(this page was written in 1996 by Greg McMurry)

One day I was discussing an interesting issue with my best friends and famous Video Technical Director. Monte Swann and I were discussing the most commonly asked technical questions. We both agreed that as a Video Technical Director, there is a tie somewhere between:

Now that I am a Visual Effects Supervisor, the most common question I am asked is:

This question is so often discussed and asked I thought I would include it in my "SANDBOX".

When you photograph a subject in front of an, "hopefully", evenly lit, pure blue, or colored, background, either you want to have the subject seen on the pure background or you are probably shooting bluescreen.  This means that it is probably intended that the background will be replaced with another image that is already photographed or a background that will be photographed in the future.

Before I get into a discussion of blue vs. green, from now on, and I mean forever, when discussing this compositing technique, I shall use the term bluescreen to mean any type of "contrast screen" background shot for the purposes of some future compositing.  By the way, in my work, I do this all the time.

One can make a bluescreen composite using optical / photochemical techniques for motion pictures or still photos, electronically in live video, and digitally to digitized images.  These digitized images can either be electronically generated or transferred from film.

First of all, the term bluescreen is so commonly used, that I prefer to use the term as one word.  One of the beauties of the English language is that we can create words through common use.  I use the word all the time so I think the term deserves its own word.  Not hyphenated and not in quotes, just bluescreen.

The decision of which color to use is usually based on one simple fact.  What color is the subject?  If the subject is green, you would probably choose not to use green as a background color that is to be later separated and replaced.  The same goes for blue.  The simple truth is that green has become so commonly used, that many of even call it "bluescreen" and for that matter, many people refer now to bluescreen as "greenscreen".  For me, I probably use green more often because the green we use in a screen backing is quite far from any normal green that might appear in an actors costume or props.  The point is that we use a color that is as different from other colors in the scene as possible.

Technically, most people I know would rather use bluescreen.  It is said to be an easier separation and if you get a little "blue spill" it is not so unpleasant as "green spill".

The compositers at most visual effects facilities, can do bluescreen compositing work just as well with green color screens.  The things they have trouble with are usually either uneven lighting of the screen or spill, not the actual color

Before I was a Visual Effects Supervisor, I was involved in the video business for almost 20 years. Chroma Key was my first introduction to the use of bluescreen compositing.

Back in the early 70's I was an Assistant Director at WTOP TV Washington D.C..  One of my jobs was to "A.D. the news". We all have seen the images behind the newscaster.  "FIRE" it would say over Gordon Peterson's shoulder as he would read a story about some idiot that tried to warm their apartment by burning newspaper inside their electric oven during the winter.  "ARMS TREATY" with a picture of an M1 Rifle would be the graphic we would put behind Max Robinson as he would lead into a story of some international arms discussion.

In those days, Chroma Key was used for a variety of graphic purposes during a news show.  Having the weather man stand over the weather map was only one of them.

This reminds me of a funny story that happened during a news show. I can't remember if my friend Steve Howard was working with me on this show, but I think I was the "floor director" during this evenings newscast.

Our weather man had a blue panel that he should slide over the weather map behind him in on the side of the news set.  I was standing right near him.  He had just finished explaining the high and low pressure areas and he slid the panel over the map.  There was supposed to be a satellite image of clouds and other stuff on the blue panel that was chromakeyed into the panel.  He gave the panel a little too much of a shove and it slid over and right out of its track.  The shaking that the set wall took began to topple the entire set.  The weather many quickly finished and the director cut to another camera, just as the board fell on the floor.  The other two news casters tried to pretend like nothing was happening but they couldn't sit there too long.  I could see that the whole set was going to fall on top of both of them.  I opened my mic on my head set and tried to get the attention of the director.

"Earnie" I said, "the whole set is falling over!!!"

Of course nobody really knew what I was talking about, so I had to speak louder and louder.  While on the air, it was forbidden to speak while on the set.

"THE SET IS FALLING OVER ! !" I said in an even louder voice.

Just then, the director began to see all the walls moving behind the talent.  He told me to cue them to commercial and he cut from the live camera to the bumper slide.

"YOU'RE OFF", I shouted.

Then suddenly, both of them leaped over the desk as the set fell on their chairs and desk.  I remember one of them was J.C. Hayward but I don't remember who the other one was.  I was amazed how JC could jump over the desk in a skirt so easily.

As it happened, the news set was built by an outside contractor and was properly installed with big steel ingot stage weights, braces and all the necessary support.  Over the years, though, this began to be the place that people would go to borrow a brace or a weight. After a while, all it took was a whisper to knock the set over.  We all learned our lesson on this on.

Anyway, this set was mostly blue.  The words EYEWITNESS NEWS was printed along with our Channel 9 LOGO in two different shades of blue behind the newscasters.  All the blue was the hue but it was a different density.  This seemed to be fine with the chromakey system we were using.

