			{
+				return frame->frame;
+			}
+			// this frame is not useful, release it
+			pthread_mutex_lock(&m_cacheMutex);
+			BLI_remlink(&m_frameCacheBase, frame);
+			BLI_addtail(&m_frameCacheFree, frame);
+			pthread_mutex_unlock(&m_cacheMutex);
+		} while (true);
+	}
+	// come here when there is no cache or cache has been stopped
+	// locate the frame, by seeking if necessary (seeking is only possible for files)
+	if (m_isFile)
+	{
+		// first check if the position that we are looking for is in the preseek range
+		// if so, just read the frame until we get there
+		if (position > m_curPosition + 1 
+			&& m_preseek 
+			&& position - (m_curPosition + 1) < m_preseek) 
+		{
+			while(av_read_frame(m_formatCtx, &packet)>=0) 
+			{
+				if (packet.stream_index == m_videoStream) 
+				{
+					avcodec_decode_video(
+						m_codecCtx, 
+						m_frame, &frameFinished, 
+						packet.data, packet.size);
+					if (frameFinished)
+						m_curPosition++;
+				}
+				av_free_packet(&packet);
+				if (position == m_curPosition+1)
+					break;
 			}
 		}
-		// this is the timestamp of the frame we're looking for
-		targetTs = (long long)(((double) position) / m_baseFrameRate / timeBase);
-		if (startTs != AV_NOPTS_VALUE)
-			targetTs += startTs;
+		// if the position is not in preseek, do a direct jump
+		if (position != m_curPosition + 1) 
+		{ 
+			double timeBase = av_q2d(m_formatCtx->streams[m_videoStream]->time_base);
+			long long pos = (long long)
+				((long long) (position - m_preseek) * AV_TIME_BASE / m_baseFrameRate);
+			long long startTs = m_formatCtx->streams[m_videoStream]->start_time;

-		posFound = 0;
-		avcodec_flush_buffers(m_codecCtx);
+			if (pos < 0)
+				pos = 0;
+
+			if (startTs != AV_NOPTS_VALUE)
+				pos += (long long)(startTs * AV_TIME_BASE * timeBase);
+
+			if (position <= m_curPosition || !m_eof)
+			{
+				// no need to seek past the end of the file
+				if (av_seek_frame(m_formatCtx, -1, pos, AVSEEK_FLAG_BACKWARD) >= 0)
+				{
+					// current position is now lost, guess a value. 
+					// It's not important because it will be set at this end of this function
+					m_curPosition = position - m_preseek - 1;
+				}
+			}
+			// this is the timestamp of the frame we're looking for
+			targetTs = (long long)(((double) position) / m_baseFrameRate / timeBase);
+			if (startTs != AV_NOPTS_VALUE)
+				targetTs += startTs;
+
+			posFound = 0;
+			avcodec_flush_buffers(m_codecCtx);
+		}
+	} else if (m_isThreaded)
+	{
+		// cache is not started but threading is possible
+		// better not read the stream => make take some time, better start caching
+		if (startCache())
+			return NULL;
+		// Abnormal!!! could not start cache, fall back on direct read
+		m_isThreaded = false;
 	}

+	// find the correct frame, in case of streaming and no cache, it means just
+	// return the next frame. This is not quite correct, may need more work
 	while(av_read_frame(m_formatCtx, &packet)>=0) 
 	{
 		if(packet.stream_index == m_videoStream) 
@@ -632,10 +980,22 @@ bool VideoFFmpeg::grabFrame(long position)
 		}
 		av_free_packet(&packet);
 	}
-	m_eof = !frameLoaded;
+	m_eof = m_isFile && !frameLoaded;
 	if (frameLoaded)
+	{
 		m_curPosition = position;
-	return frameLoaded;
+		if (m_isThreaded)
+		{
+			// normal case for file: first locate, then start cache
+			if (!startCache())
+			{
+				// Abnormal!! could not start cache, return to non-cache mode
+				m_isThreaded = false;
+			}
+		}
+		return m_frameRGB;
+	}
+	return NULL;
 }