Compositing is combining images from different sources into a single frame that reads as one photograph. Almost every effects shot you have seen is a composite, and almost every failed effects shot failed at the compositing stage rather than at the creation of the elements.
Layers and the stacking order
- Layers stack in order — the top layer covers what is beneath it, exactly as in image editing.
- The alpha channel stores transparency per pixel, which is what lets a layer be partly see-through.
- A matte or mask defines which parts of a layer are kept and which are cut away.
- Blending modes control how a layer's pixels mathematically combine with those below — multiply darkens, screen lightens, add is used for light and fire.
Chroma key
Chroma key removes a specific colour from an image so whatever is behind it shows through. Shoot a subject against an even green backdrop, tell the software to make that green transparent, and the subject can be placed anywhere.
Why green, and when blue instead
Green is furthest from human skin tones in the colour spectrum, so removing it is least likely to eat into a face. Digital sensors also capture more green information than red or blue, giving a cleaner edge. Blue is used when the subject wears green, or for a blonde subject where green spill is especially visible — the rule is simply to key the colour the subject is not.
| Problem | Cause | Fix — on set, not in software |
|---|---|---|
| Spill | Green light bouncing onto the subject's edges and hair | Move the subject further from the screen; add backlight |
| Uneven key | The backdrop is brighter in some places than others | Light the screen separately and evenly from the subject |
| Hard, jagged edges | Over-tight key settings, or a compressed source | Shoot at higher quality; soften and choke the matte slightly |
| Shadows on the screen | The subject is standing too close to it | More distance between subject and backdrop |
| Wrinkles and creases | A cloth backdrop hung badly | Stretch it taut, or use painted board |
A bad key cannot be rescued in software
Every hour spent lighting the screen evenly saves several hours of frame-by-frame repair later. The two things that matter most are even lighting on the backdrop and distance between the subject and the screen — and both are free. Compression is the other enemy: heavily compressed footage discards exactly the fine colour detail a key depends on.
Making a composite believable
Removing the background is the easy half. Making the result look like a single photograph is the half that separates convincing work from obvious work, and it comes down to matching four things.
- Match the lighting. The direction, hardness and colour of the light on the subject must match the background plate. A subject lit from the left cannot stand in a scene lit from the right.
- Match the perspective. Camera height, lens angle and horizon line all have to agree. A subject shot at eye level will not sit into a background shot from above.
- Match the colour and grade. Apply the same colour treatment to every element so they share one look.
- Match the grain, blur and focus. A pin-sharp subject in a softly focused background announces itself instantly. Add matching motion blur and film grain.
APractical effects
- Built and filmed for real — models, makeup, pyrotechnics
- The camera photographs actual light, so it sits naturally in frame
- Actors can see and react to them
- Expensive to repeat, and hard to change afterwards
BDigital effects
- Created or altered in software
- Infinitely adjustable and repeatable
- Can achieve what cannot be built or filmed safely
- Can look weightless if lighting and physics are not matched
Most big films use both
The usual approach is a practical element to give the shot real light and real interaction, extended digitally beyond what was built. A real set for the first few metres and a digital extension above it will out-perform a fully digital environment almost every time, because the part the eye scrutinizes most is genuinely photographed.
Plan effects during pre-production
An effects shot decided in the edit will cost far more than one planned before shooting. Knowing the shot is a composite tells you where to place the camera, how to light the subject, what reference to photograph on set, and what to measure — and those decisions cannot be recovered afterwards.