night green screen questions

[Tom Lowe]

I am very curious to learn how I might be able to shoot my actors in front of a greenscreen, and then insert them into a night timelapse shot.

To give an example, how can I take one or two actors, maybe just standing there, or perhaps a couple kissing or something, and comp them onto a moving 24p timelapse of stars moving across a night sky like this plate below (minus the sunrise on the mountain peak) which I would shoot on my DLSR or RED at 4K...

http://farm1.static.flickr.com/115/276743296_de9363e329.jpg

What would be ideal for me is if I could bring along, say, a 12x12 greenscreen and shoot my actors on location, in the very places where these timelapse plates would have been shot, and somehow correctly light and expose them to fit perfectly onto a composite with the timelapse plate. This, rather than trying to match wardrobe, hair, etc, a month later in a greenscreen studio in LA.

It is more or less difficult to composite against a dark or even black night vs against a glowing sunset or regular blue sky?

Even with all the technology George Lucas has at ILM, I have to say that the final sunset shot on Tatooine in Sith did not look convincing at all to me.

It seems like this type of greenscreen shooting might be tough, because if you are outside on location and it's dark, and you are lighting a green screen behind your actors, wouldn't you be getting some "backlighting" around their edges and hair that would seem unnatural for a shot supposedly taken against a night sky?

thanks!

[Steven Parker]

Sounds like a sweet shot!

Don't know if this works with your visual style for these night exteriors, but I would go with a soft toplight (at key or just a bit under) on your talent with minimal fill light, almost like creating silhouettes of your couple. Motivated by the stars themselves. That way they merge with the very dark foresty feeling in your example.

As far as halos, I think I would pull them as far away from the greenscreen as possible and use as long a lens as possible to minimize the risk of spill. Soft light on the greenscreen to minimize bounce. Depending on the shot size you need of your couple, hopefully you can get just enough green around them to pull a garbage matte.

It could be weird to have a wide< plate and compressed foreground element, but if you're close to your talent on a wider lens you'll need more than a 12x or you're going to have spill/reflectance issues.

Wow, the night exterior aspect does make it more difficult! ;)

My greenscreen work has always been day oriented, so hopefully others will chime in...

[Jim Arthurs]

Tom, I personally think that trying to work with the actors on location AND deal with the time-lapse aspects of your shots is more trouble than it's worth... besides, if you bring out a green screen, then you have to light it on location, as well as light the talent. No aspect of the "natural" environment (sky light, etc.) will impact or improve the photography of the people, as it's two completely different ranges of exposure. I guess what I'm saying, is that your people lighting setup will be "artificial", no matter if on location or in studio.

One tip for this sort of subject... string up a large black in front of your greenscreen and light your subjects to suit in front of that. Then remove the black and illuminate the screen to the proper level. Trying to judge the lighting with all the green spill is difficult, so make it easier on yourself by using the black. Remember, the green spill, from the software's point of view, is actually negative illumination, and needs a certain amount of white light fill added to simply read as black and not an area of the image with the green stripped from it crawling with noise.

As Fovean mentioned, back away and keep the size of the screen as small as possible to cover the action and no more...

Regards,

[Steven Parker]

Well said, Jim, and the 12x solid is an excellent, excellent idea.

I think the whole nature of the shot is artificial, but meant to give an emotional impact. In that case I think I would still want to do it on location, provided schedule and support and so forth allows for it, because actors and crew respond to that emotion as well.

It is quite an endeavor for what - 6 or 8 seconds of screentime? But what seconds!

[Tom Lowe]

thank you so much for these suggestions, guys!

keep the knowledge coming please.

Yeah, for me, knowing actors, it will be better, if there is any way to do it, to get it done on location, rather than trying to get actors who are working for SAG ultra-low scale to show up in LA for greenscreen a month later in the same wardrobe, and hoping their mood, facial hair, etc, is all the same.

