A scan line is one line, or row, in a raster scanning pattern, such as a video line on a cathode ray tube (CRT) display of a television or computer. A scan line represents a row of picture elements (pixels) in the image being displayed. The term is used, by analogy, for the representation of a single row of bitmap data in raster graphics. Scan lines are important in representations of image data, because many image file formats have special rules for data at the end of a scan line. For example, there may be a rule that each scan line starts on a particular boundary (such as byte or word; see for example BMP file format). This means that even otherwise compatible raster data may need to be analysed at the level of scan lines in order to convert between formats. On older CRT screens, the horizontal scan lines were visually discernible even when viewing from a distance, as alternating colored lines and black lines. This is sometimes used today as a visual effect in computer graphics.
