Chromakeying
Most modern TV news programmes use computer generated sets where it is
possible that only the presenter and the desk are real. Everything else in the
frame is added digitally usually by a process called chromakeying. Chromakeying
is basically a process where an object or person is filmed in front of a blue or
green screen. The background colour is then rendered transparent allowing the
person or object to be superimposed on a background image. With modern
computerized nonlinear edit suites chromakeying is well within the realm of the
home video, and with some attention to detail excellent results can be obtained
Having said that modern digital edit suites are suitable for producing chromakey
effects, actual digital video is less than ideal for chromakeying. The signal
compression produces JPEG-like artefacts, and sometimes even motion blocking,
which is not good. Although the artefacts may seem non existent, during the
process of chromakeying the compression results in jagged edges around the
foreground objects.
If at all possible, it is best to shoot the bits that involve chromakeying on
Betacam. If you have to shoot with digital video, but have the possibility to
capture uncompressed from a good digital video player, it's recommended to
capture from the analogue (preferably component) output of the digital video
player. This may seem strange, but a good digital video player uses advanced
filtering algorithms that will reduce the problems caused by the in-camera
compression. It is possible however to create high quality mattes from digital
video footage.
Video cameras often feature image enhancing circuits that artificially increase image detail. This results in overly sharp edges between the foreground and the background, which will be the blue screen when shooting for chromakeying. The problem is accentuated by the compression. This feature is often called aperture control, sharpness or detail. If it is possible on your camera, turn this feature down or even off. To improve the matte even more, use depth of field to create a focused image of the foreground, but leave the screen as out of focus as possible. This will effectively hide the inevitable imperfections in the screen, making keying much simpler.
Screen ColourTraditionally blue has been favoured as the colour for the screen because it
is the complementary colour to skin tone, and thus easiest to separate from
actors. Film also used to be most sensitive to blue light. With current video
effects software any colour can be keyed out. Blue, red or green, all channels
can be used. The choice of the screen colour is usually determined by what you
want to film in the foreground. Choose a background screen that is an opposite
colour to the colours in the foreground. Alternatively avoid the screen colour
in any of the foreground objects. It sounds simple, but watch out for ties,
shirts or eyes which can disappear in the key, additionally be careful of
anything white or shiny that reflects the background colour.
Technically a green screen is better for video. This is because green is
represented in the luminance channel of the YUV signal. In 4:1:1 or 4:2:2
compression ratio's, green is sampled four times. This means more information is
used in the green channel then in blue or red making it better for extracting a
high quality matte.
The television and film industry use special green and blue paint, but other
paints can be used. Vivid hues that come close to pure green, blue or red are
best. Practically use Photoshop's colour palette and look up pure green as
(0,255,0). Alternatively get some colour swatches from the local DIY store, film
the different colours and test which creates the best matte then buy a can of
that colour. Make sure your screen material is matt because a glossy or too
reflective surface will cause green light to bleed on to the edges of the
foreground preventing the creation of a good matte.
It's best to keep the actor or foreground objects as far away from the screen as
possible. This can mean a pretty big screen, although it's not always necessary
to fill the entire frame with the screen: you only have to place the screen
behind your foreground, no more than necessary. Reducing the size of the screen
will prevent your subjects from casting shadows on it and it will also reduce
the amount of reflection of the screen colour on the foreground. Just make sure
the actors stay in front of the screen.
It's important to light the screen evenly. If you are using fabric, make sure
to remove all of the creases. Giving the screen surface a slight curve on the
vertical axis, if possible, will help soften the light and lessen visible
highlights on the screen and if there is one available use a video waveform
monitor to measure the brightness and colour of the screen to make sure the
screen is lit consistently.
Professional lighting rigs will give the best results, but it's quite possible
to use flood lights from a DIY store to light the screen. Better results may be
obtained reflecting the light onto the screen (white polystyrene or something
similar a 2.4m by 0.6m is about £3.00) and give a more uniform and diffuse
screen colour than when lighting the screen directly. Don't over light the
screen though, the aim is to achieve an even, saturated colour and an over lit
screen often results in the background colour spilling onto the foreground.
When lighting the foreground, make sure the light doesn't hit the screen. This
is achieved partly by keeping some distance between the actor/object and the
screen. Barn doors on the lights, if available or pieces of cardboard on a stand
can keep the light away from the screen. If coloured lighting is used, make sure
it is not the same as the keying colour. When filming against a green screen do
not use a green gel on the foreground lighting.
It's common practice to use a colour filter on the backlight when filming for keying. When filming against green screen, use a magenta gel on the backlight. For blue use an orange or amber gel. This increases the separation of the foreground from the screen and reduces the reflection of background colour you get around the edges of your subjects. This should prevent fine objects like the hair and fingers from disappearing in the key. Don't overdo it though. With too much backlight the subject will end up with a white halo around it which could look odd.
No matter how successful the matte is, if the foreground doesn't look as if it belongs to your digitally inserted background, the effect is going to look rotten. If the foreground lighting does not match the inserted background, even the greatest composite film will look un-natural. If possible, use a reference shot of the background footage when setting the lights for the foreground. If the background footage is an exterior shot, consider filming the foreground in natural sunlight to. This will improve consistency and make the overall film look much more convincing.
The following clip shows an example of what can be achieved without too much effort. The subject is me, taken while I was setting up for a proper shoot with a real person (which is why I am concentrating on a monitor in front of me). The background is a shot of a computer monitor which was the first file I found. Both clips were shot on a Sony PD 150 camcorder. The composited clip was done in a few minutes (plus render times) using Adobe After Effects. The two clips were imported and added to a new composition. With the layers set to 3D, a light was added to cast a shadow to roughly match that from the real light. A camera layer was added with depth of field enabled to slightly de-focus the background and finally the foreground levels were reduced to better match the background.
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The following pictures show frames from the original video clips that were used to create the composite.
