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Hello all;

I found a web page years ago that explained the traditional photo-chemical blue screen process in brief and easily understood but fairly complete detail.  It talked about how a basic composite required something like 14 film elements, the issue with blue spill and how a green/blue difference element needed to be created in place of a conventional blue separation for the foreground.  I can't seem to find the article now (which had great illustrations as well).  Does anybody know where it is or if there is another resource that does into that level of detail?  There's hundreds of sites out there that talk about the process but are dumbed down to the point of not really representing how it was done (many say the green separation is simply used in place of the blue separation, which was rarely true.)


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Excellent, thank you!

While I was hoping to find the static web page explaining the process in detail, that video goes into a lot better detail than most, including the often overlooked cheat for the blue separation.

I thought there was another trick where a blue/green difference separation was created to use in place of the blue separation...or something like that.

I recently regained interest in this process because I was doing a green screen comp where spill was really obvious on a VERY fuzzy, mostly gray sheep when it moved quickly.  The green spill suppression wasn't as effective as simply replacing the green channel in this case and the change in color was barely noticeable.  That got me to thinking about how to change the channel matrix to get better color in other situations, should the need arise.

Thanks again!

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Thanks for the links.  If I was smart, I would have downloaded the entire page when I found it.  There's at least enough information between the links you guys provided to refresh my memory on how it was done.

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I found this explanation on a site that's largely dedicated to matte paintings https://nzpetesmatteshot.blogspot.com/2016/01/mattes-maketh-musical.html

"any color that had more green than blue could not be reproduced.  Green and yellow, for example,  were missing in action. Yellow turned white, green turned blue-green (cyan). That's because, by definition, green and yellow have more green content than blue. (Yellow = green plus red.)  Excess blue was poisoning those colors.

Vlahos realized that if he could add just a little density to the green separation in those areas only when it was being used as a replacement for the blue separation, he could get those colors back.  He needed a film record of the difference between blue and green, thus the Color Difference System's name.
In a pencil-and-paper thought experiment, Vlahos tried every combination of the color separations, the original negative, and colored light to see if he could achieve that difference.  The magic combination was the original negative sandwiched with the green separation, printed with blue light onto a third film.  The positive and negative grey scales cancelled out, but where there was  yellow in the foreground (for example) the negative was blue and the green separation was clear. The blue light passed through both films without being absorbed and exposed the third film where there was yellow in the original scene.  The same logic produced a density in green and in all colors where there was more green than blue.  This was the Color Difference Mask, which in combination with the green separation, became the Synthetic Blue Separation."
I've learned so much about Hollywood effects from that site over the years.  It's really worth a look if you haven't seen it already.  It actually inspired me to do background (oil) paintings for a recent series of promos I shot instead of finding real backgrounds or making them with CGI.   They weren't exactly 100% realistic but the cartoonish action of the video made the paintings work well in this case.  Plus, it gave a unique look to the final product.

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