Character and code reading need contrast, position control, and a pass definition that matches the process need. Human-readable text must sit in a location window, remain readable in deformation, spacing, and size, and match the expected string. Characters that look alike are a font problem. Laser on a consistent substrate usually beats ink. Two-dimensional codes must decode on a similar background. Offline graders can score a code under controlled conditions. Online verification is less controlled and more prone to drift, so a grade on the line should not be treated as a lab grade.
Fielding, minimum character size for periods and dashes, and the diagnostics for height and width are the usual reader corrections. If a fixed mark can fixture the reader, the search window stays honest. An oversized window will eventually read the wrong line and do it with high confidence.