How to make good keying?

[Olivier Madar]

I tried an unique alpha 4 from K5600 a few days ago for lighting a big green screen. I needed something powerfull, daylight balanced, because I was shooting à 300 fps. The alpha has 160° aperture in open face and 1/8 diaph between the center and the edge.
You can see a picture here :

www.vecteurred.com

 

[Donald Strubler]

Sharp lens helps a lot... Especially with hair.
Good colour separation helps,
Minimize the spill onto your subject.
Touch more luminance in the screen helps too...
Motion blur can kill ya...

But mind you... I keyed a guy standing against a brink wall the other day and it worked? go figure

Keying is getting very easy these days. Practise makes perfect.

All of these are true, if it isnt your job. Remeber, people were able to pull good mattes and integrate with DV footage. Its a process, independent of using master primes or shooting at 90000K resolution.

 

[Sam Edwards]

As a 20 plus year compositor and now VFX supervisor I say hire a professional if you want it to look professional. If your keys look bad the most obvious explanation is that you think you can wing what actually takes years of training and experience. My current crew has a minimum 5 years experience and they all make mistakes every day. The crappy shots that you see in major VFX projects are usually done by the newbies that are brought on at the end of the project when it's already too late, or they were farmed out.

At ILM they always shoot blue screens unless the subject has a ton of blue, even for digital. The reason is that the matte is secondary and shouldn't be a problem with any well lit scene. What is really important to the look of the final composite is the suppressed foreground. Suppression replaces the process channel - your screen color. Why would you want to replace your best channel, green, with your worst channel, blue? Also Blue is generally much closer to the colors it will replace, so less suppression is generally needed. You can see this because your people get all grainy after you key them. That's what you get when your replace the green channel with a mix of red and blue.

If you can't hire a pro, I would highly recommend trying to barter with a VFX artist or facility to supervise the shoot and do composites. I'm sure they'd love to have a few RED camera days in the bank. Or just put 'TEMP VFX' in the corner of the frame until you get some distribution money to finish your film.

Sorry for the rant.
Sam Edwards
VFX Supervisor, LA

 

[PatrickFaith]

Sorry for the rant.

Not a rant, I found that HUGELY helpful . From my low budget perspective, which is to shoot at a very low cost as a kind of long "pre-production", then bring shops in if I really "have" something ... I was wondering about filters. Currently I'm thinking of only using polarizing filters and shooting everything at QHD, that way the VFX guys can key at 4k (i.e. so I am not going to use any "black magic"s and other "softnening" filters). That make sense? Also I'm leaning to not even blue screening, but leaving that to vfx compositing (i.e. backyard of the house shots, vfx will modify what is above fence lines, sky, etc ... ). Thanks for the time.

 

[Gavin Greenwalt]

At ILM they always shoot blue screens unless the subject has a ton of blue, even for digital. The reason is that the matte is secondary and shouldn't be a problem with any well lit scene. What is really important to the look of the final composite is the suppressed foreground. Suppression replaces the process channel - your screen color. Why would you want to replace your best channel, green, with your worst channel, blue? Also Blue is generally much closer to the colors it will replace, so less suppression is generally needed. You can see this because your people get all grainy after you key them. That's what you get when your replace the green channel with a mix of red and blue.
Sam Edwards
VFX Supervisor, LA

This is a perfectly valid theory and I've heard it for years but I've got my doubts... I just worked with The Mill who had the same philosophy and I found it simply introduced more problems than it solved. I would say blue gave me twice as much work.

Green spill is slightly less aesthetically pleasing than blue spill but you still have spill and I find that motion blur comes out worse from blue than green. But I would argue that the noise question is almost non-existent now. In de-spilling you aren't going to be gaining much blue with green spill so your noise level should remain static--I haven't seen any extra noise introduced from green vs blue. I definitely wouldn't use blue if you are going to a virtual white cyc.

When I get some time I really want to do a large test to see just how true many of these old practices still are true and which are simply superstition. I suspect what your background ends up being would affect it dramatically. Lots of exterior shots probably go to a blue sky anyway in which case you could leave a little blue spill to look natural.