In the early days of television, all the cameras had three or four imaging tubes.  At Channel Nine, we had 4 each, GE 250 color cameras. These used four plumbicon imaging tubes.  Red, Green, Blue and a monochrome tube for luminance.  Some day there will be a television technical section in my sandbox for you to read, for now I will get away with just mentioning this.  One of the engineers at channel nine designed a clipping circuit that we used to do our chromakeys.  There were currently available chroma key systems, but they were tricky and expensive.  Television equipment was really expensive in those days.  It still is today but even in those days, it might cost $150,000 for a single television camera.  Ours was a circuit that would be manually switched into the desired camera and connected to the blue channel.  It would clip out the dark areas of the blue image and create a high contrast monochrome image.  The camera control operator would switch that output into the key input on the production switcher and we would have chroma key !!!  It was cheap and dirty but it worked.

We always felt that blue was the best color as it was the least color that was used in flesh tones. We had to be careful what colors the newscasters would wear.  There is a lot of blue channel in many colors in television.  Flesh tones were easy to separate though.

The television cameras we had at Channel 4 in D.C. were RCA TK-44s.  RCA designed a chroma key module into each of the cameras.  The key signal would be fed separately to the studio switcher so we could do chroma keys.  I actually purchased ALL of the TK-44s from WRC-TV / NBC Washington and used them on the MURPHY BROWN SHOW for 10 years.  Afterwards, I think they were probably never used.

Later, chroma key became a function of the video switcher only.  Switchers would take RGB feeds from each of the cameras and we soon had the ability to do a chroma key from any color we wanted.  Then some chose to do strong reds and even green sometime, but blue was still the favorite.

At NBC, we were slow to adopt chroma key for all our "over the shoulder" graphics in news. We had these rear screen projectors called "VISMO" that were used for years and years. I think it was sort of a Union problem that kept them on line.

Someone finally designed a choma key system that would work with composite video. It didn't work too good, but every switcher would soon have one. I think all these systems did was have a decoder built in to separate the RGB then the job was the same as we had been doing for years.

The basic chroma key concept is to create a "third source key signal" from a the combination of a particular hue and intensity gathered from a particular color in a video signal. Most chroma key systems use conventional monochrome television key sensitivity and clipping controls in conjunction with the controls used to set the color selectivity controls in the chroma key system.

Ultimatte is a registered trademark of the Ultimatte Corporation. Petro Vlahos, did a bunch of work in the 60s attempting to invent a better soft edge traveling matte system for use in motion pictures. Not enough resolution for film, but the R&D was revolutionary for video. The first Ultimatte was the result in the mid 70's and you had to be a complete "transistor head" to operate one.

I have asked several people about the using Ultimatte and Chromakey and have included their descriptions here.

Bob Kertesz bob@bluescreen.comBobs Description

Steve Bradford's Bluescreen Page

For me, the term "bluescreen" has become a terminology for a technique, not the color or type of screen. Usually these discussions come up long before the actual type of screen can be chosen.

If during a production meeting I were to say:

"I will use a composite screen"

I would be explaining myself for much longer than anyone would want to listen.

My favorite lighting for bluescreen is fluorescent. I built all my own fixtures but most people use Kinoflo 2 or 4 bulb fixtures. They can be rented easily and you can get the bulbs you need form Kinoflo as well. I have tested most every bulb I could get my hands on and see little difference in the bulbs that people have had their names printed on. Basically, to light green screens you use green bulbs. To light blue screen you use the super blue diazo blue print bulbs. Most all the fixtures used use the power normally reserved for 8 foot bulbs on 4 foot bulbs. Sure the bulbs burn out faster, but you get more " poop " out of the them.

I don't put much stock in all these special expensive bulbs that everyone is selling.

It is a good idea to use electronic ballasts for this process. The old magnetic ballasts can drive you crazy with hum and flicker, particularly when you over crank the camera. The new electronic ballasts are lighter and most run at 2000 plus hertz therefore providing little trouble with your camera shutter angle or speed.

I like to light the screen at the lens stop and always attempt to keep the screen even to within 3/10 stop when measured on my Minolta Spot Meter

It is always a good idea to black out all screen that is not needed. It is easier to garbage matte out the remainder of the picture than try to remove a ton of green or blue spill. Grips usually have a bunch of black "dube" that can be draped over unnecessary screen or floor. Remember, you only need to cover the area behind the subject, not the whole image area.

It is also my practice to not panic when there are a few frames that our subject might violate the screen border. Roto of a few frames is trivial and when balanced with the time it might take to re-hang a screen is a good and cheap solution to a 5 frame problem.

With a little practice, you can train your crew to light your screens for you quickly and efficiently. When I get on a big project, I always try to use the same couple of electricians form shot to shot as the learning curve is short and by the time you do you third screen shot, you can be at the craft service table for most of the process, only coming in at the end to make some final adjustments.

Finally, most people make a big mystery out of a process that is really the easiest part of the complete process. You do have to get it right, however, but once you get the technique down, you can spend your time on a much more important part of the job, setting your camera angles and designing the shot so it looks real.

