[Interest] [Qt3D] Render to video

[Andy]

On Mon, Apr 18, 2016 at 9:05 AM, Sean Harmer <sean.harmer at kdab.com> wrote:

> Hi,
>
> I think it's possible, although it's not an out of the box feature yet, as
> Harald pointed out.
>
> Have a look into this kind of approach:
>
> * Implement something similar to the Scene3DItem that allows to embed Qt 3D
> into a Qt Quick 2 scene
>

Thanks Sean.  I wouldn't have thought to go through Qt Quick since I've
never used it and it's not on my radar.


>
> * This requires using a little bit of private (for now) API to feed the Qt
> 3D
> renderer your own OpenGL context and to ask it to render when needed.
>
> * Use an FBO (as the Scene3D item does) as the render target - take care to
> tell the framegraph the size of this "external" render target.
>
> * Then, once the scene is rendered on your custom context, either:
>
> a) use glReadPixels to grab the contents of the texture you attached to the
> FBO and pass it to a software encoder.
>
> or
>
> b) use the texture id to pass the data to a hardware encoder such as nvenc
> without needing a round trip from the GPU->CPU->GPU.
>
> I think that can be made to work.
>

nvenc looks like it would be a big win for those with NVIDIA cards, and I
see AMD has the analogous VCE.


Thanks everyone for helping me get a better sense of the problem I'm trying
to solve!


>
> Cheers,
>
> Sean
>
> On Monday 18 April 2016 08:38:21 Andy wrote:
> > On Mon, Apr 18, 2016 at 3:26 AM, Harald Vistnes <
> harald.vistnes at gmail.com>
> >
> > wrote:
> > > Hi Andy,
> > >
> > > An alternative is to use the FFMPEG encoding library instead of the
> > > command line tools. Then you can pass each frame to the video encoder
> as
> > > you generate it without writing them all to disk first.
> >
> > This is non-GPL software, so I can't use the lib directly (I should have
> > mentioned that).  I haven't found a BSD or MIT lib that encodes the
> common
> > formats - is anyone aware of one?
> >
> > > The natural way to generate the QImages would be to render to an
> offscreen
> > > surface of the desired resolution, and then pass each frame to the
> video
> > > encoder.
> >
> > Great - so I'm on the right track!
> >
> > > Unfortunately, reading back the content of offscreen surfaces is
> missing
> > > in Qt3D. According to these it will not come before 5.8.
> > >
> > > https://bugreports.qt.io/browse/QTBUG-52136
> > > https://bugreports.qt.io/browse/QTBUG-52074
> > >
> > > So AFAIK we just have to wait with generating images and videos with
> Qt3D.
> >
> > Thanks Harald.  Your report is exactly what I'm looking for...  That's
> > unfortunate though - a real showstopper.
> >
> > > Cheers,
> > > Harald
> > >
> > > 2016-04-18 5:46 GMT+02:00 Andy <asmaloney at gmail.com>:
> > >> Goal: generate video with a user-specified resolution, frame rate, &
> > >> container/codec format from an animation in my Qt3D window
> > >>
> > >> (Disclaimer: I've never worked with video files before!)
> > >>
> > >> As far as I can tell, Qt doesn't provide a way to generate video files
> > >> directly, so I think I have to write a series of QImages to disk and
> use
> > >> them to generate a video using ffmpeg.  This seems like it will take a
> > >> large amount of disk space, be pretty heavy on the I/O, and generally
> be
> > >> slow.  Are there better solutions?
> > >>
> > >> If I need to do it that way though, I must generate QImages from my
> > >> existing Qt3DCore::QAspectEngine in my QWindow-derived class.  I don't
> > >> see
> > >> a clear/elegant way to do this.
> > >>
> > >> I think I need to create an offscreen surface? window? with the
> correct
> > >> resolution and then somehow render & animate my scene to it, saving
> > >> snapshots as I move the camera.  (I am already using
> QAbstractAnimation
> > >> to
> > >> move the camera, so I would use it to grab the snapshots as well.)
> Can I
> > >> use the same root entity in multiple QAspectEngines? (i.e.
> > >> setRootEntity()
> > >> to my root entity in the new offscreen and tell it to render.)
> > >>
> > >> Has anyone done this before?  Is this even close to the right
> approach?
> > >>
> > >> (I'm using straight C++ - no QML.)
> > >>
> > >> Thanks!
> > >>
> > >> ---
> > >> Andy Maloney  //  https://asmaloney.com
> > >> twitter ~ @asmaloney <https://twitter.com/asmaloney>
> > >>
> > >>
> > >> _______________________________________________
> > >> Interest mailing list
> > >> Interest at qt-project.org
> > >> http://lists.qt-project.org/mailman/listinfo/interest
> >
> > ---
> > Andy Maloney  //  https://asmaloney.com
> > twitter ~ @asmaloney <https://twitter.com/asmaloney>
>
> --
> Dr Sean Harmer | sean.harmer at kdab.com | Managing Director UK
> KDAB (UK) Ltd, a KDAB Group company
> Tel. +44 (0)1625 809908; Sweden (HQ) +46-563-540090
> Mobile: +44 (0)7545 140604
> KDAB - Qt Experts
>

---
Andy Maloney  //  https://asmaloney.com
twitter ~ @asmaloney <https://twitter.com/asmaloney>
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