 

[Sam Edwards]

Hey Gavin,
I've done some pretty recent test. I threw both Green and Blue test shots at several senior compositors and they all chose the blue. Maybe it depends on who is doing the comps. What are you using do do your suppression?
For every rule there are a thousand exceptions. My main point is that it is naive to expect great results without experience or training. You cannot distill years of experience into a forum post.
Cheers,
Sam

 

[Gavin Greenwalt]

Hey Gavin,
I've done some pretty recent test. I threw both Green and Blue test shots at several senior compositors and they all chose the blue. Maybe it depends on who is doing the comps. What are you using do do your suppression?

Personally in Nuke I usually just rely on the good ol' fashioned HueCorrect. I find Ultimatte/Primatte style 3D LUT spill suppression tends to over bake skin tones. But in my experience generally both blue and green tend to get to the same place in about the same amount of time. Where I lean towards green over blue is in semi-transparency where my experience is that skin tones go too dark.

For every rule there are a thousand exceptions. My main point is that it is naive to expect great results without experience or training. You cannot distill years of experience into a forum post.
Cheers,
Sam

Agreed! And I would love to see the results of your test if you ever had time to put together a blog post. God knows how easy it is in this industry to settle into a rut of our own comfort zones. Always great to see real data and tests challenging assumptions.

 

[Petr Dvorak]

It´s hard to expect perfectly blended instant one click key from so completely diferently lit elemets, shot from diferent angles with diferent lenses. Oh and I almost forget worst thing is complete diferent DOF. Like in SW where some Jedi is sharp in the foreground, then the edge of windows is naturaly getting out of focus and then the town outside the window is unexpectably strikingly sharp! ) Yea I know all the financial and personal reasons why it is this way, but this obvious mistake always distract me so much.

If you polish sh*t you still get sh*t not a gem.

 

[Marc Wielage]

When I get some time I really want to do a large test to see just how true many of these old practices still are true and which are simply superstition. I suspect what your background ends up being would affect it dramatically. Lots of exterior shots probably go to a blue sky anyway in which case you could leave a little blue spill to look natural. When I get some time I really want to do a large test to see just how true many of these old practices still are true and which are simply superstition.

I would again suggest reading the book The VES Handbook of Visual Effects: Industry Standard VFX Practices and Procedures. This explains all this stuff in extraordinary detail. I suspect you'll eventually discover that what you call "superstition" may just be "experience" to people who have been doing this for a long time.

I agree 100% with Sam Edwards comment above: the best thing you can do is hire a good VFX supervisor to consult on the job and stop you from making mistakes before they happen. Good experienced guys like that are worth their weight in gold -- they can save you lots of time and money at the end. There are no short cuts to pulling great keys, beyond good lighting, good lenses, and evenly-lit backgrounds, plus following all the rules.

 

[Patrick S. Marshall]

VFX expert Steve Wright strongly, strongly advises against using colored backlights on actors.

So would I. I was not talking about backlights. The backlight should match the light that you would get from the background you are going to use for the shot as close as possible. But you can compensate a bit for the green spill by using colors on key and fill. Not much of course, you don't want to get the skin tone all unnatural... But if the final scene would have frontal light of any kind, that should be matched instead. Always try to match the background and talent as much as possible.

Ummmmm I have to actually correct to a little bit of misinformation here. Distance is not key. The reason people think distance is important is because they've been taught "Brightness = 1/n^2" but that's only true of a point light.

While it may not be 1/n^2, the further away you are, the less light you will get, no matter what. That said, it is also true that if you have a well lit GS (and you should), the light you see, is what you get and less light means less green, which in turn reduces the key-ability. The actual reason - at least for me - for a good distance is more about separating the GS from the talent with light. If I can differentiate the lights between Talent and GS without them interfering with each other, I can much better match the Talents light with the final composition, without reducing the quality of the green. But in a confined space or small GS setup, a few feet distance is enough.

And for some shots you will have to dig deep into the post production tool box. from masking of single frames, rotoscoping, several layers of keys and much more can all help. And the hardest parts are those where Talent and the green touch (e.g. if you need to shoot feet on a green screen floor).

 

[Patrick S. Marshall]

As a 20 plus year compositor and now VFX supervisor I say hire a professional if you want it to look professional.

