[Cin] User Defined Blend Algebra

[Georgy Salnikov]

On Fri, 14 Feb 2025, Andrea paz wrote:

> I was thinking of adding the following note to the manual, in the
> chapter on overlays. Do you think it is correct or should it be made
> more precise?

Right! Overlays can be really confusing, unless you have 3D vision from
birth. Any additional note would not be superfluous.

> 1- The colors of the 2 tracks that will be blended. We often consider
> only these channels to guess the final result.
> 2- The Alpha channel of the 2 tracks that can interact with each other
> in ways that are intuitively (but not mathematically) unexpected.
> 3- The choice of Source track and destination track (top or bottom).
> 4- The presence of a third black background track (or even the color
> of the canvas, which in CinGG is black by default but whose color can
> be varied manually), which can interfere with viewing the blend in the
> Compositor and show unexpected results.

There exist even more factors.

If the footage material in a track has transparency by itself, it might be
modified by overlay formula and develop some color, hidden by transparency
before overlay was applied.

Overlays can be applied at two places: via the patchbay button and in the
Overlay plugin. In which sequence in the video processing pipeline plugins
are processed, is defined. But when are processed overlays from patchbay? I
am sorry, I don't know this.

There may be mask. The mask can be applied either after plugins, or before
them. The results of Overlay plugin depend on it.

There is fader control. To my feeling, fader is applied after all plugins
and overlays, but I am not 100% sure.

Effects from some other plugins can have transparent parts (Titler, for
example).

There is a strange behavior of transparency generated due to attached
effects, highly resembling a bug. So far we apply a workaround by placing an
additional track with opaque background on bottom. But I am still confusing
by the fact that without additional track this transparency effect is not
always the same. Sometimes really the background, as configured in
Settings->Preferences->Appearance->Composer BG Color, is visible through the
transparency, but sometimes the original image itself, I cannot yet find an
origin or pattern.

There are two buttons in the patchbay, Play track (can be switched off), and
Don't send to output (can be switched on). In both cases video from the
affected track disappears, but the background left can be different in both
cases.

> Ultimately, if we get a result that looks wrong to us, we need to look
> at the formula for that blend and calculate our mathematical result.
> Then compare our calculation with the result we see on the screen."

Yes. In principle, the user, if interested, could now attach Blend Algebra
in addition to Overlay plugin, switch one of them on, another off, play with
formula in Blend Algebra how the modifications affect the result, even print
and analyse the intermediate color values. And using some specially drawn
regular color patterns instead of a natural working footage.

_______________________________________________________________________________

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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