Adaptive thresholding

Adaptive thresholding uses an image threshold that varies with position, often calculated from a neighborhood around each pixel. It can help separate marks from background when illumination changes across an image. A global image thresholding rule instead uses the same threshold everywhere.^[ref]

For a simple real-arithmetic local-mean model, putwhere ${\overline{I}}_{\mathcal{N}}$ is the neighborhood mean and $C$ a fixed offset. OpenCV documents local-mean and Gaussian-weighted alternatives, with a declared neighborhood size. Its binary comparison assigns the high output value only above the threshold.^[ref]

Consider a three-by-three patch with eight values of $100$ and a central value of $80$. For $C=10$,so the centre receives binary value zero. Adding $60$ to every intensity gives eight values of $160$ and a centre of $140$. Now $T=1330/9\simeq 147.78$, so the centre still receives zero. A fixed threshold of $110$ would classify the two centre values differently, despite their identical contrast with their local backgrounds.

The invariance follows algebraically. If every intensity in a neighborhood increases by the same $b$, its mean and threshold also increase by $b$. Hence $I+b>T+b$ exactly when $I>T$. This proves an additive-shift property of the stated model, assuming no clipping or rounding; it is not a claim about arbitrary spatial gradients or multiplicative brightness changes.

Neighborhood size and offset affect the result. A very small neighborhood can follow noise or include too little background, while a large one can span changing illumination. Integer rounding and image-edge handling must also be specified for an exact implementation comparison. The resulting binary image remains an intensity-based selection; connected-component labeling and semantic interpretation are further steps.