compositing refractive motion blurred objects.

[super gnu]

ive got some flying water droplets in a scene. i need to render them seperately.

if i render them using rendermask without alpha they come out ok as expected.

if i render with alpha, i lose most of the droplets...

its a really tricky one, client is not at all happy with how they look in-render "they look like little crystals" so i need to render them seperately in the hope of getting something they like in post... however im not having much luck there either.

cant show whole project unfortunately, but here are:

the droplets without alpha (so on black)


the droplets on background cut with alpha ( note there are tons missing)


the droplets on background without alpha, set to screen.



any suggestions, i have to finish this week and im a bit lost.

[super gnu]

nobody? ive now got a day to dfinish this job and ive got no better idea so far than rendering out my ae comp as an image sequence, loading it as environment bg in max, and rendering droplets directly onto it. i can then use a multimatte to mask them out and paste them back into the comp at the top. works fine unless they wanna change the background. which they do.

[joconnell]

So you've two things - the distorted bg / refraction part, then the reflections over the top. Using a simple alpha won't give you enough as you've to distort the bg you're comping over using the droplet shape, you've gotta darken down your background using the reflection filter as a mask (energy conservation stuff) and then plus your reflections over the top - if the clients wanna change stuff they'll need to do it in afx at least, a simple alpha in a video editor won't have enough control to pull it off :/

[super gnu]

So you've two things - the distorted bg / refraction part, then the reflections over the top. Using a simple alpha won't give you enough as you've to distort the bg you're comping over using the droplet shape, you've gotta darken down your background using the reflection filter as a mask (energy conservation stuff) and then plus your reflections over the top - if the clients wanna change stuff they'll need to do it in afx at least, a simple alpha in a video editor won't have enough control to pull it off :/

ok so let me see if i understand. currently ive got "affect all channels" on for the bdroplets, and a matte object there which gives me a red multimatte where the bg refracts thru. so currently the reflection channel has the refraction in it too.



so i should turn off "affect all channels" and output seperate reflection and refraction elements. then i lay the refraction element over the bg/product, and use refraction filter to mask it.. right? if not how do i mask it.. do i need to use a blending mode for that? then i darken it using the reflection filter to stop everything getting too bright.

then i put the reflection channel over that.. (set to screen?)

anything else?

how do i make the droplets transparent where the bg is, but not where the product is refracted int the drops? to get a multimatte of where either the bg or the product are refracted by the drops, i need affect all channels on, which means i cant seperate reflection and refraction passes.

do i render on black and output as exr? should i have the bg image showing in the refraction? distortion of the bg is not a big problem as its fairly featureless, however as you might have imagined, theres gonna be a product in the middle. i guess i should have that showing in the refraction as i have now...?

(sorry lot of questions but ive gotta get it right first time tomorrow.. )


(ps thanks Jo, youre a lifesaver.. ive gone from a life of archvis into the world of topline advertising agencies and national ad campaigns, with rather too much confidence. (animation? yeah i can do that no problem!") we used the entire original project time up just discussing reflection shapes. now 3x over deadline and ive been doing 16 hr days for 2+ months. tomorrow is supposed to be the last day! )

p.p.s yeah as per the other discussion regarding comp alpha, we have agreed with agency just to provide the arteereffects comp and source renders. cant be arsed to figure out an alternative.

[joconnell]

Do they expect to be able to change the product in the refractions too or just the featureless background behind the product?

[super gnu]

just the background - consider also that we pre warned them that they are not going to be able to go from a blue bg to a bright yellow one or something.. they insist that the blue is fixed, but they want the ability to lay it over different background formats, move it around, or tweak background for client.

[super gnu]

one other thing to note, my comps are all 16 bit, dont wanna risk screwing up the rest of the corrections by changing to 32 bit.. i guess ill need to compensate for that when comping the spritz?

-one problem ive got when trying the nethod above, there ar some nice glinty highlights on the drops, i dont get them when i comp. checking, they are refractions of area lights, not reflections. i guess another pass, either with just the area lights refracting, so i can add a screen layer, or a render with affect all channels on and mattes refracted thru drips for bg and product so i can cut those out of a copy of the refraction layer and screen it on?

ugh all seems so hacky.

my main issue is how do i mask in the refraction layer? using a multimatte of the drips is what kills those nice glinty refractions, as theya re at the edge of the drips. i can use matte choker to bring them back, but then i get an ugly border round the drips. i cant use the alpha to mask in that layer, as it misses all the more blurred and smaller drips entirely.