Mask

A mask in post-production is a black-and-white image (or grayscale, for soft edges) that defines which regions of an image are affected by an operation and which are not. White (or fully opaque) regions of the mask indicate where the operation applies; black (transparent) regions are protected from the operation. Masks are used throughout the post-production pipeline: in color grading (a power window is a mask that limits a grade to a region), in compositing (a roto mask isolates a foreground element for a clean composite), in VFX painting (a clean-plate mask isolates the area being cleaned), and in motion graphics (a mask reveals an animated element over time). Masks differ from alpha channels in usage: alpha is the transparency channel inherent to an image, whereas a mask is typically generated externally for a specific operation. Masks can be hand-painted (rotoscoping), automatically generated by qualifiers (HSL key, depth key, luma key), tracked over time (animated rotoscoping for moving subjects), or derived from 3D scene data (cryptomatte, depth pass). Modern tools blend several approaches: Mocha Pro for planar tracking masks, Silhouette FX for fine roto work, DaVinci Resolve's Magic Mask for AI-assisted masking, Nuke's RotoPaint for traditional vector roto. Masks are stored typically as image sequences alongside the main plate, or as embedded vector roto data in tool-specific project files. Reference: Foundry Nuke documentation on roto and mask conventions.