Is there a way to give the appearance of volumetric haze in a stopmo scene?  The straightforward way seems doomed to failure in a stopmo environment (unless there's some magical haze product out there that maintains a perfectly consistent density for hours on end...).  Shooting in layers and applying progressively a progressively hazier color grade on the more distant layers digitally could work for some scenes, but that's assuming each layer is narrow enough to be approximated as 2D relative to the haze gradient, and the particular scene I'm considering involves a very large character visibly fading back into the fog.

Is it simply as impossible as it seems, or is there some genius trick to make it happen?

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You're right Nick. Depth matte creation from stereo pairs is far from perfect. It would need some cleaning up.

For a computerless solution, this one is a classic (from 1975, no less)

Ok, I did a rough test of one method -- clearly room for some refinement, but I think the basic idea could be made to work.  Once I have a chance to mock up a 3D slider rig, I'll give that method a shot too.

The test creature (monochromatic puppet, so I don't need to worry about UV light for the test):

Basic test setup for the fog mask exposure was a half-silvered film clamped in front of the lens at a 45* tilt, with an LED light set up directly under it, so it was reflected into the scene from the perspective of the sensor.

The raw fog mask:

The fog mask with some rough cleanup:

Final result, using the refined fog mask as an inverse luma matte for a solid grey layer over the main frame:

Big things to improve upon based on the quick test:

  • I didn't bother closing off the camera-side of the tilted reflector, so there was some contamination by the ceiling reflected back into the camera.  Cleaning that up lost a lot of the fall-off in the raw shot.
  • The light wasn't *quite* aligned with the lens correctly, so you can still see some shadows in the mask shot -- ideally, spending more time on the setup, I could eliminate those (or at least, significantly reduce them) with a more precise light/reflector placement.
  • The lighting on the creature itself is too harsh -- it would look a lot better with properly ambient light you'd expect in a dense fog.

But overall, I feel like this approach has some real promise.  Any other ideas?

Nice test results! Yeah, I think if you lose the hard toplight and go with more diffused lighting you'd get rid of those hard edges and end up with a good fog effect. 

Ok, that's looking really promising, if not exactly right.  Enough to think, maybe I do need to get my head around what it is you are doing.  I still have an old half-silvered mirror from when I used to shoot front projection, well actually I think it was more like 40/60 but I'm not sure which way around.  I think it had more reflection than see-through.

Here's a basic sketch of the setup, if that helps -- 

So you've got a half-silvered mirror at 45* in front of the camera (ideally with very dark material enclosing the area between the mirror and the lens, both to keep external light off of the camera side of the mirror, and to absorb the portion of the fog light that passes through and hits the top surface, which would otherwise bounce back into the mirror and into the camera).  The fog light is set up under the mirror, ideally placed such that, when viewed from directly in front of the camera, the light appears to be coming right out of the lens.

Each frame gets two exposures -- one with the fog light turned off and the rest of the lights on, and one with the scene lit only by the fog light.

The big thing standing in the way of this being useful for your hansom cab scene is color -- if you, for instance, had a grey cab pulled by black horses, that would totally throw off the fog matte, since even though the horses would be closer to the camera, they'd show up darker in the matte.  No bueno.  Hypothetical solution is to use a blacklight for the fog light, and spray down the scene and puppets in invisible UV paint... which is the biggest reach in this method, and is very likely to fail spectacularly.  So we'll see!  A stereoscopic based solution could very well be the better approach.

Hmm... it's a black cab, to be pulled by a dark brown horse with black mane.   So at least most of the horse is slightly less dark than the cab, but it's all dark.  Except the two carriage lamps and the cabby's face.  

Thanks for drawing it!  The set-up is similar to front projection, except the slide projector (or light in this case) was off to the side rather than below in my setup.  My mirror is in a frame that pivots sideways.

I may need to go with lighting it the way I do for atmospheric perspective, with soft blueish light on the back parts of the set and cab, and sharper light up close.  That never really washes out all the hollows and undersides where you still get shadows, though.  I have plenty of time before I get onto this one anyway. 

   

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