FFmpegImageStream thread safety

[Ben Meijering]

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

[Robert Osfield]

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.

[Ben Meijering]

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....@gmail.com>:

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[Rob Spearman]

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/

[Ben Meijering]

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 <r...@digitaliseducation.com>:

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[Robert Spearman]

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

Rob

[Ben Meijering]

Hi Rob,

Ok, thanks for your responses.

Cheers,

Ben

Op di 11 aug. 2020 om 17:21 schreef Robert Spearman <r...@digitaliseducation.com>:

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