At ILM they always shoot blue screens unless the subject has a ton of blue, even for digital. The reason is that the matte is secondary and shouldn't be a problem with any well lit scene. What is really important to the look of the final composite is the suppressed foreground. Suppression replaces the process channel - your screen color. Why would you want to replace your best channel, green, with your worst channel, blue?

While I can not compete with 20 years, you will have to admit that the technology changed very much in that period.
For me it is the other way around for exactly the reason you are mentioning. There is more green information, which makes a digital key much better in terms of details and separation and less noise. I can fine tune with about the double in terms of color range from the green than I would be able to do with the blue. And blue is much more part of our skin tone range than green ever will be (veins for example). I don't know the exact technology behind the old blue screens, but I think it had to do with blue having more detail on film then the other colors, which is basically the same reason as today with the bayer pattern having more details in the green. Besides, you need much less light to properly light green than blue, which is a big advantage.

 

[M Most]

I don't know the exact technology behind the old blue screens, but I think it had to do with blue having more detail on film then the other colors, which is basically the same reason as today with the bayer pattern having more details in the green. Besides, you need much less light to properly light green than blue, which is a big advantage.

Not correct. Blue was used in part because it represents the smallest component of flesh tones, and in part because it turns black when a red filter is used (remember, blue screen processes began in the optical compositing era). Today, it's primarily used because it is less reflective than green and thus causes less spill given the same lighting setup, and also because digital spill suppression algorithms cause far fewer problems, especially with flesh tones, than they do with green screens.

How "easy" it is to create a matte is not the only, or even primary, issue in high end compositing. The final composite is the only criteria, and for many visual effects supervisors, the belief is that blue screen is superior in that regard. Of course, the usual rules of color difference matting still apply in either case: if wardrobe selects blue, you can't use a blue screen, and if you're shooting on a grass field with lots of trees, green is a poor choice.

 

[Donald Strubler]

I suspect you'll eventually discover that what you call "superstition" may just be "experience" to people who have been doing this for a long time.

No. The first thing you learn over the first few years of compositing is that there is no right way to do anything. The more experience you get the more tools you have to solve mutliple problems. I have worked with a great many compositors that are superstitious and only use the one way they know to do things.
Its not good practice.
It is superstitious.

Also wanted to point out for the thread that the intended background should not affect the integration whatsoever. To be properly integrated, fg edges should be unbiased, thus ready to layer over anything, Anyone who tells anyone differently is only using a despill and not preparing their edges correctly.

 

[Patrick S. Marshall]

Not correct. Blue was used in part because it represents the smallest component of flesh tones

This is one of the things I never agreed with.
While the skin tone color may be far away from blue, blue is still part of your skin and green is not. If you are caucasian, I suggest you take a light and look at a pale part of your skin and then tell me if you see some blue, or if you see some green. Just look at a vein under your skin, what color does it have? So yes, red is the dominant part of skin, but blue is there and green is not (except maybe if you have a hematoma, but that is mostly blue and yellow, which makes some parts look green of course). btw I can not vouch for the details on other skin tones as much as I can on my own, but in my experience, I find more blue spots, parts and elements in skins than green. Talk to a make up artist on skin color...
With todays media, you can see many more details, if you want, while film blurs colors out - and make up flattens the color too (but does not have to if you go for that harsh and rough style). And I just checked, one reason really was that blue has more details on film. The crystals are much smaller... But never mind.

Green usually works better for me, but I use what I need to use in a given situation and I would recommend to get some practice with different colors (not only green and blue) and objects and light. Understand what you do, even if you get a pro to do it for you - which is always advisable, if you can afford it.

 

[Gavin Greenwalt]

While it may not be 1/n^2, the further away you are, the less light you will get, no matter what.

The less light you will get but the more light you will need to light your greenscreen. The two cancel each other out with a constant intensity screen per m^2. A 100' green screen at 100' away will be the same spill as a 10' green screen at 10' lit to the same exposure since they'll in respect to your subject matter be the same angular size.

 

[Marc Wielage]

No. The first thing you learn over the first few years of compositing is that there is no right way to do anything. The more experience you get the more tools you have to solve mutliple problems. I have worked with a great many compositors that are superstitious and only use the one way they know to do things.

Uh... nothing you say conflicts with what I say. I merely said that experienced VFX people generally know what they're doing. No question, there are many different methods to handle different problems. But it does help when the people shooting the initial material follow all the rules.