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Ghost Walking Through Glass Door


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#1 Tim Unger

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Posted 25 March 2008 - 02:47 PM

I am thinking of doing a sequence in which the subject appears to walk through a glass door without stopping. In addition, I would want to lower the opacity of the subject's layer to apply a "ghost effect."

For the most part, I've decided on using a static angle, as a moving camera would make the editing much more difficult.

Right now, this is what I've come up with in terms of a shot list:

1) Empty frame, without a subject (door closed)
2) Subject walking through (door open)

I was thinking of masking out the closed door in shot 1 and placing it in the second shot, but I don't think that will accomplish what I want. I'm not sure how to make the transition from one side of the glass door to the other in order to make it look convincing.

Any suggestions on how to achieve this effect would be greatly appreciated.
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#2 Jim Keller

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Posted 25 March 2008 - 08:18 PM

How were you planning to composite the shot? Different software would result in a different workflow, and hence dictate a different way of shooting it.

Also, it may be easier than you think to do the effect physically, without compositing, if you're planning to work on a set you'll be building rather than on location. I've seen the effect done (on stage) by sliding glass in and out of the way (setting the lights so they catch glare as another person opens it, but there's effectively no glare when the door is closed -- the glass can then be slid out of the door, the "ghost" passing through it, without the audience seeing it) and by mounting lights in a doorframe to pick up some scatter off of atmosphere pumped down through the top of the door, though I'm honestly not sure how well those would translate to an on-camera application.
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#3 Jim Keller

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Posted 25 March 2008 - 08:25 PM

Oh, I should go ahead and add that I tend to use After Effects for my work, and there I would simply create three layers. Layer #1 and Layer #3 are identical -- the actor without the door, and Layer #2 is the door in between. You would then need to do a mask (either simple or complex depending on your angle) on both Layer #1 and Layer #3 to transition between them as the actor passes the plane of the door (e.g., a mask which starts surrounding the portion of the actor that is leading, and then expanding and changing shape as other parts pass through the door). If you have a higher-end compositing packing, this is probably easier, but I don't have that kind of budget to play with. :)
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#4 Tim Unger

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Posted 25 March 2008 - 09:29 PM

For compositing, I was thinking of using After Effects for the masking, while my main editing would be done in Final Cut. Also, I should have probably mentioned that I will be shooting on location, so I won't be able to mess with the door at all.

Since it is a glass door though, will I need to change Layer #2's opacity so that we are able to see the ghost before he has walked through (in the beginning of the shot)?

Thanks a lot for your help.
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#5 Jim Keller

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Posted 27 March 2008 - 03:58 PM

Since it is a glass door though, will I need to change Layer #2's opacity so that we are able to see the ghost before he has walked through (in the beginning of the shot)?


That would depend on the effect you're going for. If you want the door to look "solid" then, no, you'd want to keep the opacity at 100% on the middle layer. But don't be afraid to play with it in AE. Its still-image previews are quite accurate. Not knowing much about your project, I'd be inclined to select an angle where the actor was visible through the glass on the far side of the door, leaving the opacity of the "door" layer at 100% but setting up a mask where the window is, and then removing said mask once the pass through of the door is done. (Having the actor appear through a solid door is easier because you wouldn't need to mask). Seeing the actor this way through a window by a door is relatively easy, since that window will be present on all three layers. You may have to play with mask opacity to get a good effect to see the actor through the glass door itself. All that said, depending on the specifics of what you're dealing with, you may be able to "cheat" by setting the "door" layer at a lower opacity. That trick was popularly used in the days of optical compositing to hide mask edges, and very few people noticed...

Shooting on location is going to give you one more wrinkle: You want to make sure your actor never obstructs or is obstructed by the open door, since you're going to need to mask it out. Ideally you would want to remove the door for the shot with the actor, but since you can't do that, select an angle where the open door (and its shadows) will truly and sincerely be out of the portion of the shot you will need to composite.

If your actor is good at repeating action, you may also try shooting a version that is split into three shots:
1) Actor walks up to closed door. 2) Actor walks through open door (with door to be added later). 3) Actor walks away from closed door. It requires a truly *amazing* actor to be able to splice these together into a single shot, but audiences are quite used to seeing many cuts around an effect shot (rendering the effect shot -- the actor actually passing through the door -- much faster and therefore cheaper and easier to get away with sloppiness).

And, finally, if you have access to a greenscreen, it's even easier (though not necessarily better) to shoot the actor walking on the greenscreen and to the composite as outlined above, but without the worry of getting the architecture perfectly aligned.
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#6 Stuart McCammon

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Posted 31 March 2008 - 08:38 AM

I have to vote for glass on this one. if you shoot the "ghost" as a reflection in glass (could be easily done in a stage or garage), then you can composite it without applying an effect, having the actor stop, etc. - it will also allow you to quickly get your location shot and eliminate the hassle of lighting and relighting to match the changes in sunlight through a sliding glass door, which would probably be the biggest single headache of this shot if done differently.
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#7 Markus Manninen

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Posted 03 April 2008 - 10:42 PM

Hm. There are so many interesting variables.
I would say the first one... if I may be so bold... does the static camera work for you creatively for the shot?
I like finding creative solutions that fit the the vision, and it sounds to me like you are giving something up by how you worded it.

1. Your transition effect.
If you shoot without green screen you will need to roto a shape that creates the transition from behind the glass to infront the glass. You will most likely need to add some type of visual effect (bloom, glare, color, blur,...) where your matte outline animates to make the transition from behind to front visually believable. You wil need to do that by probably widening the outline of the matte to an area that you can work with. If you are really a composite freak, you can add some noise effect on the plane of the glass door to make it look like the ghost is actually affecting the door as it goes through it.

If you can use a green screen approach, then you will actually already have a matte and can use matte choking instead of rotoscoping your transition. There's several ways to do that in After Effects. Just don't use the "simple choker". Brrggh. As far as interaction with the door and the transition the above is the same (using an effect with the matte outline).

2. If you are using a green screen, and if it is not important to see floor contact (shadows on the floor from the ghost), then you have more flexibility (I think someone else stated that). You don't have to perfectly line up the two shots, as long as they "line up" visually in the composite in After Effects.
The green screen approach has one other positive - you can play around with the character lighting, a lot - and one negative - you need to get a clean green screen setup to not have to jump through hoops later in post.

My 2 cents.

Best of luck.
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