multimatte to use forgrading and cc

[manodzit]

here is the situation:

i am using the multimatte (i have quite a few of them) element with a channel copy node and creating a new channel from either R,G, orB and using the newly created channel as a mask input for grading different parts.
its working like a charm! apart from the following issue and some other secondary issues:

primary issue:
1. i can seem to use it on a grade node, however i am unable to use a color correct node

secondary issues:
1. since i have a lot of them i am kind o daisy chaining them which is a bit lengthy process
2. get a slight edge fringing at the edges after all those grades


i could really use some help on the primary issue. (for now i can live with the secondary issues)

thanks in advance
m.

[Severin]

Primary:
1. You mean you cant connect it? Or what happens? Color correct should be able to use masks just as the grade or any other node. Did you chech the ranges tab?

Secondary:
2. Have you remembered to unpremult your image before grading, and then premultiplying again after? (Always grade on unpremulted images)

[pingking]

for the second issues:
1. you could render to a multichannel exr where all your channels come in one image and then you can choose directly the mask channel in your nodes
2. because the multimattes are antialiased the will get less opaque to the edge. if you do now some hard grading you will see these edges (the unpremult should help there)

[manodzit]

hi,

thanks for the replies...
@ severin... thanks i found it in the ranges panel!
@ pingking thanks for the tips.

please pardon my ignorance to basic stuff. when you say unpremult...
does it mean

way one>

A> BG -i should use a unpremult node right after the read node and then start shuffling>grading>colors>rotos>etc> premult - finished BG
B> FG -same process as above - finished FG
C> FG over BG merged
D> motionblur and DOF

way two>

A> BG - unpremult > shuffle>grade>etc> finished BG
B> FG - same process - finished FG
C> FG over BG merged and then premult
D> motionblur and DOF

let me know which one generally yeilds better results. (i use half float multichannel EXRs )


one last noob question:
how do i cookiecut an image with the alpha of another image


thanks plenty for the help
m.

[Severin]

The general workflow is always grade unpremulted, and filter premulted.

Way one vs. way two: If you merge with an "over" you need the sources to be premultiplyed. If you merge with a "Matte", the merge-node does the premult for you.


I usually use the channelmerge-node to do alpha operations. Try the "stencil"-operation to cookiecut.

[manodzit]

thanks again servin!

one thing more i am having some problem with:
i have this multimatte pass and i am plugging it as a new channel to grade.

now what i want to do is affect only a part of this mask... hence i plugged in a roto node (i am far from comprehending this node yet)
and it works with the grade node but doesn't seem to work with the other nodes like hue correct/ color correct.

i am sure i am making some mistakes/missing something.


m.