Cecilia's Excellent Blue Screen Adventures
-or- How I learned to love ImageFX's CineMatte Hook
[note: click on any of the image thumbnails in this article to see them in full size]
The use of trick photography and special effects (a term first used in the credits of the 1926 film, What Price Glory) walked hand-in-hand with the infancy of still pictures and later with film. Those who were lucky enough to pioneer this new technology, built their own cameras and projectors and invented new techniques as they went along. Many films made a hundred years ago, were just excuses for showing off special effects. Things haven't changed much it seems.
Early film pioneers realized that they could amaze viewers by placing unlikely events together on one screen, such as a human standing on the moon (well, it WAS unlikely then). In the 1890's, movie goers began seeing lots of new effects, including: glass shots, double exposures, models and miniatures, rear and front projection and in-the-camera matte shots. "Wow", the audience would shout! And that "Wow", is what art is all about.
Essentially, all of these different techniques had one goal, to get two or more visual elements to appear in the same frame. The use of computers has not altered this simple reality in any way. Computers, for the most part, just emulate what is done in the real world.
Move forward in time about fifty years to when travelling mattes, using blue screen, began coming into popular use. Petro Vlahos was commissioned by MGM to devise something similar to the sodium matte system (yellow) he had created for Disney. According to him, it took six months of thinking before he came up with the blue screen idea. No joke. (He describes sitting at his desk looking out onto Hollywood blvd spending hours everyday just THINKING!) After all this brainpower they first used this clever technique in the 1959 film Ben Hur and the rest, as they say, is history. In 1976, Vlahos founded the Ultimatte Corporation.
Move to the present.
Those of you lucky enough to have a Video Toaster Flyer and who want to make your own epics would do well to learn about blue screen. First, a few rules for shooting people against blue. The point is to solve most of your problems from the start, so when you are done shooting the rest can go smoothly.
Rule 1: Complete Separation
The whole point of blue (or green) screen technique is to physically separate the erstwhile actor from his blue background so that you can plop him onto any environment you like. However, before you start up ImageFX, intelligent lighting and subject placement will avoid that most horrid of Hollywood activities: "Fixing it in Post". Only 20 year olds in suits say such things. I expect better of you all.
Rule 2: Sharp Edges
You can actually buy Rosco blue and green paint and make a blue room or wall for your epics. The paint is expensive - about $50 a gallon. Ouch! But if you find a place that will mix paint for you, it's possible to create a nice not-too-dark, not-too-light, blue. Just make sure that no red gets added to it.
There is paper that's made for this purpose, too. I've read somewhere, though, that different kinds may have red in it due to chemical additives in the fire retardant that gets applied to the paper.
The CineMatte Hook (ImageFX v2.6)
The CineMatte Hook was first introduced with ImageFX v2.6. Since then, it has been much improved; but, this earlier version can still get the job done. First, you need to have a good understanding of the hook's basic parameters - and then we'll get into the hands-on tutorial.
The options available in the CineMatte Hook:
When tweaking numbers in CineMatte, think of it as a fight between the Forces of Dark and Light (go ahead and laugh but you'll all remember this). After darkening the background and lightening the foreground, the "fight" is localized in the edges of the actor. Look at the fingers, the hair (ack!) and other troublesome areas. These numbers will pull the darker and lighter greys over and around these edges. When you can no longer see the blue, but before the hair gets clipped off - that's when you've got it right. And peace reigns in the land of the good. Hurrah!
This older version of CineMatte may seem "simple" but I used it for the film, Hologram Man. We (AI, Effects) were contracted to do 140 shots in three months and ended up doing 191 shots in the same period of time. We were given digitized film footage of the actors against a blue screen. I had to pull them off the blue and save the resulting "Beauty" pass (beauty means it has the color information) and the resulting matte. There were four people on the job, with three Amigas. The next time someone says Amigas can't be used for professional jobs, tell them THAT!
Nova Design has also made a version of CineMatte for use with PhotoShop and Aura. It works basically the same way. When I have a moment I may write a mini "how-to".
The CineMatte Hook (ImageFX v3.2)
Let's put the new version of CineMatte to the test, shall we? Because pictures can express a thousand words, we'll look at two examples of blue screens.
