What is Mask?
Also known as: Alpha mask, Selection mask
A grayscale image that defines which pixels of a primary image should be edited (white) vs. preserved (black) during inpainting or compositing operations.
In detail
A mask is a single-channel grayscale image, the same dimensions as the target image, that defines per-pixel edit intent. White pixels (255) mark regions to fully edit; black pixels (0) mark regions to fully preserve; gray values blend between the two. In AI inpainting, the mask tells the diffusion model which region to regenerate — only masked pixels go through the denoising loop. In seamless tile creation, the mask is typically a vertical or horizontal band centered on the offset seam: white in the band (telling the AI to heal the seam), black outside (preserving the user's original middle pixels). Mask design matters: too narrow a mask gives the AI insufficient context to heal naturally; too wide a mask causes the AI to invent content that didn't exist in the source.
Example
A 1024×1024 tile is shifted by 512 pixels horizontally so the seam is in the middle. A 1024×1024 mask is generated: black everywhere except a vertical band 200 pixels wide centered at x=512, which is white. SDXL Inpaint heals only the white band, blending the seam into the surrounding context.