Uses the fast noise variance estimator of Immerkær (1996): the image is convolve...
https://bild.halowerk.com/v1/noise-estimateOverview
Uses the fast noise variance estimator of Immerkær (1996): the image is convolved with the 3×3 mask [1 −2 1; −2 4 −2; 1 −2 1], which cancels linear structure, and sigma = sqrt(π/2) · Σ|response| / (6·(W−2)(H−2)). Computed for BT.601 luma, the R, G and B channels and the JPEG Cb/Cr chroma planes. Because edges inflate the estimate, a second luma value excludes the 10 % of pixels with the strongest Sobel gradient (Tai & Yang 2008), and a tile median over up to 8×8 tiles is given.