Green Screen Questions

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  07 July 2013
Green Screen Questions

Hi All

So I'm a veteran when it comes to Maya but something that i have never done is, live action footage into CG environment with a green screen.

So what we are doing is taking some footage with actor that are in front of a green screen.
After searching for many days i have not found the answers to the questions i was looking for.

1. I know previously you would take a mirror ball and use your camera to take a full range of f-stops in your environment (Studio or out door scene) to apply to your material, is this still the case?

2. Does the temperature setting on my cameras effect the quality of my composition. i.e. do i need to know this for my materials to apply to it in any way?

3. Any other tips?
I've got a pretty awesome team helping me shoot my short film but i'm the only one with a CG background. Besides having tracking points and the walls lit up correctly, could you give me any other pointers to help me in my post production process with CG smoother.

  07 July 2013
Measure twice cut once =)

What i mean is measure stuff on set carefully.
Write it down, distances from camera to object/ characters.

Write down every lens for every shot you take. All camera settings should also be written down.

Make a drawing or take a photo of the lights location so you could easily recreate their position in 3d if needed.

Basicly, the more info you gather the better you are off, nothing worse to get back from the studio and realizing that you didnt write down the lens info and distances and now you need to track it.
Voxels for dinner.
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  07 July 2013
Since you are doing the shoot yourself and friends, what Schempp said is absolutely helpful. In a lot of productions we will get a shot as is with nothing but the few tracking markers in the footage. If you are new to green screen shot, I'd do as many lockdown shots as possible. Taking HDR is also still done in many cases even just for lighting reference.
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  07 July 2013
Great pointers, very much appreciated.
Thank you Schempp & BruceCLin

I'll deferentially be doing HDR then.
I'm going to have to get 2 guys to do my measurements during the shoot because the scripts are improv. The actors have a story line but are getting a concept for each 7 min scene, so i'm using them for ideas on what to create for them. I'll have about 2 of them where they are not moving position

I'll have a 4 - 5 camera men team who do this sort of shoot and tracking every other day, we been planning this and working it out for 2 weeks and still have 3 to go, you guys have given us some more fruit for the table. It's going to be fun but super challenging, it is going to broaden our imagination for future shoots.

  07 July 2013
On a GS heavy shoot a mirror ball is only marginally useful (YMMV) as it will mostly pick up a lot of green, which you don't really want to use to light things.

It's useful to take gray (18% gray useful) and reflective probe shots with and without the GS, more so without, for lighting purposes.

What is more important however is knowing your cameras to get the best response and edge contrast in the green you possibly can (green varies greatly between cameras depending on sensor, codec and so on), make sure you get even lighting and no chromatic interference (it's a lot easier to offset colours from a perfect neutral shoot with no spill and punch than it is to have a perfect romantic photography you need to remove spill and punch from), and start getting used to using proper photometric equipement and knowing what the measurements read like.

Countrary to what a lot of people give in terms of advice good GS is a lot more about good neutral photography and target alignment and comp than it is about 3D CGI.

Fifty years of Maya experience aren't worth a fraction of six months of strong photographic, colour and comp experience here.

Recording data might or might not be useful, but will be so more for tracking and matching than for the green screen component, especially if you have a full BG replacement.

Sure, take them, especially on moving shots, and if you need tracking make sure you steer clear in the shoot of as much as you can that confuses tracking (zooming, especially with heavily breathing lenses tends to be brutal), but that's a different set of problems entirely from the specifics of shooting GS for the chroma/roto side of things.

Lastly, always leave yourself some overscan space. Don't frame EXACTLY to final if edge elements in the shoot matter, or the moment you will need even just a minor pan&scan style adjustment you will have screwed yourself up.
Same for undistort/redistort.
You also usually get much clearer data centre lens and centre sensor.

A really solid, crisp 1800 pixels width available in a shoot will work up to 2k perfectly fine, a perfectly framed final lensing composition spanning the whole lens and sensor with heavy breathing and some vignetting and a ton of spill at 2k will need a lot of work.

Shoot for post when you shoot GS heavy.
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  07 July 2013
Excuse my ignorance JacO could you explain what you mean by this.

Originally Posted by ThE_JacO: It's useful to take gray (18% gray useful) and reflective probe shots with and without the GS, more so without, for lighting purposes.


  07 July 2013
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