[Interest] [Qt3D] Render to video

Sean Harmer sean.harmer at kdab.com
Mon Apr 18 15:56:53 CEST 2016


On Monday 18 April 2016 09:28:00 Andy wrote:
> 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.

You don't necessarily have to go via Qt Quick, I just meant to use the Scene3D 
item as inspiration for tying it into your own render loop and having QT 3D 
render into an FBO.

Cheers,

Sean

> 
> > * 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>

-- 
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



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