THE SMA FORUM ARCHIVE
Posted by Brickfilms, on 2001-05-12 00:36:53
Anyone here had experience with this (Chroma Key)?
I'm having trouble getting hte lighting/materials to work.
Any suggestions on a material that gives an absolutely even color? What if you want to film something that has either blue or green in it? Do you use a third color?
Also, what about lighting? If I want to film something against a chroma key but light it dramatically, say from underneath, how do I get it to come out properly?
This is my first foray into the world of chroma key. :)
Posted by MovieStuff, on 2001-05-12 10:41:27
You can use any color that is very vibrant as long as that color does not appear in the shot as well.
To get good edges with green screen, it is advisable to put pink or red rim lights on the model. Depending on the exact shade of the green screen (which produces an expected "spill" on the edges of the model) and the exact shade of the reddish gel on the rims, the two will combine and produce a white rim. This will go a long way toward getting a smooth edge.
There is actually a paint made for chroma key work. It is not cheap, but video supply houses generally stock it as well as cloth. However, if you go to the art supply store, they sell flourescent green poster board that works perfectly. Cheap, too. About 25 cents for a 20 x 30 inch sheet. You can over lap a bunch of them to make a decent green screen. Just make sure the overlaps don't produce a shadow. If you start at the top and work your way down, then the overlap will be up. This will avoid creating a shadow as you would get if the over lap were down. Again, any flourescent color will work; blue, green, orange, etc. Yellow is a no no. Not enough chroma.
Posted by sam_liberto, on 2004-10-23 13:19:46
i was checking out some stuff at vinton studios a couple months ago and they do it, the "real" way... i guess. i don't know if it's better but the animato i talked to acted like it was the serious way to do things.
so they do one frame with the normal lighting for the shot.
then they hit a button that switches the lighting so that there's just one light in front of the white background that gives you a dark sillohuette of the characters against a white background.
this gives you, not a green screen, but an alpha channel, i think is the term.
if you're working digitally, you would then seperate out the two videos. then important them seperately into premiere or after effects, layer them right on top of each other, and that will give you a perfect alpha channel.
for me, this was hard because i shoot digitally and i end up with just a series of numbered photos. so i made a template macromedia flash file. you kind of have to know flash to understand this: so i made an object of a graphic, where i can import the pictures. and then on the main stage i set it up so that each consecutive frame on the timeline references every other picture in the object. then you can just export it as another avi and it'll just be the one video stream. this was obviously kind of a pain in the ass to set up, but like i said, once the template is done, then you have it. of course, if you knew actionscript you could probably set that up with like three lines of code.
Posted by Nick H, on 2001-05-13 02:57:56
My trick is to paint a canvas screen blue or green, then use blue/green coloured lighting on it as well, to really reinforce the colour and avoid amy possible white shine. And like Moviestuff says, a complementary coloured (amber for bluescreen, pink for greenscreen) rim light on the subject to kill the blue light bouncing back from the screen. Evenness comes from balancing the lights on the screen, checking with a lightmeter to keep hot spots and falloff to a minimum.
Posted by Jim Aupperle, on 2001-05-13 11:05:41
You also want to be careful not to overexpose the bluescreen. As a general rule about 1/2 stop under 18% gray is the exposure you'll want to aim for when you use your spotmeter. Greenscreen is even more reflective so care must be taken. See if you can find the edition of the ASC Manual with the article by Bill Taylor on photographing blue screen. Bill is THE MAN when it comes to bluescreen photography.
Posted by opticalguy, on 2002-06-01 16:58:56
Well Jim does know his blue/green screen photography and I agree that Bill Taylor's advice is going to be top quality. I feel that the question wasn't detailed enough. Are you shooting on film and if so, which film stock? How is the shot being composited?
If you are shooting on film avoid the Kodak "Vision" stocks 5263, 5284, 5289, 5246, 5274, 5277, and 5279. They deliberately created these stocks with cross over in the color layers to give a desaturated look to the image. Really bad when you are counting on getting a very pure color to pull mattes. Use the Kodak SFX 200T film stock or the slower Fuji stocks such as 8521 or 8531. If you use the "Vision" stock you will have a hard time even in digital putting the shots together.
