PHYX Blog

How to shoot a Green Screen.

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Visual Effects artists often receive poorly-shot footage, and must spend days doctoring it with multiple keys and rotoscoped masks. This process can cost producers thousands of dollars for one shot. The "fix it in post" mantra becomes reality when the cost of "shooting it right" is higher. Your project's budget may be less than one shot of a major blockbuster film, yet you still want the final product to be realistic and believable without selling your car or house, or spending weeks to achieve that goal.

If there were one word for shooting green screens properly, it would be TESTING. The brand of camera and lens can be important to the final result, however, a poorly-lit green screen, shot with an amazing camera and expensive backdrop, can be far more tedious and difficult to key than well-lit footage shot with more modest equipment. Testing your set-up can save hours or weeks of frustration. If possible, plan to shoot some tests BEFORE your actual shoot, and make adjustments where needed. Whether you are shooting with Red EPIC, HDSLR, P2 or other format, proper lighting is crucial to obtaining a good key quickly.

 

LIGHTING

You must light two distinct and critical areas: the green screen, and the foreground elements. The foreground may be an actress, dog, car, etc. - whatever you want to key. The key (pun intended) to lighting the green screen itself is to light it as evenly as possible. If you use cloth, make sure it's GREEN (not bluish or yellowish), and make sure that there are as few wrinkles visible as possible. Look through the camera at your monitor. If there are "hotspots", move lights around, remove or bounce lights, etc., to get the smoothest and most even green possible. Frame your first shot as wide as possible, and shoot some footage.

Load your image into After Effects, or save a still frame as a jpeg at high quality (100%), and import it into Photoshop, GIMP, etc. We will be talking about RGB levels in 8-bit, where 0 = black and 255 = white. Take your Color Picker and move it over the image. Inspect your values in the brightest and darkest areas of the image. You want the G (green) values to be ideally around 155, and R (red) and B (blue) ideally around 10. In reality, 135-175 for G (green), and R (red) and B (blue) values around 20-50 is acceptable. Do not overexpose the green screen (above 235), and try to make the values as consistent through the image as possible. We have been sent "green screen" footage from people asking how to improve their footage. The footage was actually an over-exposed bluish green screen. The G (green) values were clipping (255) and their B (blue) values were over 200 in bright areas! That's more than enough blue to be considered a blue screen !

Next, light the foreground elements however you want. In past years, it was recommended to rim-light foreground elements with magenta filters to neutralize green spill on actors from the backdrop. That is no longer necessary with the Despill filter found in PHYX Keyer.

Check out the images below:
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IMAGE 1 (above) is the green screen footage keyed over black, with no other processing applied. Note the harsh green halo called 'spill' created by reflected light from the background.
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IMAGE 2 (above) is the result of only the Despill filter in PHYX Keyer. Note how the green spill is properly neutralized without color-shifting the skin tones.

On the big day, plan to shoot a "Clean Plate" if possible, which is simply the green screen background without the foreground elements. This short take can further help to smooth your shot with the ScreenCorrector plugin in PHYX Keyer. Check out the images below:

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IMAGE 3 (above) is the Clean Plate, which is the green screen without the foreground talent,
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IMAGE 4 (above) is the 'Foreground' or 'Action' Plate (the model in front of the green screen),
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IMAGE 5 (above) is the result of ScreenCorrector, which evens out the green screen.
In the next image, we compare the mattes:
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IMAGE 6 (above) is the key generated without using ScreenCorrector. As you can see, there is "white" on the sides of the frame, which means this will be keyed as well. To fix this, we can boost the contrast and crush hair detail, and make a very harsh matte, or (see next);
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IMAGE 7 (above) is the key generated using ScreenCorrector. Note how the ScreenCorrector output is vastly superior. We can key the entire background without destroying hair detail.
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After ScreenCorrector, we apply Keyer, Despill, ColorMatcher, and LightWrap. All plugins are tweaked to taste. Above is the final key, which was created and fine-tuned in less than a minute.

Other points to consider: We recommend using the highest quality camera, lenses, and recording settings possible for shooting green screens. PHYX Keyer supports After Effects, Premiere Pro, Motion, and Final Cut Pro (X). For more information, click HERE.

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