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# pseudo-tech explanation of the effect named echo and it's operators?

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 pseudo-tech explanation of the effect named echo and it's operators? on Dec 24, 2014 at 8:33:29 pm

I am asking about the effect named echo and the echo operators and about the lesson taught in the COW tutorial about echo. I am not asking how do I achieve such and such visual result.

Following the COW tutorial about echo, I see no result. A difference is that I am using video while the tutorial is using a shape, I think.

Someone on the adobe forum says that there needs to be transparency for composite to work.

I want to understand why that is true and how the echo operators work.

The AE help says that add adds pixel values, blend averages values, screen "sandwiches" layers and composite in front causes successive images to appear in back.

rgb(1, 2, 3) + rgb(10, 12, 13) = rgb(1+10, 2+12, 3+13)?

Does average mean:

rgb(1, 2, 3) + rgb(10, 12, 13) = rgb((1+10)/2, (2+12)/2, (3+13)/2)?

What does "sandwich" mean?

How are images "composited" when you composite in front or back?

If someone wanted to use one of the additive operators, is there a formulaic way, i.e. without prior knowledge of the content being echoed, to make the result not appear jarringly brighter or darker?

 Re: pseudo-tech explanation of the effect named echo and it's operators?on Dec 25, 2014 at 1:03:10 am

i'm not sure which echo tutorial you are following, but this one addresses some of the issues you first mention:

https://library.creativecow.net/articles/rabinowitz_aharon/Echo_Time/video-...

in short, composite in back or in front can only work on keyed layers, the others are similar to ae's layer blending modes and will work on footage or other layers. add, screen and blend will make layers brighter. if you are applying to footage layers it is often helpful to set the echo time to the frame duration of the footage -- ie, 1/24 or 1/29.97, 1/59.94 etc.

if you want to blend frames but not effect the brightness as much as echo, cc time blend and cc wide time may be a better choice. wide time is pretty straight forward, but time blend isn't as intuitive...

time blend uses cached frames, which is very fast, but you have to express the number of frames to blend as a percentage of your cached frames. i don't have a way to calculate that, but higher values belnd more frames and lower values blend fewer frames. you'll also always want to ram preview from the first frame of the comp and after making any change to the settings you'll want to hit 'clear' at the top of the effect settings to purge the cached frames, otherwise it will blend your previously cached frames with the new settings, and things start to get wonky fast. also, before rendering, i'd choose edit>purge>all to avoid getting bad frames in the render.

Kevin Camp
Art Director
KCPQ, KZJO & KRCW

 Re: pseudo-tech explanation of the effect named echo and it's operators?on Dec 25, 2014 at 1:46:51 am

Yes, that is the tutorial I am talking about.

I figured out that you can use the echo composite operators with video that has no transparency by setting the "start intensity" parameter.

I think the explanation is hinted at by the Cycore FX manual:

And this definition of composite:

http://en.wikipedia.org/wiki/Digital_compositing

The cycore time effects rely on having access to rendered frames and so they require that input to be a composition.

Composite on that wikipedia page means deriving a value from pixel values including opacity.

The starting intensity parameter sets the opacity of frames that are being composited. So, setting it to less than 1.0 odds opacity that allows the echo to be perceptible.

I still don't know what the screen operator does.

And, I don't know how to adjust colors of additive echos to appear to have the same "brightness" without distorting the balance of colors that result from the echo. I would really like to know that. But, I hope I can learn that as a result of learning about color.