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Comping Live Video in to CGI

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Eric Kirk
Comping Live Video in to CGI
on Dec 9, 2010 at 7:09:16 pm

Hello,

This is just a rough still shot of a longer hallway that leads in to this chamber. I am trying to comp a live video of a guy walking through the doorway and having some difficulty in the workflow and wonder if someone could help.

I can make a person walk up and down the hall all day long but when it comes to crossing the threshold of the doorway, for obvious reasons, I have difficulty.

To do this, I shot myself on green screen, walking in the direction of the door, lifted my foot as if to step over in to the room and that is where I have my problem. Not quite sure how I place the layers so that it appears he has actually stepped through the doorway.

I uploaded this file of a still shot. Hopefully it worked and appreciate the help ahead.

1373_chamber.jpg.zip

Eric

Eric D. Kirk
http://www.kirkproductions.com


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Dave LaRonde
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 7:26:35 pm

You'll need a big green screen set and a GOOD (i.e. not DV or HDV) camera:
  • Build a chroma-key green threshold on the floor of the green screen set for the actor to step over.
  • Match lens focal length, angle & height between the CG Image and green screen.
  • Shoot the actor walking the hall and stepping over the threshold.

You'll want to run tests to make sure you get a good key. You may find you'll have to do some roto work.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Eric Kirk
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 7:39:07 pm

Thanks for the quick reply.

I'm tracking with everything you said I believe. So, just to get a little more info, it sounds like the key thing I was lacking is really just the green screen threshold?

So, as I think through it in my head now, the exact matching of size, etc is basically so that the green screen would block out my foot at the appropriate spot where I pass over.

Does that sound about right?

Eric

Eric D. Kirk
http://www.kirkproductions.com


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Kevin Camp
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 7:58:35 pm

i think you just need to do a split layer at the point where the subject appears to step though the doorway (edit>split layer)

then you'd layer order the clips something like this:

1. walking-down-the-hall layer
2. the hallway layer
3. walking though the doorway (split layer)
4. back wall layer

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Eric Kirk
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 8:10:10 pm

Appreciate it. I believe we are making progress. My next comment regarding the green screen threshold thought was that since it is just a few frames, I could mask the heel of the foot as it passes over to accomplish that.

I like the sounds of this split layer idea though. I could use just a tad more explanation.

I will just add comments to your 4 layers below and if you don't mind.... :)

1. walking-down-the-hall layer: Ok, this would be the layer of my actor (me actually) walking down the hall. Check.

2. the hallway layer: Are you suggesting extracting the hall/cutting it away from the doorway layer?

3. walking though the doorway (split layer): This would then be the same clip as number one but a copy as a new layer?

4. back wall layer: Sort of same question as 2, are you suggesting I chop out the back wall that is visible through the main door?

And then, a final question, when you say "Edit, split layer", is that an actual command? (not at my work desk at moment to see in AE)

Thanks,

Eric

Eric D. Kirk
http://www.kirkproductions.com


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Kevin Camp
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 8:38:29 pm

[Eric Kirk] "2. the hallway layer: Are you suggesting extracting the hall/cutting it away from the doorway layer?"

yep, the hallway (and doorway) would be separate from the back wall. there would be a hole in that doorway that you'd see though to the back wall.


[Eric Kirk] "3. walking though the doorway (split layer): This would then be the same clip as number one but a copy as a new layer?"

correct. using split layer it's in point would be trimmed to start exactly where the end point was on the original layer... essentially the layer gets split into two layers, each getting trimmed to the current time indicator (cti).

[Eric Kirk] "4. back wall layer: Sort of same question as 2, are you suggesting I chop out the back wall that is visible through the main door?"

yes.


[Eric Kirk] "when you say "Edit, split layer", is that an actual command?"

yep. choose edit (from the top menu bar) and select 'split layer' and it will split the currently selected layer at the cti.

the idea is that you'd place the keyed clip over the scene, scrub though until you get to the point where is seems like the subject would be stepping though the main doorway, then do a split layer and move the new split layer below the open doorway layer so now it will be behind the doorway.

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Eric Kirk
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 8:53:42 pm

Kevin,

Hey great run down on this procedure.

It sounds relatively easy. This step below might have me a little. I mean, I understand how you say to do it, mentally, trying to understand what takes place here so that Layer 1 (the guy) appears to walk thru.

Really appreciate your help.

Eric



[Eric Kirk] "3. walking though the doorway (split layer): This would then be the same clip as number one but a copy as a new layer?"

correct. using split layer it's in point would be trimmed to start exactly where the end point was on the original layer... essentially the layer gets split into two layers, each getting trimmed to the current time indicator (cti).

