[osg-users] FFmpegImageStream thread safety

OpenSceneGraph Users osg-users at lists.openscenegraph.org
Tue Aug 11 08:21:01 PDT 2020

Sorry, I don't know off hand.  I didn't work much on the code myself.


On 8/10/20 11:38 PM, Ben Meijering wrote:
> Hi Rob,
> Thanks for the link.
> In this version I also see that m_pImgStream->setImage is invoked 
> in FFmpegRenderThread::run(), another thread.
> 2015 is a long time ago, but do you perhaps remember why this is safe ?
> Thanks in advance.
> Cheers,
> Ben
> Op ma 10 aug. 2020 om 23:03 schreef Rob Spearman 
> <rob at digitaliseducation.com <mailto:rob at digitaliseducation.com>>:
>     We made changes back in 2015 to fix threading issues with the osg
>     ffmpeg plugin.  I don't think these changes ever made it into OSG.
>     Source repo: https://bitbucket.org/digitalis/osg-ffmpeg-plugin/
>     On Monday, August 10, 2020 at 3:46:32 AM UTC-7 bjs.me... at gmail.com
>     <mailto:bjs.me... at gmail.com> wrote:
>         Hi Robert,
>         Thanks for your response.
>         If FFmpegImageStream::publishNewFrame is invoked from a
>         separate thread.
>         Is it then somehow guaranteed that (for example)
>         Texture2D::apply is not using/reading from the image object,
>         while FFmpegImageStream::publishNewFrame is modifying it ?
>         Cheers,
>         Ben
>         Op vr 31 jul. 2020 om 21:24 schreef Robert Osfield
>         <robert.... at gmail.com>:
>             Hi Ben,
>             I'm away from my dev computer so just commenting off the
>             top of my. FFmpegImageStream should be double buffered so
>             that the ffmpeg thread writing to the image will be
>             writing to one part of the data, while the other half is
>             available to be read by the graphics thread.  This should
>             avoid threading issues.
>             The threading used by the ffmpeg plugin is separate from
>             the viewer threading so settings related to the viewer
>             won't be important, nor will general settings of the scene
>             graph.
>             You should be able to just add the ffmpeg genearted
>             imagestream to a texture, start the steam going and then
>             largely forget about it.
>             Robert.
>             On Friday, 31 July 2020 at 20:15:45 UTC+1
>             bjs.me... at gmail.com wrote:
>                 Hi,
>                 I was just looking at FFmpegImageStream, I am not very
>                 familiar with this code, but I have some questions.
>                 It is not immediately clear to me how
>                 FFmpegImageStream::publishNewFrame is thread safe.
>                 It seems like the image data is set (setImage) from
>                 the video decoder thread.
>                 The image then uses a pointer to one of the buffers of
>                 the video decoder (of which the contents might also
>                 change ?).
>                 FFmpegImageStream also doesn't seem to override
>                 requiresUpdateCall, which I believe will result in the
>                 texture not being dynamic (Texture2D::setImage).
>                 Which could be used, for example in
>                 StateSet::computeDataVariance(), to determine whether
>                 the StateSet should be dynamic (which is needed for
>                 multithreaded rendering).
>                 If anyone could shed more light on this subject, it
>                 would be very much appreciated.
>                 Thank you.
>                 Cheers,
>                 Ben

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