I'm using a simple DirectShow graph to convert some videos to WMV format, which is working fine. I'm now trying to use a filter based on the Synth Filter sample to supply a silent audio track to the videos and I'm running into some problems.
Essentially, I don't know how to stop the graph when this filter (the synth filter) is connected. I guess because it just provides samples forever until somebody tells it to stop, the usual approach of calling IMediaEvent::WaitForCompletion on the filter graph doesn't work (the graph never stops). What I want it to do of course is stop as soon as the video source filter is finished.
I've tried tracking the position of the graph with IMediaSeeking::GetPositions and then manually stopping the graph when this exceeds the duration of the source file, but the accuracy of the stop time with this approach isn't great.
Can anyone think of a better way to do this? Do I need to have another filter that monitors the output from the video source and also has a pointer to the audio source so it can stop it as soon as the video source delivers EndOfStream? Is there no way to accomplish this from purely application-side code?
I've done something not too different myself in the past. I added support for IMediaSeeking to the silence generator filter, and then you need to make sure that you set start and stop times for the conversion (even if it's just 0 and duration), so that the silence generator can generate the right amount of audio and then send EOS.
G
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Normally seek commands are executed on a filter graph, get called on the renderers in the graph and calls are passed upstream by filters until a filter that can handle the seek does the actual seek operation.
Could an individual filter seek the upstream filters connected to one or more of its input pins in the same way without it affecting the downstream portion of the graph in unexpected ways? I wouldn't expect that there wouldn't be any graph state changes caused by calling IMediaSeeking.SetPositions upstream.
I'm assuming that all upstream filters are connected to the rest of the graph via this filter only.
Obviously the filter would need to be prepared to handle the resulting BeginFlush, EndFlush and NewSegment calls coming from upstream appropriately and distinguish samples that arrived before and after the seek operation. It would also need to set new sample times on its output samples so that the output samples had consistent sample presentation times. Any other issues?
It is perfectly feasible to do what you require. I used this approach to build video and audio mixer filters for a video editor. A full description of the code is available from the BBC White Papers 129 and 138 available from http://www.bbc.co.uk/rd
A rather ancient version of the code can be found on www.SourceForge.net if you search for AAFEditPack. The code is written in Delphi using DSPack to get access to the DirectShow headers. I did this because it makes it easier to handle com object lifetimes - by implementing smart pointers by default. It should be fairly straightforward to transfer the ideas to a C++ implementation if that is what you use.
The filters keep lists of the sub-graphs (a section of a graph but running in the same FilterGraph as the mixers). The filters implement a custom version of TBCPosPassThru which knows about the output pins of the sub-graph for each media clip. It handles passing on the seek commands to get each clip ready for replay when its point in the timeline is reached. The mixers handle the BeginFlush, EndFlush, NewSegment and EndOfStream calls for each sub-graph so they are kept happy. The editor uses only one FilterGraph that houses both video and audio graphs. Seeking commands are make by the graph on both the video and audio renderers and these commands are passed upstream to the mixers which implement them.
Sub-graphs that are not currently active are blocked by the mixer holding references to the samples they have delivered. This does not cause any problems for the FilterGraph because, as Roman R says, downstream filters only care about getting a consecutive stream of sample and do not know about what happens upstream.
Some key points you need to make sure of to avoid wasted debugging time are:
Your decoder filters need to be able to queue to the exact media frame or audio time. Not as easy to do as you might expect, especially with compressed formats such as mpeg2, which was designed for transmission and has no frame index in the files. If you do not do this, the filter may wait indefinitely to get a NewSegment call with the correct media times.
Your sub graphs need to present a NewSegment time equal to the value you asked for in your seek command before delivering samples. Some decoders may seek to the nearest key frame, which is a bit unhelpful and some are a bit arbitrary about the timings of their NewSegment and the following samples.
The start and stop times of each clip need to be within the duration of the file. Its probably not a good idea to police this in the DirectShow filter because you would probably want to construct a timeline without needing to run the filter first. I did this in the component that manages the FilterGraph.
If you want to add sections from the same source file consecutively in the timeline, and have effects that span the transition, you need to have two instances of the sub-graph for that file and if you have more than one transition for the same source file, your list needs to alternate the graphs for successive clips. This is because each sub graph should only play monotonically: calling lots of SetPosition calls would waste cpu cycles and would not work well with compressed files.