The foam backed blue and green screen fabrics are pretty good theses days and pretty darned durable as well. A random search yielded this source at http://shop.store.yahoo.com/cinemasupplies/chromkeyfab.htm for the stuff.
Posted by 1, on 2002-06-01 17:56:50
By no means an expert on blue/green screen, and only have a little experience with it. But what I did was go to the paint shop and look for the two closest colors that matched a blue and green screen paint in the swatch section. I had a gallon made up of each and both colors have worked well in premiere with no problems.
The Chroma Key paints are just so overly expensive for something that may or may not be a benefit. I mean the accuracy of the tone does not need to be that precise since different lighting and films and digital cameras all see things different anyway. At least thats how I feel about it.
One last tip is to find someone who has some green/blue screen paint, dip a paint stick in it, and send it to you to be matched by the paint machines.
Posted by opticalguy, on 2004-08-18 16:52:10
I just was e-mailed a query from someone about locating a good book on "chroma key" (I assume the person wishes to do bluescreen or greenscreen composites in digital video) and I thought I'd ask all the mavens in these boards.
Is there a good book that covers this topic or should I suggest he track down several books and articles on the various aspects of the topic? It might not be easy for anyone outside of the Hollywood area to get a hold of the ASC Manual with Bill Taylor's article on lighting bluescreens which would be what he needs to actually light the setup.
Posted by opticalguy, on 2002-06-04 14:14:14
>One last tip is to find someone who has some green/blue
>screen paint, dip a paint stick in it, and send it to you to
>be matched by the paint machines.
Marc, that may work OK but make sure that the paint doesn’t have any of the sparkly stuff (in some paints it's actually fish scales) that are often put into hose paints. They will reflect white light back to the camera and that will screw things up a bit even for digital compositing. Still try filtering the light on the blue/green screen as it doesn’t hurt to have a well exposed blue/green screen to work with. Mind you , you can cove for a lot in digital. I remember in the film RED CORNER they shot the lead actor (in a scene on a rooftop) against a blue-ish screen. No kidding! It was a "Williamsburg Blue" and not even close to a pure color. The folks at Digital Domain actually pulled it off (I think it was Jammie Friday who did it but don't hold me to that) so it is possible. It's also really stupid.
Posted by Jim Arthurs, on 2004-08-18 21:46:44
I'm not aware of any one book dealing with high end matting (there should be one, IMO).
A good starting point on the web is Bluescreen Bob's site...
This guy knows his stuff and has links to other sites with tips and such.
Posted by Jim Aupperle, on 2004-08-19 08:11:35
The two best books that I know of that deal with digital compositing are
"Digital Compositing for Film and Video" by Steve Wright (many step by step examples). Focal Press
"The Art and Science of Digital Compositing" by Ron Brinkmann (a bit more on the theory side). Academic Press.
The two books compliment each other well.
For blue screen photography I'd still go with Bill Taylor's article in the ASC Manual. That book cam be found on Amazon.
Posted by webster_c, on 2004-08-20 16:33:19
This is an old still from PDI's work on "Forces Of Nature". One of the compositors sent this out as an example of really bad bluescreen. Note how the production ran out of bluescreen fabric so they continued on with greenscreen fabric!
I haven't learned Shake yet (I'll be doing so soon), but one really great compositor who I worked with at PDI, Erik Winquist, showed me how you can use different color channels and pull various color difference mattes, even contrast mattes, in digital and layer them up to avoid having to do roto.
I don't know what company ended up doing the comp on this- these are frame enlargements from workprint of a Monkeybone shot. I'm still amazed that they were able to pull mattes for this. And in the end the shot was cut anyway (but it made it onto the supplemental section of the dvd). Justin Kohn and I were on the set, and there were three passes shot on consecutive frames: the bluescreen shot, the hero/shadow pass, and one pass not show which had specific lighting for the clear plastic woman.
