aftereffects "set matte" wierdness

[super gnu]

trying to use a multimatte in ae.
i use "set matte" on the layer in question referencing the green channel of my multimatte.

however i get a great big fringe around the masked element.

i assumed this was due to the gamma of my multimatte being wrong ( the main render needed adjusting to gamma 0.45 to look correct)

so i put an exposure control on the multimatte and adjusted to 0.45. i clearly saw the edge of the green element shifting slightly and looking more correct.

HOWEVER

the "set matte" that references the multimatte (with exposure control on) ignores any adjustments done on that layer and only looks at the raw footage for its matting. so i can adjust exposure, levels, whatever i want on the multimatte, and the "set matte" stays exactly the same.

this seems a bit weird.


workarounds?

[Morbid Angel]

are you rendering into exrs? Set matte do you mean track matte?

[super gnu]

no i mean the "set matte" effect where you use one channel from a specified layer as a mask for another.
my issue is it takes the raw footage as the mask, not the final result of the specified layer, effects and all.

this has always been my method for using masks in ae. please if there is a better way let me know.

ive begun to realise that using multimattes with HDR scenes is a bit useless. trying to mask an overexposed window is a nightmare. the aa, and hence the edge, is completely different in the render and the matte. what am i missing?

[cgrant3d]

Instead of "set matte" use track mattes. Anything can be a track matte - a b/w image, text, a solid, etc & it's always evaluated *after* any effects on the matte itself. To do this just drag the b/w matte above the layer you want to mask. Then on your layer you want to mask then either use the menu (Layer / Track Matte / Luma Matte) or in the comp next to the layer mode for that layer you'll see a column "TrkMat" and click the drop down menu...

[super gnu]

kewl. ill have a fiddle when im back at my desk. im assuming i am able to use any channel from an image, not only greyscale..?

and.. Dmitry, yes im using exr's.

so how do people deal with masking in hdr scenes?

in this case im trying to add a stronger contact shadow to an already rendered scene with a character walking across a sunlit floor. i rendered the ao pass with floor and character, and used a multimatte of the floor to mask the shadow i wanted. trouble is where the characters feet (black) hit the bright floor ( probably overexposed) i get quite a harsh aa, as expected. this isnt the case in the matte of course. so my shadow has a lovely line of bright pixels between the dark shadow and the black feet, where the bright floor is poking through the edges of the mask. obviously rendering the character seperately would have been ideal but this is not an option.

ive tried using various matte adjustments but none of them seem to fix the issue properly. . is there a standard way to generate masks that match the aa of an overexposed area...?