From: good@pixar.com ("Time is a pie."  -- Ed Catmull)
Subject: Re: Matte Shots?
Organization: Pixar - Pt. Richmond, CA USA
Date: Sat, 19 Dec 1992 07:37:20 GMT

In article <1992Dec17.152431.23667@gserv1.dl.ac.uk> g.coulter@daresbury.ac.uk writes:
:
:Now what I want to know is when you see a shot of a spaceship
:say on StarWars how do they compose the shot (ie how do they do
:it?) 

I'm going to try to keep this very simple, since this is a news posting
and not a full-blown article about compositing.  You should be able to
find books about the blue screen process in a good library or film-oriented
book store.  The process isn't terribly new.

:From what I know they first film the stars ie a background plate
:then they film the model against a blue screen. Why?

There are many reasons for compositing, including the differences in scale
and exposure between the desired background plate and the blue screen
element.  It might not be a simple star field but footage shot in some
earth-bound location (ie: "Close Encounters").  A typical "Star Wars" kind
of motion control shot has the space ship sitting still and the camera
moving past it to give the illusion of motion.  The exposures are very long,
and the machinery moves in slow motion.

With the blue screen background, they can pull "mattes", which are black
cookie-cutter shapes matching the space ship.  The film makes several passes
through a machine called an optical printer.  On one pass the star field is
printed but with the matte "holding out" an unexposed area where the ship
will go later.  On another pass, the ship is printed into the "hole" left
for it, and a matte that is the inverse of the first one keeps anything
from printing where the star field is.

You have to do it that way because if you just plunked the space ship on
a star field you would have a double exposure with stars showing through
a semi-transparent ship.  In fact, this is what happens when a blue screen
matte is too "thin", meaning it's not opaque enough to hold out its region
of the image.  Motion blur can make this a difficult problem.  The one time
Alfred Hitchcock tried blue screen, "The Birds", he was quite unhappy with
the thin mattes which let you see flying crows throw semi-transparent
children.

:Next they do the same to get just lights and engine outlets?
:Why not do this in the same shot as the model or are the lights
:rotoscoped onto the film later?

The lighting and exposure requirements for the "beauty" pass and the
"engines" and "lights" passes are very different, which is why this
multiple exposure is made.  Often this is handled right in the camera
by simply backing up the film and having the motion control rig run
the program again.  The brightly-lit beauty pass is done first (probably)
and then a second pass is done with the dim little bulbs in the model
turned on.  They aren't bright enough to compete with the lights used in
the beauty pass.  I watched the "lights pass" of a star destroyer from
"Jedi" which took nearly all night to shoot.

Some engine effects, as you have guessed, might be rotoscoped in by the
animation department later.  Using an animation stand they can produce
their own mattes for their elements, and the rest of the compositing
process is much like blue screen compositing.

:My final question is do you have to
:go through any of this when creating a computer animation of
:say a starship, or is it just a simple case of pasting a ship
:onto a backdrop of stars.

If you have 4-channel images (Red, Green, Blue, and Alpha) it is just
about that simple.  The alpha channel, you will not be surprized to learn,
is also known as the matte channel.  It is used algebraicly much the
way a blue screen matte is used optically.  Digital compositing, when given
good enough equipment, is "better" than optical in many respects for this
and other reasons.  As usual, input/output to and from the digital domain
is still the weak point, but it's getting better.  If you're doing video
and not film, then you have no trouble: Digital compositing is king there
already.

		--Craig
-- 
	good@pixar.com
	Make Welfare as hard to get as Bulding Permits
Xref: sparky comp.graphics.animation:1381 comp.graphics:13065 comp.sys.sun.apps:2822 comp.sys.sun.hardware:6381 comp.sys.sun.misc:5924 comp.windows.x:20407