Eric D. Kirk
http://www.kirkproductions.com


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Kevin Camp
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 10:03:11 pm

so, you have a keyed clip of you walking away from the camera. when you drop that over your plate and scrub thru the timeline, at a certain point it will look like you should be walking thru the doorway, but you are still on top of the doorway...

let's just say at the 3 second point it looks like you should be walking thru the doorway.

it's at that point that you would split the clip layer. leave the first 3 seconds of the clip above the hallway/doorway plate, so it will look like you are in front of that plate.

move the split portion (the clip from the 3 second mark on) to be bellow the doorway, but above the back wall. so as you play thru the timeline it will look like you move from in front of the doorway to behind the doorway at the 3 second mark.

dave's right about the set and the plate/virtual set... it will be tricky to get this to look correct perspective-wise. if that is important, you may want to try shooting your green with regularly spaced markers on the floor. you can then use those to help line up the perspective in the plate (this would most likely involve modifying the plate).

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Eric Kirk
Re: Comping Live Video in to CGI
on Dec 10, 2010 at 1:50:53 am

Hi Kevin,

Thanks. I am considering both points. You both have a wealth of knowledge.

I am first trying the laying technique since I've already got the footage shot, etc.

I feel like I am missing some critical point that you are mentioning but have not figured it out yet.

I've layered like you mentioned and did the splitn at the appropriate time but I'm not getting the effect. The foot of the guy still plops down on the front side of the door.

I'm sort of at a loss here.

Eric

Eric D. Kirk
http://www.kirkproductions.com


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Dave LaRonde
Re: Comping Live Video in to CGI
on Dec 10, 2010 at 3:54:35 pm

I think the fundamental problem is that in order to cross a raised threshold, a person normally has one foot on the floor at all times. You step OVER a raised threshold, put the foot down on the other side, then the other foot steps over.

Unless the subject hops over the threshold, there will always be a time, no matter how short, when one foot's on one side and the other foot's on the other side.

See how much fun it is to think effects through? That's why Disney animators had mirrors next to their drawing tables: so they could look at their facial expressions.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Dave LaRonde
Re: Comping Live Video in to CGI
on Dec 10, 2010 at 7:27:13 pm

[Dave LaRonde] "I think the fundamental problem is that in order to cross a raised threshold, a person normally has one foot on the floor at all times. You step OVER a raised threshold, put the foot down on the other side, then the other foot steps over.

Unless the subject hops over the threshold, there will always be a time, no matter how short, when one foot's on one side and the other foot's on the other side."


This line of thinking is also why I originally suggested building a chroma key green threshold to match the CG threshold. You've got a virtual set. You need to build chroma key pieces that mimic the behavior of what's in your virtual set.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Kevin Camp
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 7:59:45 pm

i think you just need to do a split layer at the point where the subject appears to step though the doorway (edit>split layer)

then you'd order the layers something like this:

1. walking-down-the-hall layer
2. the hallway layer
3. walking though the doorway (split layer)
4. back wall layer

Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW


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Dave LaRonde
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 8:15:48 pm

[Eric Kirk] "...the exact matching of size, etc is basically so that the green screen would block out my foot at the appropriate spot where I pass over.
Does that sound about right?"


That's correct. It's a pain to set up, and the floor perspective will be a killer if you don't build this virtual set carefully, but if you're able to give yourself a little up & down wiggle room on that threshold, it gets a little easier.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Eric Kirk
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 8:55:29 pm

Dave,

Thanks. Sounds like a I have a couple options to explore. I will first try the layering I think and see if that helps since it appears to have less to do with trying to perfect the size, etc as you mentioned.

Shooting in HD, I typically end up reducing the size of my subject in AE.

Eric

Eric D. Kirk
http://www.kirkproductions.com


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Dave LaRonde
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 9:10:01 pm

[Eric Kirk] "I will first try the layering I think and see if that helps since it appears to have less to do with trying to perfect the size, etc as you mentioned."

I don't know about that. I think Kevin might have taken the proper construction of a virtual set as a given.





[Eric Kirk] "Shooting in HD, I typically end up reducing the size of my subject in AE."

Perhaps. But since the subject is standing up, you might also want to think about turning the camera 90 degrees, giving you a 9x16 image instead of a 16x9 image, so you use more of the picture. I use that trick all the time.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Eric Kirk
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 9:16:30 pm

Dave,

Sorry, that's not what I meant. I have plenty of real estate with my wide angle Panasonic HMC150. Actually what I meant is I capture the entire figure but because I end up reducing the size while comping, it becomes of little importance then to have to worry about size perspective when I am inserting him in to the hall.

So, I end up reducing the scale, maintaining the HD.

Hope that was more clear. :) But I will tuck your idea in my mind for later use. That is fantastic for close proximity shooting, like in my basement.

Eric

Eric D. Kirk
http://www.kirkproductions.com


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Dave LaRonde
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 9:24:15 pm

Heck, I use the 9x16 trick even when I KNOW I'm going to scale the footage down, and scale it down a LOT, too. I'll use that trick even when I make a 4x3 spot in SD!
The reason: if there's any problem with the edges of the key, you can hide much of it just by scaling the green screen layer down.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Eric Kirk
Re: Comping Live Video in to CGI
on Dec 9, 2010 at 9:27:59 pm

Dave,

Excellent point and fully understand. That really makes sense!

Thanks,
Eric

Eric D. Kirk
http://www.kirkproductions.com


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