[Interest] [Qt3D] Render to video

Mon Apr 18 15:57:25 CEST 2016

On Monday 18 April 2016 15:46:14 Harald Vistnes wrote:
> If anyone try out Sean's suggestion and get it to work, it would be great
> it they share an example so we don't all have to reinvent the wheel :)

Or even better, contribute it to Qt 3D so everyone can benefit from it out of 
the box. :)

Cheers,

Sean

> 
> Harald
> 
> 2016-04-18 15:28 GMT+02:00 Andy <asmaloney at gmail.com>:
> > 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>

-- 
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