Compositing artifact: edge shimmer or color fringing around keyed objects from imprecise rotoscope or bad alpha channel. Looks like cheap '90s greenscreen work.
You know the drill: a character is shot against a green screen, the keyer does its job, everything looks clean — until you notice in the edit that the edges are flickering, discoloring, or developing a greenish/bluish halo. That's quonking, and it destroys otherwise solid VFX work in seconds.
Quonking almost always stems from two sources: either your rotoscope is too imprecise — the mask isn't pixel-perfect on the object's edges — or your alpha channel is damaged. The alpha channel is the invisible fourth channel of an image (alongside RGB) that defines transparency. If this channel is rough, transition values aren't cleanly interpolated, or you're working with 8-bit instead of 16-bit, your edges will look like they're oscillating between two values 50 times per second. The flickering is then particularly visible when the edge moves or when there are high-contrast backgrounds behind it.
In compositing, you avoid this by doing several things simultaneously: First, work with high-resolution masks. If your rotoscope operates at half resolution and you then upscale, you're guaranteed to see aliasing and flickering. Second, use feathering and edge expansion not as emergency tools, but as precise fine-tuning. A buffered, soft edge is better than a sharp, unclean one. Third, explicitly check your alpha channel. In Nuke or After Effects, you can visualize the alpha in isolation and immediately see if there are artifacts. Often, you'll need to apply a simple Dilate/Erode operator to close small gaps without becoming grainy.
Another common cause of quonking is chroma spill — green or blue color remnants clinging to the edges. The keyer didn't cleanly remove it, and if this spill flickers, the problem multiplies. Here, a dedicated despill operation after the key helps, not before. And more importantly: pay attention to your keyer's output settings. Some keys only give you a hard mask, others a weighted, semi-transparent alpha. The latter is usually better because it allows for more natural transitions — provided you calculate it with enough precision.
In the edit itself, you'll recognize quonking immediately: it looks cheap, unrealistic, like bad 90s green screen. Modern viewers subconsciously sense it instantly. The solution isn't to ignore it and hope for the best — the solution is to go back to the compositor and recalculate the mask accuracy or the alpha. Sometimes, you'll also simply have to demand a higher bit depth or reshoot the shot with better lighting.