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What do these things have in common?

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Blue/green screen compositing

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Process (overview)

• Filming of a subject in front of an evenly lit, bright, pure blue (or green) background

• The compositing process, whether photo-chemical or digital, replaces all the blue in the picture with another image, known as the background plate

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How does it work?

• Imagine a photo:

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Select a colour or similar range of colours by the magic wand tool

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Delete the selected areas

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• After doing this, overlay the image over something else

• Now imagine same process for every single frame of a video clip

chroma keying

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The filming process

• Traditional method:• Film subject over homogenous green

background• Make sure subject does not wear any

green clothing. Why?– Will disappear when chroma key is applied

• Subject must be drowned out with strong white light (from all directions). Why?– To eliminate shadows on green background

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Traditional method

• Advantages– Relatively easier to adjust and fine-tune

during filming

• Disadvantages– Background may not be homogenous green

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New method• Shine green light on a subject over a special

background• Advantages:

– Homogeneity of background does not matter– Potential to produce very well done greenscreen

effect

• Disadvantages:– Subject cannot wear reflective clothing / have

reflective objects (eg. spectacles) as green light will be reflected

– Subject cannot be too close to camera or green light will drown out the subject, no matter how strong the white light is

– Takes a lot more adjustment of lighting

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Setting up the background

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Setting up green light (stick to camera)

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Preparing to film

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Filming

22Note the white lights all around to drown out the shadows

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Examples of badly done greenscreen

Shadow (will cause a green border to remain around character after chroma keying)

- Not enough white light in some directions

Non-homogenous / grainy background

-Green light intensity too strong / not enough white light

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Shadow (will cause a green border to remain around character after chroma keying)

Different shades of green

- Green light too focused / too close to subject

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Green light reflected off surface of subject (will disappear in when chroma is applied)

- Green light too close to subject / too strong / not enough white light on subject

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Background has black streaks due to irregular surface

- White light has not drowned out the scene (reposition the angles, increase intensity)

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Example of well-done greenscreen

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In premiere… (after capturing the shot footage)

Under effects tab Video effects Keying Chroma key

Drag to the clip you want to chroma key

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Under effects control, expand the “chroma key” effect you just inserted

Under the colour tab, click on the dropper tool and click on the background of the scene to change the keyed colour from white to the specific shade of green

In this case, clip to be keyed is overlaid on a black video

Adjusting colour of key

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Work with the similarity tab

This is what you will get when similarity is 0 (too low)

Adjusting similarity

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This is what you will get when similarity is too high

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Find the optimum similarity (note the presence of green border even if green-screen is quite well done)

Adjusting similarity

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Find the optimum blend (here it’s 100%)

White surfaces may be faded off, but that is alright if clip is overlaid over a non-moving, homogenous background

Adjusting blend

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Drag a “Color Balance (RGB)” effect to ABOVE the chroma key effect

Color Balance (RGB) can be found under Video effects Image control

Decrease green and increase red and blue slightly until you get something like this

Color balance

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Drag a “Brightness and Contrast” effect to BELOW the chroma key effect

Brightness and Contrast can be found under Video effects Color Correction

Adjust brightness and contrast until you get something like this

Brightness & Contrast

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Additional effectsAdditional effects you may want to use to finetune your chroma key:

• Lighting effects – Video effects Adjust Lighting Effects

• Gamma correction – Video effects Image Control Gamma Correction

• Color channels– Video effects Color Correction Channel Mixer

OR Color Balance – note: this color balance is different from the color

balance (RGB)• A second chroma key

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Words of caution• Method to do the perfect chroma key varies with

your footage• Need to experiment with the above effects• Play through the entire clip when you adjust the

settings for the effects (may work for one frame but not for others)

• If necessary, cut the clip into a few clips and apply different effects for each

• To make post-production work easier, footage must be well-shot (refer to abovementioned examples of badly done and well done greenscreen)

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Take test shots!

• Always take some test shots with the subject in the greenscreen room before the actual thing

• Note down where you place your camera and the lights (esp. the intensity of green lights, white lights and their angle)

• Try out different combinations but note down which is used for which shot on the tape

• Try out chroma keying on the computer and see which setup works best

• Do the optimum setup of the lights, etc on the actual shooting

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Before each take, write down the arrangement of lights on a paper, put in front of the camera, and record for a few seconds

Allows editor to see which setup gave best effect in Premiere

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So, how do I get the greenscreen room?

• Approach the art department (in the art office on third level of SRC) beforehand (ie. a few days)

• On the day itself, find the same teacher and ask for permission to get the keys

• Remember to keep the room clean, switch off all the lights, fans and air-cons after using, and lock up

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The End. Good luck!