[vtkusers] Asynchronous outputs

[David Gobbi]

Hi Tim,

A VTK source cannot serve pipeline requests from more than one thread.

In order for a vtkVideoSourceRecorder to work, the video would have to 
feed through it to the rest of the pipeline, and internally, it would 
have to be constructed so that pipeline requests coming from its output 
don't propagate to its input.  Then its input can run in one thread, and 
its output in another.  It is certainly possible to write such a class, 
but it would be a little, ah, unorthodox.  It would keep the 
vtkVideoSource from having to directly serve requests from two threads, 
though.

A long time ago I started a project where I was separating the 
vtkVideoSource class into a vtkVideoSource and a vtkVideoBuffer, which 
would expose a public interface including locking functions so that 
frames stored in the buffer could be accessed from different threads.  
Unfortunately I never completed the project and the code is pretty stale 
by now (it dates back to VTK 3.2).

What I used to do instead of continually writing out the frames was just 
set the video buffer to be huge and use a computer with lots of RAM.  
That way I could just write out the frames after I was done.  Each gig 
of memory will hold 3000 frames at 640x480, that may or may not be 
enough for what you want to do.

 - David



Tim Soper wrote:
> Hi-
>    I am still  trying to get a follow-up to a question I had regarding how to best access a continually updating output.  I have made a video frame grabber class based on the vtkVideoSource class but I want to add functionality for saving individual frames to file at a pretty fast rate.  Currently I've just added code to the InternalGrab( ) function that saves each frame after grabbing it. It would be nice to do this asnychronously though. My idea was to make a vtkVideoSourceRecorder that would continously save frames from the associated video source inside a separate thread.  What I'd like to do is something like this:
>
> videoSource = vtkVideoSource::New(); //create the source
> videoRecorder = vtkVideoSourceRecorder::New();  //create the recorder
>
> videoRecorder->SetInputConnection( videoSource->GetOutputPort() ); //connect the inputs/outputs
>
> videoSource->Record( ); //start grabbing frames
> videoRecorder->BeginRecord(); //start saving frames to file.
>
> This would allow me to save and and display at separate rates. As well, it seems more practical to separate these two functions into different classes. After trying to implement this though, the program chokes and freezes. It seems to be grabbing the output when it's busy grabbing another frame or something. Is there a more correct way to do this?
>  
> Tim Soper
>
> Research Assistant
> University of Washington
> Department of Bioengineering 
> Box 352142
>
>
>
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