Premultiply (coming from AE and Nuke)

[icedeyes]

Hi everyone,

I have been using fusion for a bit but never had to deal with any premultiplication issues. In my last project i run across a problem trying to premultiply in fusion.

First of all, i tried two methods:
1. use the subtractive merge method on a merge node (i read that this is a form of premultiplication)

2. used channel booleans to multiply the color channels by the alpha

both methods create a slim white halo around my cg elements (which are rendered over a light blue bg and not the default black which is the reason why i got into this problem).

In AE, i just used the premultiply on the interpret footage and picked the color i wanted to premut with (haven’t seen anything similar in fusion).

In Nuke i would just use a premult node to do it for me.

Anyone has any suggestions?

Thanks in advance,
George

[Wally Rodriguez]

Merge (and almost every node has a Fusion tab – the one with the little nuclear symbol)
on that tab there is an option (checkbox) called Multiply by Mask

Sometimes (on a matte control for example) it’s called Pre/Post-Multiply and directly embedded into the regular options tab.

I’m not quite sure if the mentioned options are technically the same but it sure sounds like it.. otherwise if they are not just use a matte control with the Pre/Post-Multiply option checked as a Premultiply node.

There is also a mini tutorial on the subject on eyeonline.com

http://www.eyeonline.com/Web/EyeonWeb/Techniques/tips_tricks/newtips/tip_106_F5.aspx

[Ian Anderson]

just use unmerge macro from vfxpedia to convert cg to premult on black
http://www.vfxpedia.com/

hth
jirka

[Jef Huey]

thanks for your replies guys…

i still get the halo though… the postmultiply function is completely different than the premultiplication on AE…. the premultiply works well since it contracts the halo but its still there… The mattecontrol works but erodes the edge of my cg object…

Unmerge on the other hand worked perfectly…

thanks again to both of you

[Author]

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