Green Screen vs. Blue Screen

Nathan_Shane wrote on 5/28/2004, 9:54 AM
This is chroma keying on a budget mind you. :o)

I would like to hear others tech-thoughts about the advantages/disadvantages to using either a green or blue screen. Also, if you have tried using other colors screens/fabric, please let me know what might be worth trying, since fabric is fairly cheap.

I'll share my thoughts below, so please read on.

This is for footage of a person moving/sitting, etc...not just a static image.

I went out and found some fabric that was real close to being green screen in color, and it seems to work fairly well once one adjusts the Low/High Thresholds in the Vegas chroma keyer...but here's what I've noticed technically.

As I read online, better results can be obtained when keeping the subject at a distance far enough from the green screen so as not to cast any shadows upon the screen...which helps keep the green color consistent obviously.

But, since I'm in a smaller room, the subject is just barely close enough to cast some shadow, and this shadow is what is "the" problem area for the keyer. Now obviously I can add more lighting to lighten up the shadow, but my thoughts were this:

It seems to me that this same shadow cast on a "blue screen" would be less likely to show up, since a shadow is closer in color to blue than it is to green...but in my mind, if I go with using a blue screen (a color that is certainly not found in the majority of everyday objects/viewing) it would seem to me that I would run a greater risk of removing some artifacts of the keyed image I'm wanting to keep.

In other words, I can see where a green screen is such a more radical color, that it would seem the better choice for chroma key over a blue screen, but maybe not. So what's the real deal here? Why are there two different colors used in the first place? And why not other colors?

Also, if there are some additional FX I should plug-in which might help the chroma keyer produce better results, please let me know.

Comments

Chienworks wrote on 5/28/2004, 9:59 AM
I'm not sure why you say the shadow is closer to the blue color. The shadow is simply less light hitting the screen. It should still be the same color as the screen, merely darker. So on a green screen the shadow should still be green.

The only time this might not be true is if you are getting a large amount of bounced light of a specific color. For example, if your subject is wearing an eye-piercingly bright blue outfit and you have a lot of fill light in back, then this blue will bounce onto the background casting a blue haze in the shadow area. Generally though, clothing won't be that strong colored and backlighting won't be that bright. I wouldn't worry about this as a significant source of colorization of the shadows on the screen.

What you probably need is to light the screen more so that the shadows aren't as obvious to begin with.
Nathan_Shane wrote on 5/28/2004, 10:10 AM
I guess you right in regards to the shadow actually not having a color, but just producing a darker color of whatever the background color is. Yes, I'm thinking you may be right that I just need to add more lighting...but I'm still curious if a blue screen might work better. I may just have to go find some blue screen colored fabric and just try it out.
taliesin wrote on 5/28/2004, 11:45 AM
I personally only use blue screen meanwhile. It works better for me in postpro. Maybe this is related to my 3-chip dv camcorder and to the PAL signal, not sure. But I made many comparisons where I had exactly same lighting conditions, same foreground - only changed the screen - and it was more easy and more precise to adjust the blue chroma key then.
There might be many different conditions - maybe it's the NTSC signal, maybe it is 1-chip cameras - where green works better but my best way is to use blue.

Marco
Spot|DSE wrote on 5/28/2004, 12:28 PM
If you are working with DV, in the NTSC world where 4:1:1 reigns, then you should be working with green due to the higher luma content of the green.
To see what the luma content difference is, drop a color bar chart on the timeline, then drop a black/white filter on the timeline. Look at what color appears to be the brightest.
PAL is 4:2:0, and blue is a better choice in most circumstances.
Jimmy_W wrote on 5/28/2004, 12:28 PM
I like the blue also. But either way "light right to get it tight"
Nathan_Shane wrote on 5/28/2004, 1:37 PM
Yes, I am working with DV, in the NTSC world. I just experimented with using the Color Corrector (Secondary) with it's "Saturate Greens" preset plugged in before the Chroma Keyer, and that really helped to remove more of the shadow areas.

