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CG reflections form Live action footage

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Cam JohnsonCG reflections form Live action footage
by on Oct 18, 2010 at 12:45:48 am

Hopefully someone out there has done this and could give me some guidance?...here's the low down:

Live action footage of an actor is going to be shot, and 3D CG elements are to be composited flying around the actor ( CG elements are simple blocks and planes...but reflective) How am I going to get the moving reflections of the actor onto the flying objects. Camera mapping won't work because it wont reflect the actor's back/sides....

My theory is to set up 4 cameras around the performer on the shoot, and map that footage onto a box in the middle of the scene, make it cast reflections, and switch of 'seen by camera'. Is this the right approach?

I am using 3ds max / Vray for the 3d rendering.



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Graham MacfarlaneRe: CG reflections form Live action footage
by on Nov 27, 2010 at 10:14:33 pm

Hi Cam,

I think you definitely have the right approach although it might be possible to observe the block in the reflections which would certainly look strange. I would suggest the following if you run into problems:

Model and texture (using front/back photo of actor in Tpose) then rig (very very simply with biped/skin modifier) a basic 3D character. Obviously the photos should be taken on set exactly where the actor will be standing for the main shoot. These photos can be planar mapped onto the tpose character prior to rigging for a quick and dirty texturing which should be sufficient.

If the actor is performing an action you can load that footage into Max as a background and your 3D character can be animated to mimic the video in a very rough manner.

Then you can proceed to render off the reflecting items whizzing around with the 3D character only visible in reflections just as you suggested.

If you don't feel comfortable making the 3D character there are plenty of free models available. Example:

I don't think it will be too easy to find a free rigged character though. For rigging, Max help files have plenty of good info on using the skin modifier with biped, which will be more than enough for your needs.

Alternatively you could just leave the modelled and textured character unrigged and thus static and make sure the pose is as close to the average pose of the actors performance.

Hope that helps

Graham Macfarlane
3D animator and VFX specialist
London UK

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Cam JohnsonRe: CG reflections form Live action footage
by on Nov 29, 2010 at 3:13:37 am

Thanks! I had given up on a response...

I think substituting a rigged character could work, however the actor is going to be performing a reasonably complicated dance routine, so that could take some considerable effort animating.(given character animation isn't realy my area of expertise!)

The director has suggested that perhaps on the shoot we could setup some mirrors behind the dancer to 'catch' the reflections, film them with separate cameras, then camera map the footage to objects in the scene. This sounds pretty good in principal, hopefully it will work.

Thanks again.

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Graham MacfarlaneRe: CG reflections form Live action footage
by on Nov 29, 2010 at 8:05:36 pm

I doubt the level of animation of the character would need to be very accurate at all to get a convincing set of reflections, unless the reflective objects are big and slow moving.

If the rigged character is out of the question might I suggest the following:

Have your second camera positioned behind the actor a little further back and off to one side so that at no point does any part of the actor come between the two cameras from each of their points of view. Make sure that the 2nd camera sees as much of the back of the actor as possible.
You might be able to use the video as is although I would expect there to be a need to cut out the actor from the BG for best results.

If using green screens then key out the actor for both shots to create a separate black and white mask animation defining the actor. Load both Mask & video pairs into Max and apply each pair to a separate plane in 3D, (a front and a back facing plane - only visible in reflections and only visible on their front faces too.)
The back video & mask may need to be stretched horizontally to correct for perspective shortening. Sync the video pairs up and away you go!

If not using green screens then you'll have a fair bit of roto work to do although you might get away with a tight garbage matt with feathering. I would run some tests with a front and back photo of yourself in a particular pose to see what method you can get away with since you don't want to do more work than is necessary to achieve a result.

Let me know how it goes!

Graham Macfarlane
3D animator and VFX specialist
London UK

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Cam JohnsonRe: CG reflections form Live action footage
by on Nov 30, 2010 at 12:11:01 am

Hi, yes I think thats most likely going to be closest to the approach, as the CG reflective panels orbiting panels are indeed large and slow moving!
I think the saving grace is thats its going to be edited into lots of quick cuts after an initial slow opening sequence, so we will be able to get away with a bit more slightly dodgy CG. Thanks for your input, our shoot is mid Dec, so final product should be mid Jan or so I believe.


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Ali QuintanaRe: CG reflections form Live action footage
by on Jun 16, 2012 at 3:05:53 pm

Hello, I was searching through the forums and saw this old post.

I do similar projects also in Max vray. Did some before in scan line as well.

I am really interested in how your project turned out and how your experiences were.

I would normally have the actor (if dancer) do the routine twice, but once turning around and back to the camera. That way I only need one cam and have exact same light settings too.

Would love to hear about how the project went.

Kind regards,

Ali Quintana

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