[Evin Grant]

I would shoot in the same location but durring the day, probably afternoon. Shoot with a good 50 IRE greenscreen and just hard sunlight on your actors.
When you get into post do your key and then CC the actors blueish and bring down thier gamma to match the scene. This will give the impression of moonlight and you can then have control over the exact matching too. It will be very hard to get a good key if your greenscreen is significanly brighter than your subject, then again with RAW 4:4:4 Redcode maybe not.

[Jeff Kilgroe]

A lot of good suggestions here.... If the picture you posted is truly representative of what you intent to comp the actors onto, perhaps greenscreen isn't the best choice? I would consider a blue screen in this situation - it may hold some unseen benefits.

Keep the actors as far forward from the blue/green screen as possible to minimize any light reflection or spill from the screen. Afternoon lighting as Evin suggested would probably be best and should give you a lot of color latitude to work with in post. You can always darken it up and adjust color to fit the night setting.

[Steven Parker]

Hey AppliedVisual--

Can you elaborate why you think a bluescreen could work better than green?

So funny; when I started shooting greenscreen, it was "Expose greenscreen at Key Light Exposure!" Then it was a stop over key, now I hear a stop under... who knows? I do what the VFX guys ask of me. I suspect it's because post software for a shot like this has developed much more in the past few years than the on-set tools for a shot like this.... lately for me it's been Greenscreen or DigiGreen, no one ever mentions blue anymore....

Wonder how long it'll be before we're talking redscreen comps? No pun intended, it's just another color channel...

[Robert Niessner]

You can't use redscreen with humans, it would interfere to much with the skin color.

Another idea worth considering would be to shot on a blackscreen and use a blend mode to composite. As you have a very dark backplate this might be a much easier solution - but it depends on how you want to have your couple look like in terms of lighting. Ask your vfx supervisor what he thinks. You even could try this one easily out - just take a photo somewhere with a black background and try to composite it on your background footage.

[Jim Arthurs]

So funny; when I started shooting greenscreen, it was "Expose greenscreen at Key Light Exposure!" Then it was a stop over key, now I hear a stop under... who knows? I do what the VFX guys ask of me. I suspect it's because post software for a shot like this has developed much more in the past few years than the on-set tools for a shot like this.... lately for me it's been Greenscreen or DigiGreen, no one ever mentions blue anymore....

The stop is dependent on the chroma quality and the luma level of the screen. GOOD paint (like Composite Components line of paint) is designed to be the right luma when illuminated to the key light level of your setup, and is of sufficient chroma at that light level. Everything else I've found (other paint, fabric, paper) needs to be evaluated by waveform/vector if possible to hit the sweet spot. Some cheap stuff just doesn't work... pump up the luma enough to get proper chroma and you find the blue/green difference is too small and the screen is almost lemon-white with a spill that hues to the cyan when corrected.

One other factor is the paticular characteristics of your sensor/stock. I've found most of the CineAlta/700 series footage to react better at a half to full stop over for green and exhibit less noise...

Blue is better than green in many cases, with blonde hair, yellows in general, etc. You also need to light it to generally lower levels to get the proper saturation, which has the side effect of reducing the spill. Of course, when a video camera exhibits a crappy blue channel you shouldn't use it, but I've had terrific results with the HVX and blue. And of course film... I still own a little transmission Stewart bluescreen back when I did blue screen the old fashion way... with an optical printer. Which is another story for another day.

I love blue and predict blue will make a big comeback due to, ironically, the color RED.

[Finner]

Freezer

I have used red screens before with people but only in a pool for underwater sequences (There is blue and green in water and the color shifts too much kinda like smoke so red works best.).

Fovean

I use blue screens for night stuff because if I get a tiny bit of color bounce on the subject at least it is a blueish tinge which works at night. I assume this is why Jeff mentions to use blue.

Also I have found the best way to work with screens is to light them to the exact exposure of the T-stop you are setting on the lens. Then light the talent above or below that value depending on the setting and lighting of the shot you are going to compose them on.

Tom save yourself a lot of hastle and find a large building to rent that is near your location to shoot the green screen stuff. Your best off in a controlled environment, day for night green screen can be really tricky but best second choice if you can't get a building.

