Posted by on Apr 5, 2011 in Blog, Inspiration, Technology | 2 comments

I attended an event at the DGA on March 12th comparing the visual effects used in the original Tron with those of the recent Tron:Legacy. There was a lot of great information about the state of the industry, work flows, etc. All good.

But one comment made by the VXF supervisor of Tron:Legacy (I think it was Eric Barba at Digital Domain) stuck in my head. He mentioned that he “convinced” the director Joseph Kosinsky to shoot on blue instead of green screen because he felt it was more flattering to skin tones. Interesting.

When I mentioned this to a DP friend of mine, he said that he understood skin tone has way more blue in it than green. Therefore, eliminating green by keying it out will lose less information. That didn’t sound quite right to me, so I did a little photoshop test:

That certainly seems to indicate that there is less blue than green in skin tones. And with a quick glance at the color wheel, it’s easy to see why. Blue is in fact a complement to orange (which is to say opposite) and thus only factors in to increase value and saturation.

So next time we shoot blue screen.

Not so fast – that is really only one of the factors to consider when picking screen color. Putting aside clothing (blue jeans) and eye-color concerns (not really a big deal anymore – easily solved with a junk matte), one ought to consider the Bayer filter issue. Without getting too far into CMOS chip technology issues, suffice it to say that the 2 dominant CMOS cameras, Red and the Alexa, are more sensitive to green than blue light – so there will be less noise to deal with in the key. (But with the new MX chip and the Alexa, they are so low noise I’m not sure that should be of primary concern anymore… practical experience necessary here – I’ll report back.)

So maybe if I was shooting with a Genesis/F23 or F30 (which use rgb and luminance stripes instead of a Bayer pattern and thus doesn’tt favor green), I’d shoot blue. Aha: Tron:Legacy shot with the Pace Fusion system with Sony F35 cameras.

So what other issues are at play? Color suppression? Well this is, I think, where we see shooting people on blue screen has some merit. When you color suppress green out of an image, you use its complement – magenta. Now, when a pixel is fully green and you add magenta, it becomes neutral. But in the dynamic texture of human skin, right next to that green pixel is a more neutral pixel and that pixel turns magenta – not a very flattering color in human skin tone. So when suppressing the green spill off the face, you have to pick your poison – get out more green but have some magenta or leave some green. (A common approach to the magenta spill is to desaturate the image after you’ve calculated the key – which is a look we got used to in vfx blockbusters back in the 90s.) In the case of blue spill, orange is the complement – a much more natural color for human skin tone.

So why did we leave blue behind if it is more flattering to human skin tone? Well, blue screen was used in traditional film matting techniques because panchromatic film is more sensitive to blue light, thus providing a cleaner pullout matte. But that was film. Video and Digital cameras have traditionally been least sensitive to blue light – those of us who shot the Red One M chip in the early days (almost 3 years ago now…) will remember struggling against noise in the blue channel.

So with all this in mind and with the new ultra-sensitive chips, I would expect we’re going to see a return to blue screens in visual effects work.

But does it matter in the real world? Well, on most gigs we shoot fast and let post sort it out. And with contemporary keying tools like Keylight and Primatte (which do an incredibly good job at color suppression) we don’t get a lot of complaints.

But better is better, after all.

2 Comments

  1. 4-8-2011

    I saw your post on Red User. Great article. You have some excellent points about the compliment of blue being orange and being more flattering. At this point in time having a green screen stage means more flexibility in what cameras work best on them. As videographers have used a wide range of cameras from Red Ones to Sony EX1s to Canon 5Ds at our studio, and overall they tend to work better on green. The clothing restriction is another major issue — how often do any of us leave the house wearing green compared to a shade of blue.

    • 4-8-2011

      Thanks for the feedback. All three of those cameras are CMOS chips, so it makes perfect sense they are happy with the green screen. Though my green screen experience with the 5D has been pretty abysmal to date – 4:2:0 color is a mess to comp.

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