Learn to really LOOK at the screens. Hologram Man has a generally even blue screen. There's no heavy shadowing; but, there is a lot of blue spill on the guy's right arm and on his dark hair. Those silly, shiny leotards! Whose idea was THAT?! We'll look more closely at the gantry picture later, because it's really bad.
If you wish to follow along with the tutorial, you will need to first download the tutorial file archive (67K) here: blue.zip. Note that the images we have provided for this tutorial were saved in the JPEG format in order to keep the archive size minimal. Ideally, you would do your work with lossless image formats.
Bugs, Bugs, Bugs
On some systems, v3.2 of CineMatte, has a couple of bugs that you need to be aware of . Hopefully, these bugs will be fixed soon.
Examine the result. If the Force Black value is about where it should be, there will little or no blue showing on the edges of the subject. If it's too high, you'll see areas along the edges that are cut away. Pay close attention to the hair, it'll be the first to go because it is the darkest area.
At this point the white parts of the matte are probably mottled in greys. To see it, we need to look at the hidden Alpha buffer.
Next we set the Force White value.
Above is an example of the Force Black value at 96, the Force White value at 76 and everything else at default. Composited against the green leaves, you can really notice the blue edges. This matte is okay, but there is still more that can be done to improve it.
Wow! That's a vast improvement. You could leave it like that or play with a few more settings. I feel the matte is good but the edges seem just a little too sharp. The actor looks like he has been cut out and pasted on. Let's see if we can soften him up.
In the case of HologramMan, Gamma needs more room to squeeze those blacks and whites.
But there's still that annoying spill to deal with.
The Output Menu:
This works sort-of like it did in v2.6. Just make sure you've already determined what the matte will look like. Then you can go to this menu and use Protection. If you want to keep a little of the background color in the actor a 1 is usually best. However, we want to remove some of that color.
Oooh! Much better. Softer, lighter edges. Ok, I admit it's a bit compulsive. But that's what you need for this kind of work.
Let's look at the gantry picture now. Look at the extreme variations in the blue. Deep shadows angled next to much lighter tones of blue. A real mess. Plus, this shot needs a Garbage Matte. This is an old film technique to cover shmuts (dirt), pieces of the set that got into frame, heavy shadows or anything else that would interfere with making a perfect matte.
We briefly pause while I explain.
How to make a garbage matte: On a black buffer with the gantry in the Swap buffer use one of the drawing tools (like Filled Box) to put blue spots where needed. Turn on the light-table to see through to the gantry picture. You can use a pure blue (0,0,255) or pick a color from the scene you are working on. Then Composite Matte. One garbage matte will work if there is no or little movement around the "garbage". Otherwise you'll have to make several or even (horrors!) a "traveling garbage matte"!! ACK!
Just to see how bad this screen is, look at this:
CineMatte made this with all defaults at 0. Now, that's one UGLY matte. Let's see if we can do better.
Ok, I left the little red register dots. These indicate the part of this set that moves during the shot. Useful for "tracking" if other compositing has to occur. Later it would be replaced using a garbage matte.
That is without using a garbage matte, which would eliminate some of the compensation required to overcome this hideous blue screen (such as removing the bricks in the lower left hand corner). I just wanted to show you how powerful CineMatte is.
The Keying Menu:
This works similar to the way the Ultimatte process works, where you use a reference shot of the blue set without the actor in it, called a Clean Plate, to help make the foreground matte. If you are shooting all this yourself, then it should be no problem to get a quick shot of the set without the actor. Otherwise, you can just create your own "blue set" using blues from the existing image. For our tutorial, we'll just use a simple blue plate (R:0 G:0 B:255).
OOH, all pink. But this does have a real nice separation - and that's what we want. Remember, you don't have to do everything all in one step.
Why did I use only the green channel? Well, it made the best distinction between the blue screen and the actor which ended up making the cleanest matte. Using the red channel would have resulted in a green guy against blue, which is just too close in tone. While, using only the blue channel would have resulted in a yellow guy against a green screen, and that would never make a good matte.
Well, that looks like a nice healthy matte. When it comes to making a matte just remember there's more than one way to skin a cat. So to speak.
"Pulling Blue" requires some tweaking. Lots, really. I always render the hook, not just go by the previews, so that I can look at the Main buffer and the Alpha buffer. Magnify the area along those edges and see that everything is good.
Below is the NovaDesign demo reel. About three shots from HologramMan are included.
