Masking and feathering
Masking limits an image or effect to a drawn region; feathering softens that region's edge so the composite does not look cut out.
See it
What it is
A mask draws a boundary that limits which part of a layer or effect is visible. Feathering spreads that boundary across a band of partially transparent pixels, replacing a hard cutout edge with a gradual blend. Masks can be geometric, drawn with Bezier points, or animated to follow a moving subject.
Use masks to isolate a face for correction, reveal one layer through another, remove an unwanted edge, or build a rough garbage matte before keying. Add only enough feather to match the image's focus and motion blur. A crisp graphic may need almost none; a defocused shoulder may need a wide, soft transition.
Gotcha: feathering cannot repair a badly placed mask. Too little reveals a cutout line, while too much creates a glowing halo or washes the effect into nearby areas. Motion blur and changing perspective also mean a mask that fits one frame may drift later, so track it or animate its path before tuning the edge.
Ask AI for it
In Adobe After Effects, draw a closed Bezier mask with the Pen tool around the subject, set Mask Mode to Add, and animate Mask Path where the outline drifts. Set Mask Feather to 18 pixels and Mask Expansion to minus 2 pixels as starting values, then inspect at 100 percent and tune them so no hard edge or bright halo remains. Use Track Mask for the broad motion, correct failed frames by hand, and add Pixel Motion Blur to the precomp so the mask edge smears on fast movement.