Jack Grossberg, the UPM and Executive Producer for Brainstorm, was doing a film called STRANGE BREW. Yes this is the famous film with the McKenzie brothers. I met many people on that film but probably the second most memorable moment I had was this call from Jack.

Jack had met Rhonda and I on BRAINSTORM. BRAINSTORM was the first film ever that I was able to show my video expertise. I had designed many 24 frame video decks, synchronizers, and a remote control system for running many VCRs at a time. I had quickly gotten the reputation as the movie business video maven. Since I had been in broadcast engineering for many years prior to getting involved in the movie business, this small format, low resolution video stuff was a snap for me.

"I need you to bring over some bluescreen stuff to MGM next Tuesday because we are doing, you know, a "flying dog" shot", Jack said over the phone. " We need to be able to move the camera just right!"

Now, though I had been working in visual effects engineering for many years by this time, my live action filming experience was not that great. I had been discovering, however, it was not brain surgery!"

"Oh sure, I can handle that !" I exclaimed.

Of course I hadn't the foggiest idea what he was talking about, but I knew I could figure it out.

Um, "flying dog", I already knew what an "Abby Singer" was, a "gobo", a " 'round-de-round". What was a "flying dog" shot.

After talking to the camera assistant, by the way, this is where I met Wayne Baker, with whom I have become quite good friends, I figured out the equipment they needed. They wanted to take the B&W image from the video tap on the camera and superimpose it over a 3/4 inch video tape while they were filming. Now, this all seems silly now, as it is done every day, but in those days it was not only next to impossible, it took about $25,000 dollars worth of equipment, two video engineers and some notion that the video tap image was going to be worth looking at. Usually it wasn't as CCDs were only a dream, and genlock was only a dream at Panavision.

HOW I DID IT !

I had this little Panasonic video switcher that would add a black burst to a black and white video signal. This was the most important factor in the whole process. You could not genlock the film camera video tap and I had this ADDA frame synchronizer that would sync an image as long as it had a burst. I took the output of the camera tap, into the ADDA and fed it to another Panasonic video switcher that had a few effects banks, including key and mix. I then brought in my BVU-200 video deck, and my CVS-504 TBC. The CVS-504 was one of the first TBCs ever. It cost about $12,000 in 1975 and it barely worked. It could, however if tweeked just right, time base correct a 3/4 inch tape and make it possible to use it as a synced source on an effects switcher.

Now I had the whole thing working. I could play back a tape, dissolve back and forth between the tape and the non-sync video tap from the Panavision camera. I still didn't have the foggiest idea what a "flying dog" shot was, though it was not high on my mind. I had the equipment ready and Rhonda and I showed up on the set.

So Rhonda and I marched on the stage at MGM with all of our stuff, I began to set it all up. Then in walked all the grips and special effects guys. Then in walked an animal handler.

"Oh my god!", I said to Rhonda, "they are going to fly a dog".

Yes, a "flying dog" shot was just that. We were going to "fly a dog"

The other thing that was interesting about this process was what the image looked like on the video tap. We had two problems.

1. What the image looked like.

2. The size of the image.

3. You cannot key from a bright background image on a B&W camera

First of all, I was looking like a genius. I could play back the tape and the grips could move this special arm that was built to hold the dog and he really looked like he was flying. We had a huge blue screen and it was simple to see what we would get on film. I found that the best use of the process was to "half lap" or half dissolve between images. The operator could then move the fader bar back and forth to see just the amount of each image that was desired.

We did have an unusual problem that none of knew how to deal with. This was the first time that I realized one of my earliest of Greg's Rules.

"Don't be fooled by what something looks like on a video tap, video will record an image NOTHING like film".

There is this great spongy material that we use for Green and Blue screens. I love the stuff. It cleans easily, can be stretched over anything. You can use a frame or hang it from ropes And, it is not too expensive. The material can be rented in many sizes from several sources in town. I rent mine to people all the time and I keep building new ones.

If you are interested in building your own screens and fixtures, check out my bluescreen/greenscreen resource page.
 
 

With these screens I can do just about anything that might come up. If I cannot do the job with these sizes, then someone should have told me something so I could prepare for the one out of 500 bluescreen situations.

There is a particular lighting package I travel with also that I discuss in the "LIGHTING FOR BLUESCREEN" section.

There are also several good paints for use in the bluescreen process. Most work pretty good, but I would almost always rather cover with material rather than paint. Once you paint a material, you can never re-match the paint for touch up without keeping the same batch of paint around. The material is more consistent with color from batch to batch as well it is more durable. I can have someone walk around on the material and simply brush off the footprints. With the paint, you have to repaint almost all the time.

The most popular paint is manufactured by ROSCO. When I have to use paint, I use the ROSCO brand paints

At a stage I built at VIFX, I had a huge bluescreen (actually blue) that was painted on stretched muslin for about 100 feet around our stage. I have repainted it about 4 times and it still works fine. I usually have to paint the whole thing each time it gets painted as the paint never matches from batch to batch.