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How to approach difficult multiple chroma keys in one shot

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Victor Stan
How to approach difficult multiple chroma keys in one shot
on Sep 5, 2015 at 2:41:10 pm

Hello everyone,

I have been doing video editing and a bit of compositing for some time and worked on a long green screen footage project last year. However everything I did was not too difficult or advanced, but now I am in the process of acquiring a more difficult project.

The following footage was done (not by me) using almost no budget and as you can see, very poor lighting. I am trying to think of a solution to key out the windows, remove the awful spill and colour correct the scene at the end. The footage is 1920x1080 ProRes 422 at 23.9 fps

I was thinking of doing selective colour change on each chroma to get them as close as possible to each other and then apply a global keylight and work from there.
Is it better to split the footage with masks into 3 parts for each window/character and compose the final scene from those? Rotoscoping or mask animation is a no-go, the shots are very long and the final cut is very choppy.
The camera is static, only the characters move slightly. Unfortunately there is also the issue of reflections in the windshield and glasses so that will require some extra work.

What do you think it's the best approach to this?

Thanks!





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Kalle Kannisto
Re: How to approach difficult multiple chroma keys in one shot
on Sep 5, 2015 at 6:48:26 pm

That certainly is some terrible green screen footage.

Personally I would first mask the windows exactly, as if the people were not there at all. If there is any camera movement or jitter, track it and apply it to that mask. (This way you have less concern with accidentally including the green spill into the key.)

Then I would try and key just the people moving in front of the windows, one window at a time.

In other words, I would build the mask from four layers, the clean windows, masked by hand, then left, back and right (people in front of the) windows separately with some rather extreme keying to get just the silhouettes of the people, wouldn't care that much if the key eats some into the people (using choke). The result of this would be a black and white mask, which is then used as a track matte for the original footage.

Change color or perhaps Selective color for the green spill on the original footage. Or even an HLS selector in Color Finesse if that doesn't do it.

If you want some detail back in the windows, fake it and put whole new layers there to replace the glasses.


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Victor Stan
Re: How to approach difficult multiple chroma keys in one shot
on Sep 6, 2015 at 5:25:49 pm

Hi Kalle, thank you for your kind reply and suggestions.

I struggled a bit, but I think I narrowed it down. So I took the footage, duplicated it 4 times, and made a mask for each of the clips so I'm left with each of the greenscreens + a small one for the steering wheel part. I used Keylight on all, used the inverted masks from above as the inner mask for each and set the output of Keylight to Combined Matte. Then set the blending mode to all 4 layers to Difference.

I pre-composed all 4 clips into a single one which I used as the Inverted Luma Matte for the original footage. I did some more masking on the original footage to deal with un-keyed areas of the footage or ones that were never covered by the actos so to help me key the greenscreen better.

So far I have only worked on the easier shot as you can see below. It can use a bit of finessing of course, there is some spill present, but wanted to see if this workflow is going to work or not in the first place. I'm afraid for the other 2 shots I will have to rotoscope the 2 character's glasses, will have to learn how to do that first, never did rotoscoping, only some basic mask animation.

What do you think of the final result, where should I focus my attention next? Any tips to make it better?



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Kalle Kannisto
Re: How to approach difficult multiple chroma keys in one shot
on Sep 7, 2015 at 6:19:01 am
Last Edited By Kalle Kannisto on Sep 7, 2015 at 6:24:40 am

Given the original quality, that's looking pretty good. Maybe a slight softening (blur) for the edges, then fake glass on the windows and color correction so the inside and outside look more integrated. If clouds is the final background you want to use, make the outside way lighter than the inside and color correct the inside towards blue, as the light source (sky) is cold blue. And you could add some glow to the highlights. Along these lines (I didn't add fake glass or soften the edges):



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Victor Stan
Re: How to approach difficult multiple chroma keys in one shot
on Sep 7, 2015 at 1:42:58 pm

Thank you so much! No, the clouds are not going to be the final background, it was the first thing I found in my library to test the key. So far I do not know what I will be compositing this footage with, the job I got is for keying and minor colour/exposure correction (no grading or luts), it's a personal art project so the client has no budget to do more so I'm guessing they will do those parts on their own. Good tip about the softening, yes they are quite harsh at the moment, wonder if pre-blur will help me out or just blur the screen matte edges. I could also try playing with the mask feathering and expansion some more.

As an off-topic question, can you recommend me some good and complete AE tutorial or learning course? Thank you!


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Kalle Kannisto
Re: How to approach difficult multiple chroma keys in one shot
on Sep 8, 2015 at 7:17:43 am

Well, then it looks you're basically there, as long as it looks good in motion and doesn't flutter at the edges. Just a bit more blur, which you can add to the matte.

You may want to add a "light wrap." That is often added to a key, especially if the new background is lighter than the subject. A tutorial for technique should be easy to find.

For good in-depth tutorials, you might want to check out Lynda.com. They have complete courses and very specific and clear tutorials. They've got a monthly membership cost, but you can do a free trial and see if you like it.


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