From: dwilliams@oread.cc.ukans.edu
Newsgroups: rec.video
Subject: Re: Explain this effect?
Date: 3 Sep 92 18:12:36 GMT
Article-I.D.: oread.1992Sep3.131236.1
Followup-To: rec.video
Distribution: usa
Organization: University of Kansas Academic Computing Serces

In article <6139@transfer.stratus.com>, pearl@sw.stratus.com (Dan Pearl) writes:
> The areas are an artifact of the image composition process.
> 
> In the blue-screen process, the goal is to come up with the following
> items which will be combined in the optical printer:
> 
> 	A. The original footage
> 	B. The starfield footage
> 	C. Black film with a clear shuttle hole
> 	D. Clear film with a black shuttle blob
> 
> The final footage will be (B+D) exposed, then (C+A) (where "+" means
> "sandwiched together").  The problem seems to be in the production of the
> C and D components.   In the background of the C there is, ostensibly,
> black.  That black resulted from the detection of blue (in the blue
> screen process), but it also results from the application of a hand-applied
> mask, which masks out the non-blue information that appeared in the background.
> 
> The hand-applied masks, I'll bet, are perfect.  That is, they let no light
> through.  The blue-screen process is, I'll bet, slightly less than perfect,
> resulting in a not-quite-100%-black where the blue was.  The problem is that
> C is actually 
> 	
> The film-to-video transfer undoubtely affects the visibility of such
> artifacts on the small screen.
> 
> -- 
> ------------------------------------------------------------------------------
> Dan Pearl  **  Stratus Computer, Inc.  **  pearl@spectacle.sw.stratus.com

To add more to the explanation of grey blobs:

The reason that artifacts like this do not show up in episodes of _Star Trek: 
The Next Generation_ or in just about anything Lucasfilm has done in the 
last several years is that the holdout mattes are created digitally 
instead of optically.  

In the case of TNG, all the effects are composited (sandwiched) using 
digital video, where they have very exacting control over these mattes and 
can make them all uniformly black.  The "not-quite-100% black" areas can be 
accounted for quite easily.  They also do not use a blue screen, but do use 
a bright flourescent orange screen, because it's easier to work with for 
this purpose (they say, like I would really know).

Starting with _Who Framed Roger Rabbit_ Industrial Light and Magic went to a 
digital compositing system.  They don't actually composite the images in a 
computer (with one exception that I know of) but they do create the mattes 
there and then composite optically.  Still, these computer-enhanced mattes 
are much more accurate than the old optically produced mattes.

Film-to-video transfers have a tendency to heighten contrast and are usually 
made from prints closer to the original negatives than most theatrical 
prints, which is why these artifacts show up on television.

Most of this information comes from a several year old article in _Cinefex_ 
magazine.  The rest is probably a figment of my imagination.

-- 
 _ 
DaV
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