Chroma Keying dilemma - when good skies go bad

entilza72 wrote on 2/24/2010, 5:47 AM

Hi team,

Looking for some suggestions on how I might get out of a difficult situation.

The background info:
I have footage shot on a 5D Mk 2. The frame contains 30% of a graduated deep blue sky, no clouds, with talent in front. Ungraded, the sky "throbs" due to over compression (thanks 5D). The director wants a "punched" look to the scene, so I have punched the saturation and used about 6 different secondary color correctors to bring different colors up differing amounts, whilst keeping the talent normal.

The problem:
When the colors are enhanced during grading, the throbbing and compression artifacts become distractingly bad. I mean REAL bad. A little bit of grading = a LOT more obvious source compression.

The obvious solution:
Ease up on the saturation. But this would be a last resort because the original image is VERY flat due to the flat response curve used. It NEEDS grading. And the compression is still very obvious anyway.

My solution:
Misuse the Chromakeyer to key off the sky, and replace it with a better sky. I pick an average sky sample, and using "display mask only" option and the MIN/MAX and blur, I fine tune it so we have a reasonable mask. There's some bleed on the shirts, but I can superimpose an un-keyed version for the bottom half of the image, blocking out the partially keyed shirts.

Sounds good in theory!

The problem with my solution:
There's a massive color shift to green in the unkeyed parts of the image: mountains and the shirts. I have identified that both the shirts and the mountains have a predominant blue hue to them, just like the sky.

But even though I have a "reasonable" mask, the unmasked parts of the image are prone to this massive hue shift to green once the keyer is activated.

I'm guessing the chromakeyer is "enhancing" the image - it's looking for parts of the image that are ALMOST the key color, and adding an opposite (negative) hue. Probably works fine on small edges with blue screen bleed, but doesn't work at all when you're trying to abuse the keyer like I am ;-)

Any suggestions for how to clean up the sky, whilst keeping the punchy colors?

Oh, did I mention there's 2 people, both moving at different rates? A static mask won't be possible.

Cheers,
Ent

Comments

jabloomf1230 wrote on 2/24/2010, 6:20 PM

It can be done quite easily with After Effects and the Mocha plug-in. That may be more than you want to do, but here's the tutorial:

I've tried it with video that has the same problem that you are describing and it works very nicely.

entilza72 wrote on 2/24/2010, 8:28 PM

Hi, thanks for the pointer.

Unfortunately, I don't own AE, so Mocha wouldn't be an option without a considerable outlay. I'll keep it in mind as a last resort option.

Roto obviously isn't something available to Vegas - I was hoping for a solution that uses parts of the Vegas tool suite (or Vegas add-ons).

farss wrote on 2/24/2010, 8:44 PM

I think your problem with the blue cast has nothing to do with Vegas altering the image. Where I think you've gone wrong is the key isn't solid and you're getting bleed though from the replacement sky. When you check the mask the white section needs to be 100% solid white or what you're seeing will happen.

Another challenge can be objects reflecting the blue sky itself.

Whilst a full rotoscope doesn't sound too tempting you might use a few feathered garbage mattes with keyframes plus the CK FX to get it done.

Bob.

entilza72 wrote on 2/24/2010, 9:23 PM

Hi Bob,

I initially thought the same thing about the BG image altering the hue - it made sense.

But the green hue floods in regardless of the BG image. I can turn off the image (so its just black) and it will still be green. I also tested with red, and sky blue. The green persists.

Parts of the image that are solid White in the mask have the green hue. Thats not to say that there isn't problem parts in the mask with parts being less than 100% white - there ARE problems.

I will try and do up a side by side image as a demo, but sadly the forums dont allow attachments.

Your suggestion of feathered keyframed mattes might actually do me just fine, as long as I keep the replacement sky color identical to the original. The very soft feather (giving enough room for the talent to move, and feathering on the sky itself) will mean a large portion of the original sky will show through, BUT the bulk of the noise occurs in the middle of the sky, so this option should work with a couple of hours of effort.

farss wrote on 2/24/2010, 9:40 PM

I have an idea for your replacement sky.
Take a good sample say every minute as a still. Use those with really long fades between them, sky tends to change a bit and if it's entirely static it might give the game away.

Good luck, my rotoscoping efforts have been limited to a few seconds and that had me tearing my hair. I think it's a craft best suited to monks and saints.

Bob.

entilza72 wrote on 2/25/2010, 2:56 AM

Thanks Bob - great idea re: series of fadethroughs.

On a related note, I've often gotten to thinking - it would be great if the chromakeyer included a bounding box where the key was only active INSIDE the box, and the ability to inverse that of course. I could have used such a thing so, so, so many times.

farss wrote on 2/25/2010, 4:19 AM

" it would be great if the chromakeyer included a bounding box where the key was only active INSIDE the box"

What you want is a garbage matte. OK unlike some other keyers the Vegas one doesn't include one as part of the FX but no real drama.
Just use a mask in the event/pan crop to crop out the bits you don't want.

Also remember you can run more than one keyer in the FX chain. If that doesn't quite work you can duplicate the track once or many times. The use masks to select certain parts of the frame and get the best key for that part etc, etc.

Bob.

entilza72 wrote on 3/14/2010, 7:42 AM

Oops - forgot to post about my results (considered not posting at all, as I don't like resurfacing long dead threads, but I figured it's a handy learning tool for all).

I went with Bob's simple and effective solution.

For the 15 second clip duration, what I ended up doing was:

* Making 5 masks that represented 5 equal length segments. The masks cut-out the sky but allowed room for wobbling heads, and gentle fade around the heads so as to not give a sharp line.

* Making 5 skies, based off the original footage. These were cleaned up.

* I cross faded between the 5 masks (2 at a time), as well as the 5 skies (also 2 at a time).

The result was highly effective. I was worried the lack of movement in the sky would give the game away, but it doesn't - the cross fading is enough to make it unnoticable.

Thanks Bob!

Also, great idea on how to get a "bounding box" for a keyer. Thats something I want to try out ASAP!

Cheers,
Jason

Jim H wrote on 3/14/2010, 6:13 PM

I would be interested is seeing a side by side of your results. Stills would be fine.