Green Screen/Chroma Key Questions

Homersapien

Homersapien

Well-known member
I'm just doing a bit of research into making green screens and had a couple of questions. I'm a complete noob to this btw ;)

Is the green of the fabric a particular shade or can it just be any kind of green and if so does anyone know the shade reference (Adobe colour or something lol)?

Is it better if the green is very luminous so as to stand out from most other colours?

Is it essential to cutout rear light or does it not entirely matter?

Does the fabric need to be completely flat lit or would it make a mad difference if there were shadows in some parts? Does it make it harder in Post if there is?

I've noticed in some films they use small black dots on the wall as reference points, what's the best way of reproducing this for a low budget method? Are these only necessary for large tracking shots or are they just as useful for small tracking shots? What's the name for this process?

I think that's all for the moment lol... any help is greatly appreciated guys and gals
 
Homersapien said:
I'm just doing a bit of research into making green screens and had a couple of questions. I'm a complete noob to this btw ;)

Is the green of the fabric a particular shade or can it just be any kind of green and if so does anyone know the shade reference (Adobe colour or something lol)?

Is it better if the green is very luminous so as to stand out from most other colours?

Is it essential to cutout rear light or does it not entirely matter?

Does the fabric need to be completely flat lit or would it make a mad difference if there were shadows in some parts? Does it make it harder in Post if there is?

I've noticed in some films they use small black dots on the wall as reference points, what's the best way of reproducing this for a low budget method? Are these only necessary for large tracking shots or are they just as useful for small tracking shots? What's the name for this process?

I think that's all for the moment lol... any help is greatly appreciated guys and gals
Click to expand...

Here ya go:

1. It can be any shade, just try to get it uniform. Having an even shade is critical to a good key.

2. The green doesn't have to be extremely luminous, but I would recommend a bright shade. Just remember that you want material that isn't too reflective, as that will increase spill, which is bad.

3. Yes, eliminate ALL rear lighting from the screen. Evenly light it from above if possible.

4. Flat lighting is the best idea. Shadows make for a difficult key. It may or may not be able to be done, but I'd recommend against it.

5. The black dots are just tracking marks. They are useful if the camera is moving at all. The name for the process is generally referred to as motion tracking.

Good luck! Any more questions come on back. I'm sure others have much greater experience in this than I do.
 
Um... I have to slightly disagree, especially if you are shooting DV or HDV, which is harder to key.

1) You should use digi or chroma green material. Buy it or rent it, but splurge. It is a specific green designed to key better with popular keying programs like keylight and ultimatte. Plus, the material itself is designed to better absorb and evenly distribute light. You can buy paints to. But I have found that there are just too many imperfections on walls and such.

2. Luminosity is not the right thing to look at, as you are not luma keying, but chroma keying. So chromanance is what is important... in terms of reflection, that is specularity versus diffuse-ness, your material should be very diffuse, and unspecular. It should absorb the light and distribute it evenly. In terms of the AMOUNT of light you want to hit the screen, that varies in style, but generally, on an IRE meter I like to hit around 40 or 50 IRE when my subjects are up around 60-70, so give or take, I keep the screen lit evenly at about 1/2 stop under key. If you make the screen too bright, you will get lots of green spill wrapping around your actor and you also run the risk of parts of the screen being out of the lattitude of your camera.
If you try and use a green bed sheet, good luck.

3. yes, you want your screen to be lit evenly. A waveform monitor is key here, even your laptop with FCP will suffice. Make sure your subject is a good distance from the bounced light of the screen, to avoid spill. Use soft lights like kino-flos to light the screen, usually they are positioned up high, angled slightly down to light the screen. If you can't afford Kino's you can use shop lights as long as your light is tungsten balanced and the unit has an electronic ballast.

4. How you light your subject is up to you. It's all about the IRE's You can increase contrast on your subject from fill to key to your hearts content, as long as you know how the screen is lit relative to your subject. Your subject could be evenly lit, or highly contrasted. Just avoid any light that casts a similar chromanence to the green screen. also, again, basic videography, avoid blasting out your subject, or making him too dark. Also, keep in mind that however you light your subject you BETTER match it with your background matte, or else people will instantly tell that you did shabby greenscreen.

5. Use black camera tape to make plusses. Or get colored ping pong balls and affix a small amount of velcro to them, with green screen material, you can affix the balls wherever you like. Keep in mind to track you need to know your cameras focal distance, f stop, x,y,z location, etc. And you will have to roto out those trackers in post.


No wrinkles, no shadows, no spill, proper IRE callibration, proper subject lighting, and a LOT OF SPACE. Those are the keys to a good green screen experience.
 
As far as simple lighting gos, I had good results with this set up about a month ago:

1200w HMI as key for green screen (left side of studio)
600w HMI as fill for green screen (right side of room)
300w joker bug with +2 magenta (upper left side behind screen, shined on talent back)
300w joker bug with shamira as talent light (right side of camera operator)

1 click key in Avid, minimal adjustments. Had great talent shadows too.

EP
 
Eric I don't think any HMI counts as simple for the indie who doesn't want to spend over 100 dollars per light unit and ballast for rentals.
 
I took some old paint into Home Depot and had them dump green tint into it. Put 3 coats on a wall to get it fully covered and lit it with a rack of 6 - 50 watt hallogens on a track light rail. Backlight the subject +1/2 to +1 stop and bingo.... perfect keying with Keylight in AE or Avid.

For my money the backlight is the key...pun intended.

Try it cheap and spend money if that doesn't work.

By the way I have some green felt that I think was made for pool tables that would work very well. I don't have much but I bet you could find something like it cheap enough if you can't use a permanent wall.
 
zoostory said:
Eric I don't think any HMI counts as simple for the indie who doesn't want to spend over 100 dollars per light unit and ballast for rentals.
Click to expand...


Possibly. The hmi's could be substituted with tungstens. I was more referring to the lighting plot, and hmi is what I had, and of course will key better than tungsten.
 
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