= Optical character recognition
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<Optical character recognition> converts pictures of written or printed text into encoded text. The input is an image of marks; the output is a sequence of characters that can be searched, copied or processed as text. Merely storing a scanned page as a <raster image> does not perform this recognition.
A typical system must locate text regions and lines, separate or jointly interpret their marks, and decide which character sequence they represent. <Image processing> can correct skew or improve contrast; <image segmentation> can identify regions and candidate boundaries. Recognition can use visual evidence together with constraints from surrounding text. These stages can fail separately: a correct symbol classifier cannot read a line that the layout analysis omitted.<An Overview of the Tesseract OCR Engine by Ray Smith>[§2, printed p. 629, architecture: binary input, connected components and organization into lines and words; §§3–5, line finding and word recognition]{ref}
For a concrete ambiguity, the final `rn` of `modern` can resemble `m` in a degraded scan, producing `modem`. That single visual merge requires two ordinary character edits to repair: substitute the final `m` with `r`, then insert `n`. With six reference characters, the <character error rate> in this synthetic example is
$$
\mathrm{CER}=\frac{\text{edit distance}}{\text{reference length}}=\frac26=\frac13.
$$
The <edit distance> makes the scoring rule explicit; one visual mistake need not count as one character edit.<Quality Assurance in OCR-D by OCR-D>[Evaluation Metrics, Levenshtein Distance and Character Error Rate sections]{ref}
A historical implementation illustrates the engineering choices: the 2007 description of <Tesseract (software)> starts from a binary image and organizes connected outlines into text lines and words before recognition passes. That is evidence about a particular engine architecture, not a requirement that every recognizer use separate character segmentation. Accuracy depends on the images, writing system, layout and evaluation convention; fluent-looking output can still contain consequential transcription errors.
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