The filter's output pins define the entire seeking behaviour of the graph. The output sample time stamps (IMediaSample.SetTime) are implemented by the filter so you need to get them correct without any missing time stamps. and you can also set the MediaTime (IMediaSample.SetMediaTime) values if you like, although you have to be careful to get them correct or the graph may drop samples or stall.
Good luck with your development. If you need any more information please contact me through StackOverflow or DTSMedia.co.uk
I'm using Directshow SampleGrabber in callback mode to capture video frame from source file and do some processing. Also I would like to maintain the current playback rate of video and need to support both random, forward and backward seeking. For this I'm also doing some local buffering in a different thread.
I'm running graph with syn source set to NULL, so as to get maximum speed. However when I pause the graph after fixed amount of buffering. The SampleGrabber callback is getting called spuriously even when graph is paused. This is affecting my frame indexing and tracking. I want to resume the graph exactly from the same position at which it was paused. However if I run the graph with default clock it works fine but then my playback get affected. I want buffering thread to finish as soon as possible.
How can I make sure that callback is not called when graph is paused? Any thoughts or suggestion would be of great help.
Thanks in advance
Pradeep
Paused graph typically has all the same streaming internally (active state) with the exception that renderers are blocking streaming, esp. as soon as enough data is received for a preview banner. Since you removed clock from the graph, your renderer is likely to not block execution because it does not hold any clock to pause against. In your case this is the problem coming out of your intent to reuse the same graph for quick parsing through file and playback. Separate graph design looks having more chances to do better.
I have a DirectShow webcam application. I make use of Sample Grabber to get the buffer callbacks and IVideoWindow to control the display co-ordinates for the Preview. I have Preview and Capture Streams which I run as below.
g_pBuild->RenderStream(&PIN_CATEGORY_CAPTURE, &MEDIATYPE_Video,cam,g_pGrabberF,pNullRenderer2); g_pBuild->RenderStream(&PIN_CATEGORY_PREVIEW, &MEDIATYPE_Video,cam,NULL,NULL);
On certain On board cameras, IMediaControl::Run followed by IMediaControl::Stop followed by IMediaCOntrol::Run doesn't switch on the camera.
Extenal USB cameras work properly here. How can I diagnose more on this? Any pointers, please help.
Maybe its specific to a certain hardware issue in the unit.
Do a quick test by adding sleep of 1 sec between calls.
If it does help than you need to find a way to know when to unit state in idle or not.
There are two important parts of the question which you did not provide:
Filter graph topologies
HRESULTs of the method calls
A problem you might be having is that one of the filters in the topology does not handle well state transitions and fails somewhere between states. Supposedly your second Run meets it still trying to complete Stop. You might get a HRESULT there which indicates the issue (better for you) or the filter fails silently.
The filter graph's is the unlikely source of the bug itself. Chances are high that it does everything flawlessly, however since internally it distributes the calls between filters, one of the filter is letting you down.
I was using a callback mechanism to grab the webcam frames in my media application. It worked, but was slow due to certain additional buffer functions that were performed within the callback itself.
Now I am trying the other way to get frames. That is, call a method and grab the frame (instead of callback). I used a sample in CodeProject which makes use of IVMRWindowlessControl9::GetCurrentImage.
I encountered the following issues.
In a Microsoft webcam, the Preview didn't render (only black screen) on Windows 7. But the same camera rendered Preview on XP.
Here my doubt is, will the VMR specific functionalities be dependent on camera drivers on different platforms? Otherwise, how could this difference happen?
Wherever the sample application worked, I observed that the biBitCount member of the resulting BITMAPINFOHEADER structure is 32.
Is this a value set by application or a driver setting for VMR operations? How is this configured?
Finally, which is the best method to grab the webcam frames? A callback approach? Or a Direct approach?
Thanks in advance,
IVMRWindowlessControl9::GetCurrentImage is intended for occasional snapshots, not for regular image grabbing.
Quote from MSDN:
This method can be called at any time, no matter what state the filter
is in, whether running, stopped or paused. However, frequent calls to
this method will degrade video playback performance.
This methods reads back from video memory which is slow in first place. This methods does conversion (that is, slow again) to RGB color space because this format is most suitable for for non-streaming apps and gives less compatibility issues.
All in all, you can use it for periodic image grabbing, however this is not what you are supposed to do. To capture at streaming rate you need you use a filter in the pipeline, or Sample Grabber with callback.