Putting Things In Motion
Once you've made the perfect matte, save the CineMatte settings and record a macro.
Now batch processing can commence. If you are saving a Flyer clip you'll need to use IMP and in the PROC field write in the name of the script you just made "rx myscript.ifx". Save a 24-bit animation since that's what a Flyer clip is and away you go. In AutoFX you can save single frames. Just load your script, load a Save_As script and you're ready!
Having worked on other systems and with other software (AfterEffects, PhotoShop), I can tell you that ImageFX and CineMatte are swell! It's not overly complex and is a bargain to boot!
My purpose in writing this was to prove that having an Amiga does not mean we are left out of making special effects. Or making professional effects. What's required is brain power, not merely processing power. You can always make your computer faster, but you have to make sure that your knowledge is up to speed first.
Well, that's all kids! Now, go out and make a movie!
Petro Vlahos originally devised the Ultimatte process for film; creating blue, green and red separations for the negatives and then printing and compositing them in a certain order. The complexity of his technique is the reason that such amazing composites can be achieved - even around flying hair, smoke and other semi-transparent objects.
Ultimatte's plug-ins for AfterEffects and Premiere, require a reference sequence (for screen correction), a foreground scene (the action) and also a background sequence (live or rendered). Each sequence is placed in a "track" or in its own "layer", as a prelude to composition. CineMatte does not require a series of frames for screen correction. You only need a single frame, although you could write an ARexx script to load a new plate for each foreground frame. Unless you have a long shot with lots of camera or actor motion, there seems little point in making more than one reference frame. Plus, the only way you could really duplicate a complex camera move is with a track. I assuming that most Flyer users only have tripods, so you can lock down; but track, crane, and dolly shots would not be repeatable with the necessary precision. Well, don't sweat it. You probably don't need it, anyway.
One of the reasons the Ultimatte process uses a screen corrected sequence is to pull up any shadow an actor casts and apply it to the composited background. CineMatte does this without needing a clean plate. Wow! The Correction number that you get using the eyedropper works to "bluify" your screen. Just make sure you are clicking on a blue area. When I first tried this, I must have been clicking on some rather unblue parts which made my matte look incredibly mottled. It was not what I wanted at the time, but it's something to remember when I want to do that next great piece of art.
When professionals are shooting blue screen on film, the best approach is to get the screen within a particular range of the wavelength for blue. As with video, the less variation the better. In case you are dying to know, the preferred range is 420 to 450 nanometers. (Are we excited yet?) Unless you plan to have a waveform monitor and vectorscope on the set, you won't know the precise color of your screen. If you're like me, you don't own all that fancy gear - But, you've got an Amiga so, if possible, bring it on the set and make some test frames and load them into ImageFX. Use a light meter to find problem areas on the lit set - too hot, too dark, etc. And everyone's favorite - shoot under the sun. It's free and really can't be duplicated effectively. It's how the Lydecker brothers, Howard and Theodore, shot almost all their miniatures for Republic Pictures' serials and features. They looked so realistic that footage of their planes used in World War II films (1942's Flying Tigers, for example), were thought to be real planes. Flying Tigers was actually nominated for a Special Effects Academy Award; but, was denied it because some of their peers refused to believe that they were indeed miniatures. In fact, there was not one real plane in the entire film (except for a couple of very obvious and grainy stock footage shots, which are clearly separated from the "faked" shots). The large scale mock-ups, not only were incapable of flight, but even ground movement. When the planes had to taxi forward for a shot, they were in reality being pulled by cables attached to trucks!
Wow, that was some aside! Now to return to the subject... If you set up a few tests, you should be able to minimize your matte pulling problems. The only real disadvantage to shooting outdoors is that you can't position the sun (Go ahead, try it). Since you must know in advance how your actor has to be lit to imitate the lighting in the background footage, you'll have to position the set and the camera to start recording just as the sun comes to that correct position. A little tricky; but, if you miss it you'll be "burning daylight". As the Director of Photography folks like to say.
It will be interesting to see how digital cameras affect blue screen technique. Compression is not kind to blue or blue screen footage, as it has a tendency to cause artifacts. I don't know enough about how the Flyer works, to know if it's encoding method will cause some artifacting. If it is a problem, green screen may work better. If possible, make tests to see how different colors perform.
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