VFX technician isolating colors or luminance ranges for compositing — green-screen separation. Downstream task, but set execution determines the result.
The keyer works in post-production to create transparency from a green or blue background—or to specifically isolate color ranges within a shot for later manipulation. It is less a creative discipline than a technical one, but it often determines the credibility or visibility of visual effects. A bad key can ruin the best VFX work behind it.
The task begins on set: How evenly is the green screen lit? How sharp is the edge of the talent against the background? The keyer later works with software—Nuke, After Effects, Fusion—and uses various algorithms to extract the chroma values (or luminance values in practical lighting situations). A difference matte simply subtracts the reference green; a saturation-based threshold keyer works more elegantly when illumination is variable. The best keyer knows both techniques and chooses situationally.
In practice: If the screen was poorly lit—hot spots, uneven coloring—the keyer must later work with despill algorithms to remove green spill from the actor's hair. This takes time. An experienced keyer therefore speaks with the DoP and gaffer on set already—better lighting saves hours in post. Some keyers even work directly with the cinematographer when it comes to focal length or lighting setup for green screen shoots.
Besides pure chroma keying, there is also rotoscoping when the process doesn't work reliably: manual masking frame by frame. A good keyer combines automated keys with targeted manual corrections. They understand premultiplied alpha vs. straight alpha, know the difference between hardware keying (for live switching) and software keying (in post-production)—and know when a luminance key is better than a chroma key, for example, with strongly backlit elements or practical light sources in the background.