On Thu, Jul 14, 2016 at 12:30 PM Jean-Baptiste Mardelle <j...@kdenlive.org>
wrote:
> On Thursday, July 14, 2016 12:41:20 AM CEST, Dan Dennedy wrote:
> > Good question, JB :-)
> > The composite transition gives less image quality for alpha
> > blending due to subsampled chroma. This also introduces a
> > 2-pixel precision limitation, which is bad enough on its own but
> > even worse for animation. I was basically doing some
> > future-proofing for animation support and to let the user choose
> > a blending mode, which you can do now by selecting a track and
> > using Properties. It does support frame threaded parallel
> > processing, which can help; but currently Shotcut only exposes
> > that in non-realtime mode (including export). I have not been
> > happy with performance either, but still Shotcut only has one
> > track transition choice. So, I chose the one with best quality
> > and flexibility.
>
> Thanks for your answer Dan.
>
> I had a look at frei0r.cairoblend, and one problem is that it always
> performs an operation to pre-multiply the RGBA input from A frame and B
> frame, even when the B frame is completely opaque, leading to a large
> resource consumption.
>
> I made a quick test implementing a Qt blend transition in MLT, that turns
> images into QImage and uses a QPainter to composite.
>
> I was able to get a much better performance by checking if the top frame is
> completely opaque, and querying/processing bottom frame only if there is
> some transparency (which is not possible with frei0r transitions).
>
Keep in mind that not all blending modes require an alpha channel. Still,
it could use this as hint for blending modes where it does use alpha
channel.
>
> My first draft is in my git clone:
>
> https://github.com/j-b-m/mlt/commit/87943f5c7b084d57a8c5717126cc0e47462d5ae5
>
> I think it would be a useful addition for multitrack compositing. Any
> comments / ideas ?
>
> regards
> jb
>
>
I could add this check to the frei0r mixer.
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