From: john@logiconultra.com (John Martyniak)
Subject: Re: Matte Shots?
Keywords: Matte, Blue Screen, Special Effects
Organization: Logicon_Ultrasystems
Date: Thu, 17 Dec 1992 23:16:23 GMT

In article <1992Dec17.152431.23667@gserv1.dl.ac.uk>  
g.coulter@daresbury.ac.uk (G. Coulter) writes:
> Hi Everyone. I hope this is the right place to ask this question
> It is related to an artical that I read in a computer magazine
> refering to doing animations of starships in flight.
> 
> 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?) what is the process for doing this, I will tell you what I
> know and I hope that someone can help or point me in the right
> direction without flaming me. After all it is Christmas.
>
> From what I know they first film the stars ie a background plate
> then they film the model against a blue screen. Why?
> 
They film it against a screen, because they want it to have even  
coloration or lack thereof.  The screen in the background gives an even  
neutral coloring, so that when they composite it all together, they do not  
get any distortion of the background.  The other thing is I don't think  
that they actually need to use a blue screen, for example in Predator they  
used Red (The reason they used red was probably because there was too much  
blue in the backgrounds in the Jungle)

> 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?
>
I don't think that they use a blue screen, when they do the light pass,  
because the light from the Blue screen would illuminate the ship  
incorrectly.  So I think that the Light pass is done in total darkness  
(except for the lights).  The reason it is not done in the same shot, is  
that in most cases you want to add an independent effect to say the  
engines (For example in the StarTrek Films (II onwards), and even in Star  
Wars in the Opening battle with the Star Cruiser, the Engine lighting was  
enhanced so that it was extremely bright.

> Now and this is where I get really lost they create a black
> negative of the ship to use as a mask when exposing the file
> But why. Is the reason for this because the stars and the ship
> can't be filmed together. 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.
The reason for the negative is to hide the edge of the object that you are  
going to use in the composite.  Because when you composite the Negative  
and the Blue Screen, the Negative will remove the blue color, from the  
film.  Once you have create this new composite image, you then composite  
it onto say a starfield with a true representation of what is hidden and  
what is not.
There is a new movement in the industry and that is to do Digital  
composition, (Where as before you used an Optical Printer, and now you use  
a computer) it was used alot in Terminator II.  When doing digital  
composition providing the background of your image is transparent (or you  
have some way of making it such) all you have to do is paste the image  
onto the background.  But your image must have a transparent background.   
Also when doing Blue Screen work, since you are guaranteed to have an  
evenly lit single colored background, you can write some code that will  
everything that color of blue to be transparent.  But if that color of  
blue exists in the object you are shooting against the Blue Screen, then  
it will be transparent as well, and you will need to make a negative of  
the image and composite the two.  

> Thanks for Any help Gary.
Hope this helps.  A good place to get more information is CineFex, StarLog  
is ok it is more Fan oriented, and just try watch all of the making of ...  
that you can.
John Martyniak