Definition
A scan line, also spelled scanline, is a single horizontal line of picture elements (pixels) that is processed, transmitted, or displayed sequentially in raster graphics systems. In raster-scan display devices—such as cathode‑ray tube (CRT) monitors, television sets, and many modern digital displays—the electron beam or pixel‑driving circuitry draws the image one scan line at a time, moving from the top of the screen to the bottom.
Technical characteristics
| Aspect | Description |
|---|---|
| Orientation | Horizontal; runs from the left edge to the right edge of the display area. |
| Sequencing | Scan lines are generated in order, typically from line 1 (top) to line N (bottom), where N equals the vertical resolution of the device. |
| Timing | The period required to draw a single scan line is called the horizontal line time; the interval between successive lines is the horizontal blanking interval. |
| Refresh | In interlaced video formats, two fields are composed of alternating scan lines (odd and even) that together form a complete frame. |
| Data representation | In digital image files and memory buffers, a scan line corresponds to a row of pixel data stored contiguously in memory. |
Historical development
- Early television and computer monitors employed analog CRT technology that physically swept an electron beam across the phosphor-coated screen line by line, producing visible scan lines.
- The concept carried over to early digital raster graphics hardware, where memory buffers were organized as arrays of scan lines to simplify address calculation (e.g., base address + line × stride).
- Modern flat‑panel displays (LCD, LED, OLED) no longer require a physical scanning beam, but the term persists in graphics APIs, video standards, and image processing to describe logical rows of pixels.
Applications
- Video display – Rendering engines output pixel data on a per‑scan‑line basis to match the timing requirements of display connectors (e.g., VGA, HDMI).
- Image processing – Algorithms often iterate over images one scan line at a time for operations such as convolution, edge detection, or compression.
- Graphics programming – Low‑level graphics libraries (e.g., DirectDraw, VESA BIOS Extensions) expose functions that manipulate individual scan lines for blitting or page flipping.
- Signal analysis – Oscilloscope or raster‑graph displays visualise waveforms as scan lines, illustrating the relationship between time and amplitude.
Related concepts
- Raster scan – The overall method of constructing an image by sequentially drawing a series of scan lines.
- Interlaced scanning – A technique that alternates the display of odd and even scan lines to reduce bandwidth while preserving perceived resolution.
- Pixel – The smallest addressable element of a digital image, which together with others forms scan lines.
- Horizontal blanking interval – The time during which the display circuitry returns the beam to the start of the next line.
References
- Standard video timing specifications (e.g., ITU‑BT.601, CEA‑861).
- Graphics programming manuals for platforms such as Windows GDI, DirectX, and OpenGL.
- Historical overviews of CRT technology in television and computer display engineering literature.