Why use Chroma key?
Often thought of as a specialist secret of Hollywood directors, ChromaKey or
Blue screen / Green screen is easily within the ability of any photographer who wants
to be creative with backgrounds. Maybe you are working at a venue and it's raining,
taking a fantastic picture of the venue while it's nice (or come back and do
it) and shooting your subjects indoors on Chromakey will enable you to replace
the Chromakey when you return to your studio (or indeed if you are still at the
event to sell prints there and then).
Chromakey opens up the horizons of your portfolio. Event and studio photographers
can benefit financially from using this. Cruise ship photographers can slip a
photo of Venice behind images of embarking passengers, Wedding shoots in bland
and brown registry offices can be transformed by dropping a portico behind the
subjects to frame the happy couple, Children dressed in Spider Man outfits can
hold a specific pose and you can insert a wall behind them, and hey presto they
are climbing the Empire State Building, etc.
In film and television actors can be recorded in a studio in front of a green screen and in the compositing process transported anywhere enabling some of the astonishing special effects we take for granted these days.
How does chroma key work? Which colour should I use?
The principle of "keying" has been around in film since the 1940's, the idea
is to film the foreground part of the image masking out the background, and then
the background masking out the foreground. The two images are then combined to make the foreground subject appear to be in front of the background even though the two were filmed at different times in different places.
ChromaKey as its name implies, works on the colour (chroma) in the image and
this then becomes a mask. If the colour (green or blue) is masked out, then the
inverse can be used to bring in another image, so each image is masked allowing
a composite to be created showing the subject in a new background. Blue and Green
tend to be used for chroma key because they are furthest from skin tone and as primary colours were easiest to separate in the film or video process, but theoretically with
electronic compositing any colour can be used that isn't present in the foreground
Blue chromakey is used because:
- It is complementary to skin tones
- If you don't pick up all the "blue" in the background in your editing process,
the so called "Blue-spill" doesn't notice so much as green.
- Blue has been traditionally used for film due to the colour separation processes
used, but blue chroma key requires more light
Green chromakey is used because:
- Video and digital cameras are more sensitive to green with less noise making
it easier to generate a cleaner mask.
- Green requires less light
- Also Green is used where you have predominantly blue subjects (maybe you
shoot in schools where the uniforms cross into the particular shade of blue).
But the choice and preference are yours at the end of the day, and some trial
and error might be the way forward.
Once you've had a few goes, and become a dab hand at masking in your digital
suite, you're ready to start earning from it alongside your usual line of income.
So how do I use Chromakey?
The essentials you need: a camera (!), lighting, Chroma backdrop, and a good
quality digital image or images (such as TIFFs or JPEGs) to place behind the
subject in place of the Chromacolour.
If you're starting out, try a half width 1.35m x 11m paper roll in Green or Blue Chroma
Key, as paper is often easier as it's easier to light a smooth paper background
evenly, although fabric allows
you to cover certain objects, etc and some photographers just prefer it. But
if you're starting out, try the paper.
For convincing results the foreground subject should be lit to match the digital background, so if the background is lit by a low sun coming from the
left don't light your foreground subject with a high key light from the right.
The chromakey background (blue screen or green screen) must be evenly lit without shadows creases or wrinkles (won't
be a problem as you are going to use Creativity Background chroma key paper aren't you?).
Try to avoid shadow from the model / subject being cast on the chroma key background. If you
have patchy light (lightspill or fringing), you will be spending a little more
time in Photoshop (or whatever your preferred software is, see below) to blank
out the Green or Blue screen so you can drop in your required replacement background.
How do I change the Chromakey background digitally?
You can use traditional image processing tools to work with our ChromaGreen and ChromaBlue Backgrounds, try extracting the Green or Blue and placing your chosen "artificial" scene as a "background" layer
in Photoshop, Paintshop Pro, Photoshop Elements or other Paint program (not Illustrator
as this is a vector or drawing program).
In Photoshop make the background a layer, by double clicking it in the
layer palette; it will then be named "layer 0" by default. Create another layer
below this, and reselect "layer 0".
Choose "Color Range" from the Select menu and first click on the green or blue
with the default pipette, the refine the selection you are creating by changing
to the pipette with the plus sign to add further similar colours to your selection.
You can fine tune further by using the Fuzziness slider, or the pipette with
the minus sign to subtract colours.
Lastly, click the Invert checkbox. Your selection can now be used as a mask by
clicking the Make Mask icon at the bottom of the larger palette.
There are numerous tutorial packages on the market, and expert tuition run by
various photography groups. The main thing is, enjoy using your Creativity Backgrounds!
It's What's Behind Your Great Images!
ACKNOWLEDGEMENT: With thanks to Rod Wynne-Powell, author and
editor of many books including Photoshop Made Simple and Mac OS
X for Photographers; especially for his expert advice on the digital process.
Rod regularly tutors and gives demonstrations to groups all over the country www.solphoto.co.uk
Original studio image by Hoss Mahdavi