Compositing ghost shot - help!

grantg wrote on 10/29/2006, 3:14 PM
I’m trying to composite two shots for a Halloween video for my family. (My daughter, 5, has written her first "movie," "The Ghost Family.")



I shot a locked-down version of two events:

1) My family dressed up as ghosts dancing around in a circle in our front yard.

2) My family sitting in the middle of the frame.



I put the ghost shot on track 1, the family sitting on the grass on track 2. I decreased the opacity of the upper ghost track to 75%. Both images appear, and the ghosts look wonderfully see-through, but so does the flesh and blood family they’re dancing around. I have tried every combination of track order, compositing mode, parent/child relationship, but nothing gets rid of the see-through family.



All I want is for my family to be solid and the ghosts to be ghost-like. Any suggestions? I need to resolve this before Tuesday!

Comments

fwtep wrote on 10/29/2006, 3:37 PM
The problem is that you have the front yard in both shots. This means you'll have to hand roto the ghost shot. (In real movies the ghosts would be shot against blue or green screen.)

Do the ghosts move in front of the live people? If not, then it's very easy to do, because you can just use a simple mask shape to block out the area of the front yard where the people are.

If the ghosts do move in front of the people, then for those frames you'll need to really roto them with a form-fitting mask.

Fred
DavidSinger wrote on 10/29/2006, 4:51 PM
The family-sitting (lower 100% opaque) clip is not really "see through" - what you are getting on *top* of them is the 25% opacity that remains from the upper clip - which is the same grass background that the family did *not* sit in front of during the dancing around scene. The grass gets as much priority as do the people in the upper shot.

This is why the ghosts must be shot with a uniform color back ground and digitally extracted (actually, the uniform background is subtracted). The trick is to be able to digitally isolate just the ghosts, so that only ghosts are being overlaid on top of the lower track. This requires that the computer, by color, can discern what is background and what is your subject(s).

You are not trying for an Oscar here, so go to a painted room someplace where the floor and the wall is the same color. If that's white, then dress your family in anything but white. Shoot them dancing around (note that they won't disappear "behind" your lower shot family, but will always show on top - so dance only "in front" - go ahead and "touch" the areas where a family member would be sitting, even "put an arm through" from left-to right).

Then come back and search on "Greenscreen" within the past 30 days to find several nice references to how to make the upper clip show only the ghosts when you put the events on the timeline.

It ain't hard, you'll get it shot tomorrow and by tomorrow night will have believable results. You'll look the hero to the family.

To summarize,
1. The most important thing is a room somewhere with a floor and one wall that is the same color. Hmmm, you could pin up a king-sized bedsheet, draped down across the floor where the feet will be.

2. Nobody wears anything the color of the background.

3. "Good enough" will be good enough, because even with fuzzing outlines, disappearing hair, and holes in your ghosts' eyes, they will look all that more unworldly!
TeetimeNC wrote on 10/30/2006, 4:12 AM
grantg,

The other posts have done a good job explaining the reason why this isn't working for you. You may be able to salvage a workable solution IF you shot the ghosts scene from a stationary camera AND you have at least one frame that only shows the background (i.e., no ghosts visible). If that is the case, you may be able to key out enough of the background to make it believable using the "Difference Map". This approach lets you remove the background from a scene that wasn't shot in front of a green screen. I've not actually done this but I've read the technique isn't perfect but it may be good enough for your daughter's first movie. There is a tut for this on the VASST site. Good luck!

Jerry
ScottW wrote on 10/30/2006, 5:48 AM
Don't know if this will help, but it's one of my fav' tutorials.

http://matthew.chaboud.com/vegas5/twocats/

--Scott
grantg wrote on 10/30/2006, 12:55 PM
Wow, what awesome suggestions. Thanks to all. Unfortunately, I don't have the time to shoot a makeshift greenscreen or explore the wonders of bezier masks and difference mapping - at least for this project.

I was able to tweak some brightness/contrast/opacity stuff here and there and come up with a reasonable end product.

Actually, the audience in mind was my immediate family and maybe my daughter's kindergarten class - not the most discerning viewers in the world, so I may yet work in this town again.

Once more, thanks.
RBartlett wrote on 10/30/2006, 1:25 PM
Perhaps too late to help GrantG, but this compositing approach can work:

Given two tracks, one with the main track and two with the ghost to be inserted quick and dirty. So much depends on the scene, but if it was shot the way I expect it was:

Set both video tracks into compositing mode using the gadget on the header of each track (global by track):

1. Set the main track to Screen
2. Set the ghost track to hard light.

Any good? There are always many ways to achieve almost the same thing. This is just one of those other ways - by all means evaluate it and tear me to shreds over it's lack of quality!

If necessary:
Adjust the opacity of the ghost track and if that works stick with it - otherwise work around the options, add other filters or consider a garbage matte to weedle out the best bits of the ghost track. You may need to make this matte travel with the ghost - hence the reluctance on my part to recommend it on the first parse.