? about the Shift Channels effect

[Max34504507s18j]

Start position: https://i.imgur.com/lfcLwnR.png

Composition settings: WxH = 720x576

Both images are the same size = 720x576

The top image has three colored stripes and transparent borders around them.

I apply the Shift channels effect. AFAIU FullOn means (255 255 255).

This is why we get RYMg and transparent borders. Ok.

But.

Please, look at these settings: https://i.imgur.com/M02Wwsb.png

I set 'Take Alpha from' = FullOn and get the transparent borders filled with red.

Why ?

Another example: https://i.imgur.com/bYcyE5N.png
AFAIU If we set 'Take from Red' = Alpha, Alpha (in the case) means (255 255 255).
This is why we get (255 255 255). Correct ?
Ok, but why cyan borders ?
Why do we get the borders filled with cyan ?

I can't understand. Please, explain why this happens?

[ShiveringCactus]

You're taking Red from the Alpha channel.  You haven't included an image of just the top layer without the effect applied, so I'm having to go by your description.  The top image has a transparent border, meaning the alpha channel will only be "white" in the centre square.  The border will be black.  When you set Red to use the Alpha channel, that "turns" Red "off" for the border.  You then have Blue and Green channels as Full On, so the border is a combination of those two.  For the centre square you are combining all three colours and producing a white square.

[Max34504507s18j]

Thank you very much !
It is much clearer to me now.
This is the start position. The effect is disabled.
https://i.imgur.com/lfcLwnR.png

Please, look at the screenshot: https://i.imgur.com/zmcHZYg.png
Why do we get the borders filled with black ?
Green = FullOf = (0, 0, 0) i.e. black.
'Take Alpha from' = FullOn  i.e. opacity

AFAIU FullOn = (255, 255, 255)
I am expecting the borders filled with white because in AE, white defines opaque areas, and black defines transparent areas.
p.s.
It would be nice if someone could make a description of the options for this plugin and put them in the online reference manual. The description already available is very limited.
https://helpx.adobe.com/after-effects/using/channel-effects.html

[Rick Gerard]

It makes things a lot easier for the folks on the forum if you drag your screenshots to the reply field, use the Toolbar, or even copy and paste them so we can all see them along with the questions and answers. The Drag & drop here field or external links work for files that cannot be added to your post.

Your first image:

There is a background image and three colored bars. The left bar is 100% red, the middle is Yellow, and the right is Magenta. The Info Panel will give you the actual color values. Red is Red, Yellow is a mix of Red and Blue, and Majenta is a mix of Red and Blue. Here are the actual values from your screenshots (100% color equals 1 in 32-bit color):

Checking the effects settings, Alpha on, Red Full On, Green is Green, and Blue is Blue, tells me that the middle bar was originally Green. Adding 100% Red to Green gives you Yellow. That's how RGB works. You can research RGB color math to understand how subtractive color works fully. Subtract Red from Green, and you get Yellow. ShiveringCactus already shared a color chart with you.

Your second image:

Effects render from the top to the bottom of the list. That means that Alpha is full-on - which means no transparency, so the background is not visible. The middle bar is Black because it was originally Green, not Yellow, and Green is now off. The Red and Blue bars have switched places because the Channels have been switched. 

This should be fairly easy to follow. 

[Max34504507s18j]

Thank you for responding to my questions . Sorry, I showed an image with the wrong starting position. The correct one -  three strips: red, green, blue.

However, my questions are not about the mathematics of color, but about transparent borders. I received an answer to my first question from ShiveringCactus.

But my second question is about something else.

If Alpha is full-on - which means no transparency i.e  opacity, so the background, of course, is not visible. Bu why is the background shown in black when the manual says: in AE, white defines opaque areas, and black defines transparent areas ? AFAIU FullOn means 255, 255, 255 i.e. white. So, the background should be shown in white, in theory.

[Rick Gerard]

When Alpha is set to Full On, the value is 1, which is White, and that means no Transparency. Solo the layer, reveal the transparency grid, then try the Alpha Channel or set the Comp Panel to view the Alpha Channel. Full On means White, which equals no transparency. It's just as the manual says. You see black because there is no color to add to the background outside the three bars. If you set the Alpha to Full Off, the entire layer would disappear because Alpha is first on the list. There are no pixels for the other operators to work on because the Alpha turns all pixels below the control off before the RGB controls can modify them. 

[Max34504507s18j]

It seems to me that your assumption is wrong. 

You say "You see black because there is no color to add to the background outside the three bars. "

But please look at the examples in my first post. Same picture. The same transparent borders around the central part.  'Take Alpha from' = FullOn But the transparent frame is "filled" with color.

[Mylenium]

Shift Channels doesn't fill anything. It's how the effect is supposed to work. If you need to override a channel, use Set Channels or Channel Combiner.

Mylenium