[Cin] Multitrack performance issues

[Georgy Salnikov]

On Sun, 28 Jul 2024, Stefan de Konink wrote:

> > Not sure that a software should always pretend to be smarter than the user.
> > Such a software could become dangerous in some cases...
>
> The point remains. Cinerella does composition. In order to do so, it
> must know which content to place where. Something that is masked by a
> top layer cannot be seen. This is the basic of composition.
>
> So technically;
>   - for every property change compute if the layer fully overlaps the
> layers below it
>   - this should basically prevent decoding of any further tracks

Stephan, I meant the following peculiarity.

As soon as the number of video tracks is >1, and the color mode contains
alpha channel, CGG has to assume some kind of composition.

If the top track has 100% opacity, and its content has 100% alpha everywhere
throughout the whole project duration, then no doubt that CGG could (perhaps
should) automatically disable any processing for all the tracks below that
one. However the user could exactly easily do the same, disable playing that
tracks by pressing a single button in the patchbay, knowing that they are
unused in his project anyway.

But if neither track is disabled, CGG can not assume that there is no need
to play any bottom track simple because the top track has 100% opacity under
some selected point in timeline. Because under some further point in
timeline track's opacity can fall below 100%, or there can appear some mute
region (with no video) in that track, or its alpha channel can become <100%
(the latter being possible to be detected only after having played that
complete track).

The things can be yet worse. The program technically cannot start playing
video from any frame in the timeline, it can start only from so called key
frames from which further frames are computed according to motion vectors.
It means, either CGG has to assume that opacity (or alpha) can become <100%
somewhere later in timeline and somehow pre-decode in advance that tracks
which are not visible just now but can become visible somewhen later. Or CGG
has to preprocess changed parts of video tracks after each editing operation
and remember where bottom tracks can be ignored, where they cannot. In both
cases a comparable performance loss can be estimated.

It may be so that the present approach, to play everything which is not
explicitly disabled, might be not the most worse one...

I personally am not an expert in video decoding/compositing, perhaps someone
who is more experiences in this field could comment more.

Georgy
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Georgy Salnikov
NMR Group
Novosibirsk Institute of Organic Chemistry
Lavrentjeva, 9, 630090 Novosibirsk, Russia
Phone   +7-383-3307864
Email   sge at nmr.nioch.nsc.ru
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