Matte / Shadow Composition Question

[Olli96]

Hello,
i have a problem when it comes to render object seperate and comp them in ps or ae. Please bear with me, its been ages since i had to do something like this. Unfortunatly i cant show the original scene so i just made a simple one to explain the situation:

this is the final image:
http://prntscr.com/8j6thj

what the job right now requires is that i render out the background:
http://prntscr.com/8j6tui

and then render out the teapot and comp it over the background:
http://prntscr.com/8j6uo4

putting both images together i end up with this:
http://prntscr.com/8j6v04

obviously the shadow color doesnt match and all my other tests were not succesfull. i thought about using VRaySphereFade but end up getting a black outline around it which i cant seem to get rid off:
http://prntscr.com/8j72pk
i could paint away the black outline but later on it wil be quite a sequence of images and doing this by hand would be not something i would like to do.
I am doing a kind of map with animated objects on it which will later be integrated into a website thts why i would like to render out only the objects and shadow to keep the png's filesize small. i could render the complete region with the animated object and simply put it on top the map but the filesize is getting too big this way...


what am i doing wrong? must be one of these days where i cant find the solution - must be something pretty easy, any pointer is much appreciated.

[joconnell]

There's some discussion on this elsewhere about getting a shadow for one object but without the other - it's a bit of a messy one. When you render one object and it's shadow you get something correct, then you render the background and it's shadow and again get the correct result. The issue as you're seeing is when you combine the two, you'll get the shadow that'd normally be shared by the two objects used twice in the overlapping areas. It's a less trivial thing to do but it's being discussed on another post I seem to remember.

Your black line is down to photoshop being terrible with alpha handling, after effects should have a few filters that could shrink in that area around the sphere fade so you don't have to treat it by hand.

What might be the best solution is to try to have your light / shadows combined it a single pass before you multiply them over your scene. There's blending modes in nuke like min and max which you can use to take two shadow passes and take whatever the darkest value is from each pass and only use that. In areas where the values are the same, it won't double up the values. You'll have to do this on your two Raw light / Raw GI passes first, then add that as a combined shadow pass over your beauty.

Here's some simple bits in nuke with two separate shadows, the two combined using a multiply showing your doubling error and then a min operation.

Shad 1:



Shad 2:



Shadows mult:



Shadows min:




Edit: Try using darken mode in photoshop to blend your two raw light / shadow renders.

[Olli96]

Thanks jconnell, always helpful with my problems, much appreciated!
I will run a test tomorrow with the sphereFade and after effects and see if i can use this method before i try the other one.

[tambyra]

hi, have you managed to solve your problem? i was doing some work with wrapper materials and they work for me in some cases and in some they dont....
did you try to render your teapot in the same light settings like in your final image but only to make the bottom plane a wrapper?

[Morbid Angel]

both shadows need to be rendered separately then combined together before applying them to bg.