Example of the scan line effect deliberately applied to an image
PAL video signal scan line. From the left: horizontal sync pulse, back porch with color burst, signal itself, front porch, sync pulse, back porch with color burst, video portion of the next scan line; different signals of multiple lines are laid over, showing shaded areas instead of a single curve
On older CRT screens the horizontal scan lines were visually discernible, even when viewed from a distance, as alternating colored lines and black lines. This is sometimes used today as a visual effect in computer graphics.
The term is used, by analogy, for a single row of pixels in a raster graphics image. 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 a byte or word; see for example BMP file format). This means that even otherwise compatible raster data may need to be analyzed at the level of scan lines in order to convert between formats.