Compositing tricks

[Da_elf]

This is something i was never able to get done. A while back i had a project where i had a very reflective wooden table with an empty place at the table thats suposed to be filed with a 3d character. Behind where the character sits is a clock. the clock is reflected on the table so when i created the 3d character, and then added reflection properties to a matte object standing in for the table so the character would be reflected, the clock shows through the characters reflection. Any effects, compositing guys can tell me how they would get around it?

little crappy diagram to help explain

[Paul Oblomov]

Well, actually, this is not that simple, you have to recreate the reflection... Some painting I think.

P.S. Maybe some real pictures for me to try ?

[Neilg]

Is there not some way to render an alpha of a reflection pass?

edit: well, if you have the guy with an alpha channel, and the glass/wood not contributing, doesnt the reflection of the alpha appear so you can use it as a mask?

You could also make the table and rest of the scene completley black, table with the same reflection properties as before and the guy with a white vraylight material, then turn all lights off and use that as a ghetto alpha. And now you'll also need the environment reflections as a seperate channel for masking.

[Seraph135]

The way I've done it in the past is to render two passes. One pass is just the characters reflection in the table...The second pass is identical, except that the character has been made 100% self-illuminated white which gives you a black and white mask that you can use to drop your character reflection back on top of the tables reflection.

Tim J

[Da_elf]

your not getting it. if the reflection on the table was 100 percent diffuse then it wouldnt look natural. its going to be slightly glossy and slightly transparent so the diffuse of the table comes through correct...the problem now lies in the fact that in the live action background plates the reflection of the clock is reflecting on the table. when overlaying the 3d characters reflection on the table the clock will show through the reflection. in reality it would be the clock peeking over the shoulder or being totally obscured by the character there. now suppose its not a clock. the clock can be moved if its not importsnt. lets say you take footage of a calm lake and then stick a 3d t-rex next to the lake. the trees behind the t-rex will be reflecting in the water. however they are supposed to be obscured by the t-rex. however the t-rex can easily be composited with the trees behind him. but his reflection in the water... its going to be mixed with the reflection of the trees

[Neilg]

I'm lost, (going off your new example) cant you just use my method to get effectivly an alpha of his reflection to mask out the trees?

You will have needed to comp the trees reflection in seperatly though.

[Da_elf]

You will have needed to comp the trees reflection in seperatly though.

the trees are not CG. they are part of the same live action plate as the lake. so its not like you can alpha channel them out.

[Neilg]

Well yeah, if you cant seperate it somehow (get 2 shots of the scene and extract the object? -assuming its the scale of a clock) then an automatic/easy solution is pretty much out.

Masks and matte painting techniques are in.

[Da_elf]

somehow two shots wont be good to remove a tree. or a forest background. maybe they would setup green screen approximately where the creature would walk then take a second shot of lake without the green screen. beats me

[Da_elf]

here is an example. lets assume we have a 3d character walking on the little wall. his reflections are going to be "mixed" with the tree reflections. when in truth, his reflection should be obscuring the tree. now how would you go about doing that?

[Paul Oblomov]

You need to paint a mask for a trees part, that would be intersecting with your character. Then, you project your new mask to water surface, according to perspective etc. I suggest you to fake reflection of character in post too, by rotating and scaling your main character pass.

By combining this technique, you can easily blend anything into anything. But it's time consuming and it can look great, only if the footage is of a good quality.

[Da_elf]

why would i rotate my character. that wont give proper perspective on the reflection. besides thats what the reflection/shadow/matte is for.

[Paul Oblomov]

There's definitely smth you dunno about compositing

[Da_elf]

definately true since im here asking now. i know you could take a character and flip it vertically to get the reflection in a way. however lets say your looking down at a character and its standing on a mirrored ground and the character is a woman in a dress. then flipping wont work since the real thing you see the top of the dress and in a real reflection you would see under the dress (naughty naughty)

here is a test i just did straight out of 3dsmax. the wall and the water objects are set as shadow/matte with recieving shadows turned on and reflections turned on for the water. see how the red of the teapots in the reflections is blending with the background reflections. the green and white sky. since technically in the reflection those things are behind the objects then the objects should be obscuring the green and sky reflection. but its not




here i basically simulated what things would look like (simulated in photoshop) if i had on set went and setup black velvet behind the wall vetically as a velvet wall. the velevet reflected would basically get rid of the reflected background we didnt need. however from here i guess it would be a help to have an alpha channel on those reflected teapots.

[Paul Oblomov]

Simple AE composition....


First I rendered GI pass, next the reflection one (envirionment color was set to red). Opened AE, placed GI and reflection. Started keying the reflection (that's why I set env. color to red - for better extraction, but it can be any color you want, justs make sure it's not on your layer). And that's it. Oh, almost forgot - I draw a mask, to get rid of some parts of the reflection.
Now, I need to add water surface imitation... Should I ?