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Johnny
05-23-2003, 07:15 AM
I borrowed the Matrix DVD from a friend, and in watching the 'making of' portions, you see, and they mention use of, green screens for backgrounds and other things.

I wonder if anyone can answer a couple of questions about the green itself..

first, is it some particular green to which cameras are ultra-sensitive such that it can be PERFECTLY masked out later, and

second..why/how does it not cast a green tint onto the actors' skin?

thanks for any info!

Johnny

Darttman
05-23-2003, 08:58 AM
Yes it is a "special green" called chromakey green (I think there is a blue also) As for shooting scenes with it you will notice not many bright or light colors are in shot due to the fact that it would bleed. Matrix is perfect for chromakey shots because everyone wears black all the time eh? :)

You can buy this paper at most lager photography shops or a place like B&W photography online. Last time my boss bought some several years ago and it is still in use (still shots)

I am sure there are all sorts of tips for using this stuff like proper lighting is very important do a search on the net there has got to be some articles on it

mlinde
05-23-2003, 09:58 AM
There 's a great book on compositing that might help, unfortunately my copy is in a box somewhere (as I am moving on Tuesday).

In addition,when you do green or bluescreen work, you need to pay attention to the costumes, so you don't have bleed issues. Most clothes have tints of blue or green in them, and you will need to compensate for that.

Also, when you light the shot, it is usually wise to set up a backlight with the complementary color to help offset/frame the character and keep the green halo away. You will have to experiment to find the appropriate brightness for the backlight to offset but not distract.

eblu
05-23-2003, 02:02 PM
ILM likes to use highly reflective mylar in the background, and reflect the green on to it, in this way, your actors dont have bleed. Its not appropriate for every shot, but its the cleanest green screen around.

and its not the camera that is sensitive to the green, it just that the green and the blue are usually not as prevelent in any particular shot. You could easily find yourself in the position of shooting someone with green painted skin under blue lighting, and guess what.... your "green screen" might Have to be red, or orange.

Beamtracer
05-23-2003, 06:52 PM
Skin is basically red. Whether someone is from Africa or Finland there's still a basically red hue to skin.

This is why chromakey backgrounds are rarely red. Blue and green work better.

You often see bad chromakey jobs on live TV, where the "talent's" head has a blue halo around it. Especially live news style interviews where they put a fake backdrop in with some location around the world. This often results from bad lighting of the bluescreen, or bad keying equipment in the TV channel.

Weather presenters have been looking a bit better in recent years!

Darth Mole
05-25-2003, 03:23 AM
If you're using After Effects to pull the matte, I can happily recommend a little plug-in from dvGarage called dvMatte. It's really good at getting decent mattes from not terribly good source material like home-made bluescreens and DV cameras. (of course, the better the DV camera - like, if it's a three-CCD model - then the better your results).

And it's dirt cheap too.

Jimzip
05-25-2003, 07:50 PM
Mostly, there are lights directly on the actors, that don't let the greenscreen reflect light onto them. But the lights have to correlate to the lights in your fake scene. Also, usually a flouro light/s shine onto the green screen to illuminate it. (That's what I know anyway, I'm positive there are many methods to do blue/greenscreening.)
That's quite clever of ILM. Trust them to think of different methods like that.

Jimzip:D

aloysius1001
05-25-2003, 08:34 PM
its not the camera that is sensitive to the green
Actually, that statement is not entirely true. Chroma Green is generally a better choice if you are shooting in DV. Dv cameras are more sensitive to the chromanance values of green so it is easier to pull a key from a green screen than a blue screen.
As far as green spill, most keying software has spill supression to remove the spill to some degree. Usually when shooting green/blue screen shots you have your actors stand as far away from the screen as possible, this helps reduce spill and also makes it easier to light your talent to match the lighting of the scene they are to be inserted to. Just think, if you wanted to put your actors into a night scene. This would be hard to do if they were completely bathed in light. Sure, you could color correct and make them darker by boosting the blue level and cutting the red, but if you can set them up far enough away from your backdrop you can light them to match a night time shot while still evenly illuminating your green screen.

BTW....
We need more posts like this! Compositing is an important part of 3D that we all should learn more about to improve the quality of our work. How about a dedicated compositing forum Newtek?

munky
05-27-2003, 02:24 PM
I think that quite a lot of the green screens used in ads and films are self lit or backlit to avoid "spill" I've even used a grey as a chromakey in a live studio.
Agood place to check out is the How it was made section of the SPY KIDS 2 dvd
Also you can check out a company called REFLECTMEDIA (reflectmedia.com) who do a reflective material keying system with the green or blue coming from a ring of led lights fitted to the camera which means you can light for greenscreen in minutes and as the reflective stuff is a cloth it's very portable. This system was originally developed by the BBC for their virtual studio and has been taken on and developed by reflectmedia. I've seen it used and it's very good.
Special Effects the history and technique by Richard Rickitt is also a useful read.

regards

paul

hope this is useful to you guys

Halsu
05-27-2003, 04:28 PM
We use a plastic carpet, painted with dedicated blue and greenscreen paint, and light it with regular lights. With modern compositing software (we mostly use ultimatte for keying), the spill isn't that big a problem - the software can handle / suppress it quite well.

eblu
05-28-2003, 05:45 AM
Originally posted by aloysius1001
Actually, that statement is not entirely true. Chroma Green is generally a better choice if you are shooting in DV. Dv cameras are more sensitive to the chromanance values of green so it is easier to pull a key from a green screen than a blue screen.


this is probably because of the YUV color space, but the original point was that theres nothing "magical" about the paint so much as the color has just been found to be the best overall solution. In the end the only thing that matters is results.

why does the YUV color space like green? well the brain trust that invented the YUV color space found that green has most of the percievable luminance so they use the whole channel of green, when they only use half of the data from both blue and red channels. Thats right, for every green pixel you make, 1/2 of a blue pixel, and 1/2 of a red one are used. thats one of the reasons that intense red blurs (blue does it too, but its less noticeable). This is true for all Video, but not film.

aloysius1001
05-28-2003, 04:18 PM
Thats right, for every green pixel you make, 1/2 of a blue pixel, and 1/2 of a red one are used. thats one of the reasons that intense red blurs (blue does it too, but its less noticeable). This is true for all Video, but not film.
If I remember correctly, this only applies to the DV format, but I thought the compression ratio was even more. DV uses a 4:1:1 compression ratio, but I thought the ratio read for every 4 green pixels you get 1 red and 1 blue. I think analog formats use a 4:2:2 ratio, ie Beta. Mabey they are refering to bits(DV's 4:1:1 ratio), in which case I'm still not sure I have a handle on the math.

Beamtracer
05-28-2003, 04:40 PM
Originally posted by aloysius1001
DV uses a 4:1:1

This varies between PAL and NTSC, and whether it is mini-DV, DVCAM or DVCPro, or the other DVC formats appearing on the market.

In the case of 4:1:1 (miniDV in NTSC) the '4' is referring to the luminance, not the red value.

aloysius1001
05-28-2003, 04:57 PM
I thought the 4 refered to the green value combined w/the luminence value. I know it isn't referring to the red.