[Jeff Kilgroe]

Jim beat me to it and summed it up niceley. Various products out there like Chromatte still support and encourage blue screen use. It does have it's uses. One reason to consider blue screen in this night setup is assuming that you may run into chromakey issues on fine details. Of course, if you're shooting with RED, the samples so far key so incredibly clean it's not really an issue. But if you're using XDCAM or HDCAM (even SR), on fine details like hair it's sometimes difficult to pull a perfect key. Using a blue screen doesn't help any more in this respect (often it can even be more problematic due to the lower luma range of blue - hence why green is more popular lately). But any residuals or halos from the blue screen could blend better with the surroundings in a shot like this and be easier to correct or deal with in post with CC. As Jim also noted, blue does work better with certain colors like yellows, blond hair, etc... Blue also allows you to do something that green often doesn't (without a lot of extra work) and that's use other green itmes in your shot.

[Hans von Sonntag]

Nice Thread!

We can learn from each other, great!

Its all a matter of taste. This is the way I would do it.

Basically you can pull a good matte from almost anything as long as the greenscreen is evenly lit and the colour-space of your footage is not crunshed (like HDCAM, DVCProHD, etc...). I always give the greenscreen 1 stop more light and use a polafilter for enhancing the green if I can afford the loss of 1 1/2 stop. The less greenspill the better; good keyer like keylight have excellent spill reducing capabillities but this is another story. To reduce spill it is necessary to have your actors as far away from the screen as possible. If you don't want to shoot your scene with a long lens you need a big greenscreen to cover your scene. As far as I can tell your background is shot with a medium to wide angel lens. To create a realistic comp focal length of back- and foreground should be almost identical.

This said it's up to your taste how to light the scene. As mentioned above it is clever to shoot this particular scene on location (you might have some nice practicals). I would shoot it at night. I like to control lights and to see the desired mood right in the viewfinder. Fighting the sun and fixing it in the post is not my way. Some very light rimlight, nice, hardly visible fill (remember it's outdoor at night) and clear visible eyelight would be my way for a low key scene. If the actress has curly hair a more defined rimlight would be nice; could also enhance the male character/shape of the actors face.

My 2 cents,

Hans

[Hans von Sonntag]

Finner:

God point using bluescreen for night exterior. Spill is already blue and much easier to handle in already bluish environment.

Hans

[Robert Niessner]

If possible I use green gel in front of the greenscreen lights - this drastically improves the red to green ratio which you need to pull a good key. I wouldn't give it 1 stop more light, because this increases the spill. With the green gel you should be able to get away with less light on the greenscreen.

And I go with Hans and the others recommending to shoot in a studio - the sunlight would make it harder to get a nightlook in the post and you would have the same amount of afford or even more to fight it on the outdoor set than just doing it indoor.

@ Finner
Some special occasions do not count ;)

[Casey Green]

All great input. One thing I haven't seen mentioned (in depth) is the use of an LED light / retro-reflective fabric solution such as the products from Reflec Media (Light Ring/Chromatte). http://www.reflecmedia.com/content.aspx?Content=litering.htm

In this situation, you mount a Ring on the Camera lens which has many Blue or Green LED lights around the edge of the ring. The light emmitted is then refelected from the Grey Chromatte fabric which has millions of tiny class beads embedded into the material. No matter which angle the light hits it, it always refects directly back at the source, which is the camera lens. So the BG apprears solid CK blue or green.

The beatiful thing about this method, is that you do not need to light your background at all with traditional lighting (since the fabric is essentially being "lit" by the custom intense LED's light. You will only need to light the actors. And for the scene you are describing, this may be worth a test to see if it solves your problem. You will be able to have low light for the actors while getting an extremely key-able shot.

I have used this technology many times on Video shoots, but have not had a chance to try it on Film. While not a perfect solution, it does remedy certain common chromkeying issues, such as spill, and the need for expensive and time consuming lighting setups.

