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AE CS6 - 3D Compositing and Color Correction workflow

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Simon Luchini DucaAE CS6 - 3D Compositing and Color Correction workflow
by on Feb 9, 2013 at 9:55:43 am

AE CS6 - 3D Compositing and Color Correction workflow

Hello everybody,
I have some questions about work flow in AE concerning compositing and Color Correction. I saw many tuts about (f.e. Lynda.com Premiere Pro Color Correction and Enhancement... and tuts by Andrew Davis here in creative.cow).
Essentially, he have this CC work flow:

GRADING (primary and secondary)
Adjusting Tonality (Brightness and Contrast)
Adjusting Colors (White balance and fix color dominance)

MATCHING
Conforming colors of different clip shoted with different camera and lighting set up.

ENANCHING LOOK
Giving to all clips a unique "look"


But what does it happen if we have NOT just some clips but ALSO a 3D object (rendered in a 3d sw or, actually also a 3d object inserted by 3D Element by videocopilot)
or/and
a keyed clip from a green screen shot?

How does this footage (3d object and green screen keyed shot) transform
the aforementioned work flow?
We have to consider that this footage (3d object and green screen keyed shot) must to be pre-multiplied to make a right compositing but when you make CC it must be again "straight"... (NOT pre-multiplied)

Is really hard also understand how dale this work flow between AE and Pre Pro if you want to make CC in Pre Pro after compositing in AE.

I try to present a work flow for this (very common) situation:

I have a landscape clip as a background

a keyed clip (pre-multiplied) with a Character watching sky

and a 3d (pre-multiplied) object U.F.O. on sky.

When I have done compositing, how make I a right CC of all footage if I have two clip (3d object and green screen keyed shot) still pre-multiplied?

I think, but I'm not sure, that should be a good idea to send to Pre Pro a composited Comp with separate layers (by dynamic links) to have all single separate layers (Background, 3d object and green screen keyed shot.. but also the Color Warping...!). Than, in Pre Pro you should make a CC (Grading) just for a BG and than try to Match (Matching) this color corrected BG with 3d obj and keyed shoot. But we have a problem at this point cause our 3d object and green screen keyed shot are still pre-multiplied, and before make matching we have to re- multiplied theme, but how? And if we re pre-multiplied them, compositing will be also right?
Or maybe, before exporting comp from AE to Pre Pro, he have to create both layers for 3d object and green screen keyed shot, one pre-multiplied and one "Stright"? But, again, how to combine them to have a right compositing and a a right CC?

Anybody can help me to understand or maybe suggest any tuts to dale this common issue in AE- Pre Pro?
Many, many thanks!


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Todd KoprivaRe: AE CS6 - 3D Compositing and Color Correction workflow
by on Feb 10, 2013 at 1:49:53 am

You seem to be overcomplicating things. Straight versus premultiplied channels aren't relevant to your color correction and grading questions.

The color matting is removed to interpret the alpha channel when you import the footage. After that, any color correction that you do to a layer based on that footage item doesn't have anything to do with whether the channels were straight or premultiplied for the source item.

You color-correct your individual items to make them match as part of the compositing process in After Effects. Then, if you want to do a color grade of the shot in Premiere Pro along with other shots, you'd be working on the single rendered/flattened composite, so the details of the original elements that went into the composite are irrelevant.

---------------------------------------------------------------------------------------------------------
Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
---------------------------------------------------------------------------------------------------------


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Simon Luchini DucaRe: AE CS6 - 3D Compositing and Color Correction workflow
by on Feb 10, 2013 at 7:42:25 am

Hi Todd,
can you please explain better what you mean when you say: "The color matting is removed to interpret the alpha channel when you import the footage"?

If we are agree that the basic compositing formula is
Foreground * Alpha + Background * (1- Alpha)
that means we have ALWAYS premultiply Foreground (3d object):

Foreground R * Alpha
Foreground G * Alpha
Foreground B * Alpha

to make a right compositing.
When we import in AE a premulty 3d object to composite it, I don't know exactly what happen automatically but if you interpret this footage you 'll see that AE say you that it is premulty.
So you can make a right compositing. But when you have composited, you have to turn this 3d object in Straight to have the right (pure) R G B channel to make Color Correction, cause now, you have still a R*alpha, G*alpha, B*alpha color channel...
Just now, at this step I don't know how to dale my 3d object (still premulty):
maybe I have to duplicate it and interpret as Straight and make a match between Bg and this Straight 3d obj.
But I've composed with a 3d premulty object and I don't know at end how to unify compositing 3d (with premulty 3d object) and Grading with (With straight 3d object). That's my problem.


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Todd KoprivaRe: AE CS6 - 3D Compositing and Color Correction workflow
by on Feb 10, 2013 at 3:34:13 pm

When After Effects imports an item with an alpha channel, you can tell it whether the color channels are straight or are premultiplied with the alpha channel. If premultiplied, then the colors are unmultiplied from the alpha channel on import as part of the footage interpretation process, before any effects are processed.

If you've got objects that use straight and some that use premultiplied, just make sure that you've set the right footage interpretation for each. After that, you don't need to think about this at all, and it is not relevant to any color or compositing work.

---------------------------------------------------------------------------------------------------------
Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
---------------------------------------------------------------------------------------------------------


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Simon Luchini DucaRe: AE CS6 - 3D Compositing and Color Correction workflow
by on Feb 11, 2013 at 7:51:26 am

Wait, wait, : you're saying that when I import a premulti 3d obj and interpet it as a Premultiplied footage, AE automatically dales alpha channel leaving tham as premulti when I composite (R*Alpha, G*Alpha, B*Alpha) , and trasforms them in a unpremultiplied (R,G,B) when I'm going to grading?

Is it right?

Many thanks!


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