Live Action/3D Reflection integration?

#1

I’m always amazed at how high the bar for 3D graphics in film rises from year to year, and mostly I’ve been able to keep up with the times and understand how something was accomplished, but there is one area which has always puzzled me, and that is reflections.

Now certainly, I can understand putting 3d objects into live reflections using matting and spot-on camera tracking, but the opposite is a stunning illusion, putting live-action reflections onto a CGI object.

I simply can’t understand how it can be done. The closest thing I’ve come to is having a panoramic camera positioned where the 3D object is meant to be in the scene, and using the video from that camera to fake the reflections, but for some reason that doesn’t seem right, but it’s the only answer I can come close to short of building the entire set in CG, which sounds like a bit much, even for the degree of realism mentioned.

Does anyone know exactly how this feat is accomplished? It’s the one aspect of CG/live action integration that has truly stumped me.

#2

What I do is I track the scene and reconstruct it in a basic block out. Then I project the Live Action video onto my geometry as a texture. Then, using final gather on in Mental Ray, you render out the CG without the built out reconstructed scene in it, and comp it into your shot.

#3

So wait, let’s see if I got that:

You do a camera motion track for the scene…

You rebuild the objects IN the scene in 3D…

You apply the footage as a texture to all of those objects (screen mode?)

You use Final Gather to compute the resulting reflections…

You hide those objects and use the Final Gather solution to provide said reflections…

~

That sounds intriguing, but either way you couldn’t get the other side of the objects, unless it’s just a trick, and the other side of the object has a mirrored version of the projected texture on the opposite side. For something simple (ie, reflection isn’t too prominent or clear, and easier to provide as a passive illusion), that sounds like it’d work in a pinch (relatively speaking). Of course, there’s no way that would work in the case of a clearer reflection, especially one involving a live action character.

Of course, I may have misunderstood as well. And naturally, studios being as well stocked as they are, I wouldn’t put it past them to simply have a camera or a placeholder mirror prop where a character’s reflection is meant to be, to be digitally fiddled with later.

#4

I use Image Based Lighting, which is basically a picture of the original environment, to get reflections. That usually provides enough reflections. If not, I just throw things into the scene with textures and get reflections from that.

#5

mostly people shoot HDR domes on set and we use those to replicate reflections in CG.

Occassionally for really specific reflections of things we go the extra mile and contruct something in cg with a projected textrue. If that thing is moving action or say a actors refelction in a cg object, you are going to want to get that from the same or similar angle using another camera at the time.

this is often referred to as a witness camera.

they did some pretty funky scene recreations on Benjamin Button to get better lighting scene. there was an article floating around on vfx world about how they used Nuke, Maya and there in house tools to do lightings cards in a hybrid fashion between Nuke and their renderers.

#6

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