Some downsides are that occasionally eyelasses or other shiny objects in the frame reflect the light too. Also, there can be a small amount of noticable blue or green "tint" that shows up in very low light environments when the camera is close to the actors... usually a fill or key light washes this away, or it can be corrected for in post. Other issues are that you can not position any other lights directly behind or near the axis of the LED Ring or it will also reflect back into the camera lens, but this is easy to adjust for. One other factor is having to mount it on the Camera, but they do make many adapters for fitting to most lenses. Also, actors and objects in the shot will need to be within a certain distance (around 45 feet or less) from the camera to achieve adaquate reflective instensity. If too close to the Camera, the light from the LED Ring can be dialed down.

I'm not sure if this is a good solution for your setup, but it might be worth looking into and doing a film or video test on. The main advantage being you are only lighting for your actors and not your BG. Perhaps the Blue version would work well for the night scene.

[Peter Karlsson]

Hi Tom, I don´t know if chroma is the best solution for your scene. I don´t know how you planned the timelapse effect, but it seems to me shooting against the actual nightsky would be better, and pull a motion matte from the actors, and then animate the stars for a timelapse effect in post using the motion matte to block out the moving sky.

If chroma is easier for your production, I would recommend you to watch how they solved darkened scenes in "Prey Alone". Its not anything like your scene, but maybe it gives you some reference.

http://www.cgchannel.com/news/viewfeature.jsp?newsid=3950&pageid=1

/Regards Peter K

[Martin Jäger]

martin

[Jim Arthurs]

While not a perfect solution, it does remedy certain common chromkeying issues, such as spill, and the need for expensive and time consuming lighting setups.

I'm typing fast as I have to go and there will be errors... no time to proof...

Thanks for bringing this up, Casey... I've refrained from talking about it, as I know a number of people like this technique, but's it's hopelessly flawed from the get-go and NOT a professional solution. It's fine for the interview/weathermap on the go situations, but the RED's quality will quickly demonstrate the fatal flaws in the system. A couple years ago I had a long converstation about this with Jim Danforth, an FX pioneer who knows more about front projection than most, and Bob Kertesz, one of the best blue screen guys in the field.

Let me give background... front projection has been around for a long time, and this system is based on the technique. But loosely based, unfortunately. Real front projection would work like this... a SINGLE pinpoint bright light would be mounted 90 degrees off to the side of the camera, use a lens to focus it, then bounce off of a 60/40 half silvered mirror toward the subject and Scotchlite screen. The screen then bounces the light back (on-axis perfectly, because the lightsource is on the same exact optical axis as the camera lens) through the mirror and into the lens. Because the screen's gain is thousands to one when the light is close to perpendicular to the screen, the projection image is relatively dim reflected on subject areas, but incredibly bright when hitting and returning from the screen. Based on this, you can light the subjects to a fairly high level, and not worry about your set lights hitting the screen, as they too will reflect straight back to themselves and not scatter around.

Traditionally, you'd project process footage or large format slides and with a relatively small projector fill large areas of backdrop behind your set. Used in 2001:A Space Odessey for the Ape/bone sequences to provide the backgrounds, Silent Running, a complex variation of this for some of the Superman flying shots, and on and on.

Stop motion animators loved it, because you didn't get the screen fall-off like rear-projection did, and you didn't have to worry about the lights hitting the screen. Right out of high school in '83 I purchased a nice 60/40 mirror from Edmunds and a chunk of Scotchlite screen and had a ball. Okay, I'm a nerd.

Jump ahead a bit and you'll find people experimenting with projecting blue light to illuminate large blue screen areas behind talent. Dune used it a bit, and the same with 2010. Remember, this is a PROJECTED single point light source aimed exactly down the optical axis of the camera. A bit later, Apogee and Jon Erland invented the patented Reverse Front Projected Bluescreen technique. I was fortunate to be at their studio in Van Nuys on a tour the day they were doing test set ups.

