VFX color correct with matte

I have a movie with tons of car sequences done in green screen...

This is what I've done:

- I graded the foreground elements.
- I graded the plates.
- I did some basic compositing of both to be replaced later by the final vfx.

From the VFX team I'm getting a comped dpx in with the same color properties as my originals and an animated matte. Each image is composed of two images (background + foreground).

So this is what I'm doing:

- I copy the grade of my foreground original and add a matte node to each node correction then invert the node key input... So far so good.
- I create a compound node just to clean things up.
- I append the correction from the background and add the matte to each node.

This is what's happening:

The background is reacting different to the original correction. And if I do it just by itself it matches by itself. It's like if I had to choose, to use a matte for the foreground or the background but not for each.

I'm able to compensate for the difference with he color controls but I'm trying to keep the process as automatic as possible.

Also one weird thing happens. As soon as I close the session and open it again, my first node which is a color space transform gets reseted... My guess is this something related with the beta version 15

Beyond some bugs in R/Fu15 you're experiencing, its a good post to share why you should NEVER work on a creative pre-grade for VFX. Grades are very likely to change. They should only be used for display/review purposes only.

A typical vfx color workflow would be

* Footage is neutral/conform graded (find a hero shot in sequence, match all to that using OCIO tools, makes cg lighting easier and more consistent)
* VFX plates are brought into Fusion and OCIO .CC files Pregrades are applied
* Composite, add cg render whatever in linear space using a target display LUT (ie something that matches your intended target)
* For final export, you'd want to REVERSE the OCIO .CC pregrades.

For VFX, you REALLY want to inoculate yourself from any external variables. Grades are one of them.

Never work with pre-graded material unless you know 100% that its not going to change. But even still, I'd always argue that you need to work on source material. Davinci forinstance truncates color conversions because its using LUTs that don't have high enogh resolution. Nuke and Fusion does not suffer this (unless you're using the same luts and OCIO lut files) because they tend to implement the exact transfer functions mathematically instead of just a look up table.

Maybe not related to actual problem, but it is usually better to prepare fg and bg graded versions without any mattes and then mix the two variants using the matte. This prevents some funky stuff on semitransparent areas of matte where none of the grades has full effect. It also removes the need to use matte for all nodes and nodegraph will be much cleaner.

Beyond some bugs in R/Fu15 you're experiencing, its a good post to share why you should NEVER work on a creative pre-grade for VFX. Grades are very likely to change. They should only be used for display/review purposes only.

A typical vfx color workflow would be

* Footage is neutral/conform graded (find a hero shot in sequence, match all to that using OCIO tools, makes cg lighting easier and more consistent)
* VFX plates are brought into Fusion and OCIO .CC files Pregrades are applied
* Composite, add cg render whatever in linear space using a target display LUT (ie something that matches your intended target)
* For final export, you'd want to REVERSE the OCIO .CC pregrades.

For VFX, you REALLY want to inoculate yourself from any external variables. Grades are one of them.

Never work with pre-graded material unless you know 100% that its not going to change. But even still, I'd always argue that you need to work on source material. Davinci forinstance truncates color conversions because its using LUTs that don't have high enogh resolution. Nuke and Fusion does not suffer this (unless you're using the same luts and OCIO lut files) because they tend to implement the exact transfer functions mathematically instead of just a look up table.

Probably I omitted that part, but I'm not the person doing the vfx, I'm the person doing the grade.

The vfx I'm receiving have no grades applied to them. I'm just trying to apply the correction I had for the separate originals to the new ungraded composed image.

So basically I have 2 originals, that I will call A and B... I know how I want those to look and have created separate looks for both.

So as an example, let's say the vfx team has created an image that is half of the screen A and the other half B. With my preset grade I can only make one half of the screen look correctly at a time. So basically I have to choose preset A or B, not both with help of the matte as I intended.

So basically I have 2 originals, that I will call A and B... I know how I want those to look and have created separate looks for both. So as an example, let's say the vfx team has created an image that is half of the screen A and the other half B. With my preset grade I can only make one half of the screen look correctly at a time. So basically I have to choose preset A or B, not both with help of the matte as I intended.

If I understand this correctly: you can correct inside the matte but not outside? What if you make an outside node? (That's unless I don't understand the problem.)

So basically I have 2 originals, that I will call A and B... I know how I want those to look and have created separate looks for both. So as an example, let's say the vfx team has created an image that is half of the screen A and the other half B. With my preset grade I can only make one half of the screen look correctly at a time. So basically I have to choose preset A or B, not both with help of the matte as I intended.

If I understand this correctly: you can correct inside the matte but not outside? What if you make an outside node? (That's unless I don't understand the problem.)

Yes, kind of... Because I can correct, it's just that my corrections behave differently.

So if I put the matte and apply a correction to the background, the background looks as is supposed to. But if I do a correction to the foreground with the matte on (which doesn't affect the background) and then go and apply the node corrections for the background, the background ends up darker than the solo version mentioned before.

So the mattes are working to properly isolate one or the other but if I choose to use it for both instances apparently the math changes.

Thanks for the tip. I'll try it and see what happens.