I am attempting to use DirectShow to play two AVI files consecutively (one after the other) so that there is no interruption in the audio or video when the player transitions from one file to the next.
I have two custom controls on my form. Each one is pre-loaded with an AVI file, and before playback begins I set up all the DirectShow interfaces, set the video windows and resize them, call IMediaControl.Run(), then IMediaControl.Pause(), then IMediaSeeking.SetPositions to reset to frame 0, on both controls. On the form, you can see that both files are paused at their initial frames.
I then call IMediaControl.Run() on the first control, and wait for it to complete before calling Run() on the second control. Initially, I hooked into the first video's EC_COMPLETE notification message, and used this to start the second. Thinking that this event might be slow to arrive (turns out it is, but for a weird reason), I tried two other approaches:
Check the first video's current position inside a timer that goes off every second or so (using IMediaPosition.get_CurrentPosition). When the current position is within a second of the video's stop time (known in advance from IMediaPosition.get_StopTime), I go into a tight while loop and wait for the current position to equal the stop time, and then call Run() on the second video.
Same as the first, except I replace the while loop with a call to timeSetEvent from winmm.dll, with a delay set so that it fires right when the first file is supposed to end. I use the callback to Run() the second file.
Either of these two methods substantially cuts down the delay between the end of the first file and the beginning of the second, indicating that the EC_COMPLETE message doesn't arrive immediately after the file is complete (I also tried hooking the EC_SEGMENT_COMPLETE message, which is supposed to be used for looping within a file, but apparently nobody supports this - it never occurs on my machine, at least).
Doing all of the above has cut the transition delay from as much as a second, down to a barely perceptible glitch; about a third of the time the files transition with no interruption at all, which suggests there's no fundamental reason I can't get this to work all the time.
The slight delay is still unacceptable, unfortunately. I assume (and I could easily be wrong) that the remaining delay is due to a slight variable delay between the call to IMediaControl.Run() and when the video actually starts playing.
Does anybody know anything I can do to eliminate this little lag? It would also help to be told this is fundamentally impossible for whatever reason, which wouldn't surprise me. I've never encountered a video player in Windows that doesn't have this problem, so it may not be doable.
More info: the AVI files I'm playing are completely uncompressed (video and audio are uncompressed), so I don't think the lag is due to DirectShow's having to uncompress the video ahead of play start, although it may still buffer ahead as matter of course (and this may be the source of the problem). I would have though that starting play, pausing and then rewinding to the beginning would fix this.
Also, the way I'm handling the transition is to actually have the second control underneath the first; when the first completes playing, I start the second and then call BringToFront on it, creating the appearance of a single video transitioning between the two originals. I don't think the glitch is due to this, because it works perfectly some of the time, and even if this were problematic, it wouldn't explain the matching audio glitch.
Even more: I just tried starting the second video 30-50 milliseconds "early" and that seemed to eliminate even more of the gap, so I'm guessing that the lag in Run() is about that long. It appears to be variable, though, so this is still not where I need it to be.
Still more: perhaps I could eliminate this delay by loading the AVIs from memory rather than from a file. Unfortunately, I have no idea how to do this. IMediaControl only has a RenderFile() method, not something like a RenderStream or RenderMemory method.
If you call IMediaControl::Run on a stopped graph, the graph manager will post the call to a worker thread (so there's some variability). On the worker thread, the graph will be paused. Render filters only complete a pause transition once they have received data, so once GetState() returns S_OK, the graph manager knows that the graph is fully cued. At this point, it picks a time roughly 10ms into the future, and calls Run on each filter with that time as the start point. Since it takes time to tell each filter to Run, the dshow Run method has a parameter which is the refclock time at which a sample timestamped zero should be played -- i.e. the time at which the actual transition to run mode should take place.
To synchronise this with another graph, you first have to ensure that both graphs have the same clock. Query the graph (not the filter) for IMediaFilter, and call GetSyncSource on one graph and SetSyncSource on the other. Then you need to pause the second graph, so that it is cued and ready. When you want to start it, call IMediaFilter::Run instead of IMediaControl::Run, and you can pass your own start time. This still has to be a few milliseconds into the future, so the best thing might be to set the start time of the second graph to be the first graph's start time plus its duration (for an indexed container of uncompressed streams, the duration should be accurate).
Another approach is to use multiple graphs. Separating source from rendering would allow you to switch seamlessly between sources since they feed into a common render graph. There is sample source code for this approach at www.gdcl.co.uk/gmfbridge.
G