Again, as Casey pointed out there are issues. Smoke, mist, glass, metal, and any reflective surface key poorly because of the way the screen gain work. But, if CAREFULLY aligned on optical axis, with the iris and focal lengths of both the camera and projector matching up, it removes the danger of most spill.

Now enter the ringed LED do-hicky. It's not on optical axis, not as bright as it could be because it isn't on axis, and throws a broad soft shadow from EACH LED onto the screen from the subject. Since the LED's are in a circle, this broad shadow is supposed to be reduced by the light from the LED opposite it, but in practice you NEVER get rid of the halo effect from the LEDs. Viewing this on footage from DV or other mushy source hides this issue in the smear of chroma and band of edge sharpening, but move toward pristine high resolution RED and suddenly this will show itself in a big way as a matte line that will never be easily removed.

Regards,

[Jeff Kilgroe]

I'm typing fast as I have to go and there will be errors... no time to proof...

Hehe.... I'd save myself lots of headaches at times If I were to proofread my forum posts. Hahahaaa.


Now enter the ringed LED do-hicky. It's not on optical axis, not as bright as it could be because it isn't on axis, and throws a broad soft shadow from EACH LED onto the screen from the subject. Since the LED's are in a circle, this broad shadow is supposed to be reduced by the light from the LED opposite it, but in practice you NEVER get rid of the halo effect from the LEDs. Viewing this on footage from DV or other mushy source hides this issue in the smear of chroma and band of edge sharpening, but move toward pristine high resolution RED and suddenly this will show itself in a big way as a matte line that will never be easily removed.

Jim, this is great info!

I've been wondering about that LiteRing and how well it would work. I've also been really interested in Reflecmedia's Chromatte fabric as a possible solution for green/blue screen material. Anyway, since you seem to know how the LiteRing performs, I'd love to know if you have experience with the Chromatte fabric and lighting it. If so, what are your thoughts?

[Jim Arthurs]

In the old days, Scotchlite was limited in seamless sizes, and was basically a sticky-backed contact paper like product. Larger screens were made from 3 or 4 inch diamond shaped pieces overlapped in sort of a herring-bone pattern. You were helped by "busy" background plates, as the pattern was still noticable.

The good news is that this fabric based material seems to have much more durability and is seamless (at least in the size I've seen). The bad news is that it doesn't have the same gain amount as pure Scotchlight.

I've been on the recieving end of plates shot with this system, and not been happy. The fabric relies on being perpedicular to the light source, and off-axis to the lens it drops off quickly in illumination. Now, every wrinkle is off-axis, and every scuff to the surface damages the alignment of the micro-beads...

The best micro budget green or bluescreen money can buy is a roll of 12' by 36' linoleum, the least expensive, flipped over and primed on the back, and then painted with the Composite Components green or blue. Put the roll up on a couple stands. Bounce light a 12' by 12' white solid overhead as your soft source (or if enough overhead room, punch through a silk, rim to suit and fix spill and there you go...

If you need to show feet interraction, then simply pull it down, and stretch across the floor, forming a natural "cove" of large radius, better than any studio cove. I've had talent actually become dis-oriented standing on it, as they can't tell where the "floor" ends and the "wall" begins and step back too far and poke through the surface as it gradually curves upward.

[Jeff Kilgroe]

Thanks, Jim.

I'm still considering options here... I'm trying to explore options for screens that would enable us to convert from green to blue with minimal down time or storage space for what's not being used. The linoleum is a good suggestion I hadn't thougt of it specifically and it would be cheaper than other plastic sheeting options I'm looking at... I'm thinking of building a fixed green screen and then using something like the linoleum or large plastic/rubber sheet (painted or made to match the proper shade of blue) that we can hang in front of the green when blue is needed. I suppose we'll just have to experiment a bit... I'm shooting for about 12 to 15 feet wide and the same for height plus a floor area. Linoleum could work well as it's 12ft wide standard (some product can be bought 15ft wide) and length is easy enough to account for the screen height plus rolling it out onto the floor platform "cove" area.