So perhaps with some additional lighting...and using some additional plugins like this, I may be able to come up with a very decent formula for better green screen. It sure is a learning curve trying to skillfully correct for less than perfect circumstances.
RickGB wrote on 5/28/2004, 3:42 PM
I prefer blue and use a 8ft x 6ft Lastolite chromakey blue fold out background. If you happen to be left with any fringing or spill around the subject, blue looks a lot more natural than green ('blue light from the sky'). Any shadow on the background is going to reduce the effectiveness of the keying, whatever the colour. Trust me, it is far easier to light stuff well in the first place ;-) A bit of distance between the subject and the background is essential. With my background I need 4-5ft minimum between foreground and background. For a lot of small scale stuff, I use four lamps with 60 watt domestic bulbs -- one at each corner of the background. In the UK, IKEA do some neat floor lamps about 5ft high which are similar to photographic lamps. Just £7 each ($12). These are ideal. This give enough light for a modern camcorder and keeps the temperature down. Some cameras give an image that's slightly darker in the corners, with this method I can give the corners a little extra light to compensate. Some people backlight the foreground subject using a pale yellow/orange light. This helps separation from the background. Again you need your subject some distance in front of the background for that to work (backlight should be at 45 degrees to them). With a blue background you'll find you will need to throw more light on it than on the foreground. Don't forget you can mask out the edge areas and corners using a blue or green cookie in Vegas. Yellow is another possible colour for keying and sometimes used in TV. Doesn't swallow up so much light, but haven't tried it much myself.
Fleshpainter wrote on 5/29/2004, 1:42 AM
I use mostly green for normal lighting conditions, for reasons mentioned above. However, for certain special fx conditions, and/or if foliege is included in the foreground, I use what I call UV Blue. White, (cheap) polyester fabric lit with 4 ft ultra violet tubes make a very easily keyable blue. An example can be seen here:
http://www.fleshpainter.com/video.htm
Click on "Stiffff Kitties"
The interesting thing here is that there were NO fx applied to the foreground whatsoever. This was the lighting conditions as they actually were. Under these types of conditions, UV Blue works well, green would have spilled all over the stage. Out in the sun, or on a normal set, use green.
jeff-beardall wrote on 5/29/2004, 6:00 PM
I shot close to 2 years of DV (mini and DVCam) chroma key stuff with vitual sets (on a Trinity system). We shot green exclusively; we didn't need to worry about blue eye colour on talent and clothing options were much wider (I bet you have a lot more blue clothing than bright freen in your wardrobe ; ).
That said, the green cast on to the talent was sometimes problematic, but we usually had a bunch of magenta side wash going on to fight it.
Points to remember:
Get as much distance as possible between the talent and the bg. A kicker or rim light for the talent really helps to seperate them from the bg.
DV is really not the best way to do chroma keying, but if you are careful, the results can impress millions (as my work did ; )
Any green fabric will do that has a good balance between brilliance and matte reflectivity. Recently I used a bright green (I'd call it lurid!) strechy fabric that is pretty bang on colour wise for chroma keying. The stretchy aspect is good because you do not want wrinkles or folds in the fabric (course, if you're stuck you can garbage matte that out, if possible). You can even paint a wall with matte bright green if it's a permaent install.
One of the coolest ways to do chromakey in the field is with some special fabric silk screened with parabolic reflecting paint. This coupled with a chroma key colour ring light around the lens made for some very quick and futuristic chroma keying.
Hope that helps.
jeff
ps Vegas rocks for chroma keying
Stonefield wrote on 5/30/2004, 11:27 AM
Chroma Keying is something I've messed with ever since I've had the ability to edit video on my computer.

Chienworks' video hosting site has several of my attempts at MiniDV chromakeying.

http://www.vegasusers.com/vidshare/

I've used both blue and green backgrounds both to various results. I would have to say that the better key's ( for me ) came from the blue backs. With limited lighting though, it was hard for me to cover the larger green roll paper we were using for some videos. Vegas 5's Beizier curve function will become very handy for carving out a "garbage matte" of those darker corners. Much more versatile than the cookie cutter filter approach.

While watching some of the behind the scenes footage of the Lord of the Rings and Star Wars DVDs, I noticed that sometimes for keying the background color was Purple ! There's a color that you might try as well.

I agree with Spot in that the color space in MiniDV leaves you kind of lacking for uses in keying, but that doesn't stop us from learning. Remember, all the skills we now learn in keying will shine when we move up to the next HighDef format. ( Whatever that will be )

Stan
Spot|DSE wrote on 5/30/2004, 12:52 PM
Actually, keying will become MORE difficult with HDV, if you are referring to that format. We'll have more pixels, but less information contained in the sampled information since HDV is a 4:2:0 format.
The beauty of how Vegas resamples at 4:4:4 does make it easier to key on any number of color variables. I've done successful keys from hotel room wallpaper.
In our PPT VASST, we spend a quarter of the day doing nothing but keying, and it's amazing how many folks are under the impression that ONLY blue or green can be used.
General rule of thumb is that for digital, properly lit green is best. For analog, blue is generally best, but in Vegas, most any consistent color works well.
Nathan_Shane wrote on 5/30/2004, 4:39 PM
Well, I went out and purchased two different blue fabrics, one being much darker than the other for a really good difference comparison. I have to agree with Spot that I can see now that any color background could be used dependent upon what the final project may be.

I did notice that the darker blue background left an almost undetectable edge when keyed out which blended in with almost any image or video I used as a new background track below. So I can see a benefit to using a darker colored background for the screen because of a less visually noticable edge, which adjusting the blur amount worked the best overall.

Even though it seemed that the lighter blue fabric seemed to key out the easiest and with somewhat "slightly" better results, the light blue edge (like the green edge) which just ever so slightly couldn't be keyed out, made it more limiting in what I choose the use for the replacement background image/video.

Overall, I was able to get very useable results with Vegas, so I'm quite impressed. Thanks for all the suggestions and information, very helpful indeed.
jaegersing wrote on 5/30/2004, 8:47 PM
I shot some video at Spot's production seminar in Singapore last year, and was able to make a direct comparison between blue and green screens using the same subject, lighting, camera, etc, only changing the screen colour.

Using Canopus Edius for the chroma keying, I could quickly and easily get a very clean key when the background was green. With the blue background, i was not able to eliminate some blue spill at the subject's edge. Maybe it would be possible to get better results if I spent longer adjusting the NLE settings, but the ease of keying out the green background convinced me that this is better for DV than bluescreen.

Richard Hunter