A Kuper system controlled the switching of the lights. The bluescreen was standard blue paint, but coated with a clear-coat material that flouresced blue under UV light. We wore protective glasses while shooting, of course. Keep in mind that these were scanned from workprint, so the blue spill in the bluescreen pass would not nearly be as apparent were it scanned properly from the neg.
Posted by Strider, on 2004-10-23 16:32:53
This sounds like what Moviestuff calls the frontlight/backlight technique, and he swears by it. It seems like he mentioned a way to separate your individual frames but I don't remember the details.
Posted by Nick H, on 2004-10-25 01:32:42
I've been testing a "whitescreen" technique to use with my Nikon D-70, because you don't have to worry about evenly lightighting a white screen, just hit it with enough light to burn it out and it's white all over! My idea was to boost the contrast to make the subject go black, and save that as an alpha map. So far I'm getting a light grey outline around my character though, so I'm considering the frontlight/backlight option - take one with the white screen lit and the character dark, and one with the character lit but the screen dark, then split the odd and even frames into 2 sequences. To do that, you load into a program like AE (I use Aura), make frame 1 the first frame and reduce the clip to half as many frames, without frame blending. It will drop every second frame. That leaves your colour frames. Then load the original again, starting with frame 2, and cut the clip down to half again, that gives you just the even frames with the black silhouette against the white screen. That works as your travelling matte, or alpha map, or transparency map, and it can preserve soft edges and translucency.
Posted by Jim Arthurs, on 2004-10-25 21:46:01
The front-light/back-light technique was the first matting technique I ever tried back in high school with my super-8 camera... I remember zooming in with my little home-made dolly frame by frame on a little Flash Gordon rocket minature I had built, and later watching the footage on the projector with that alternate frame flicker... don't know exactly how I thought I'd ever finish the shot, but...
When I had lots of access to a real optical printer, it was very easy to separate out the frames, as the old Acme printer had a series of switches to control what pattern of frames would be cycled through before taking a picture...
I also used it quite a bit on the animation stand for matting my cell animation over live action... but in this case you could run the entire front-lit pass, then change film and do a back-lit pass on the range of cells. Worked pretty good.
Nick, that's a pretty nifty technique for separating out the frames... another possibility if someone doesn't have a compositing program is to use a batch file renaming utility after first selecting every other frame in the directory and moving them to a second directory. Then just batch rename each set of files restoring the sequential frame numbers. I'd really recommend making a copy of the original directory first before doing this... just in case..
Back in the days of the Amiga I wrote my own renamer with AREXX because nothing like it was for sale... boy, times have changed...
...Here are some renaming utilities I just Googled, there are so many of them that I think it's hard to come up with new names for them...
Posted by Betamax, on 2004-11-01 10:39:38
Err... I was wondering what software (preferably free) you can use for Chroma Keying. I really could do with some.
Posted by Strider, on 2004-12-30 23:34:20
If you use a Mac you can do compositing with some iMovie plugins from Stupendous Software. It's a lot cheaper than any oif the ones already mentioned. It won't give perfect results though, apparently vbecause it uses a codec that groups pixels into blocks of 4 rather than individually, so your edges won't be really smooth. (It's a heck of a lot easier than learning to use After Effects though!)
Posted by LittleFenris, on 2004-12-30 15:01:23
There are quite a few programs you can use for Chroma Keying. Adobe After Effects (Pro version is $1000), Adobe Premiere ($700), Discreet Combustion ($1000), Combustion's big brothers Inferno (I believe $100k or so), Flame and Flint, Apple Shake ($3000), Apple Final Cut Pro ($1000), and quite a few others. None of the ones I mentioned are cheap, but all do a pretty good job. I have personally used the Adobe products and Final Cut Pro. I find After Effects to be the best of the programs I have personal experience with when it comes to anything to do with compositing.
As for free Chroma Key programs, I have no clue.
Posted by LittleFenris, on 2005-01-03 00:24:49
To do something as simple as Chroma keying out a green or blue screen After Effects really isn't all that hard to learn. Really none of the basics are hard to learn in After Effects. Maybe for some it is, but I really find After Effects pretty intuitive.