Theory
Sweep every pixel, or draw only the lines?
A display has to move its beam (or update its pixels) to form the picture, but there are two fundamentally different strategies for doing so, and they shaped two families of graphics hardware.
A raster-scan display sweeps across every pixel, row by row, painting the whole screen from stored data. A random-scan display instead moves the beam directly between the endpoints of lines, drawing only the strokes of the picture. This lesson contrasts the two. The raster approach won and underlies modern screens, but understanding both clarifies why the frame buffer and pixel-by-pixel drawing matter.
Theory
Raster scan: sweep every row
In a raster-scan display, the beam moves in a fixed pattern: it sweeps horizontally across a row, then drops to the next row, covering the whole screen top to bottom, and repeats to refresh. It visits every pixel whether or not the picture has anything there.
The picture is stored as a grid of pixel values in a frame buffer (also called the refresh buffer), and the display reads this buffer to know each pixel's colour. Because every pixel can be set independently, raster displays handle filled areas, shading, and realistic images well. This is how TVs and modern monitors work, and why raster is the dominant approach.
Theory
Random scan: draw only the strokes
In a random-scan display (also called vector, stroke, or calligraphic), the beam does not sweep every pixel. Instead it moves directly to the endpoints of each line in the picture and draws that line, jumping around to draw the strokes in any order, only where the picture actually has lines.
The picture is stored as a list of line-drawing commands (a display file or display list), and the display redraws that list to refresh. This produces very smooth, high-resolution lines, great for wireframe drawings, but it cannot easily produce filled or shaded areas (it draws lines, not regions). Random-scan displays were used in early vector graphics systems.
At a glance
| Aspect | Raster scan | Random scan |
|---|---|---|
| Beam path | Sweeps every row, every pixel | Jumps directly between line endpoints |
| Picture stored as | A grid of pixels (frame buffer) | A list of line commands (display file) |
| Good at | Filled areas, shading, realistic images | Smooth high-resolution lines |
| Weak at | Very fine lines can look jagged | Filled/shaded areas |
Quiz
A raster-scan display stores its picture as what, and how does the beam move?
- As a list of line commands; the beam jumps between line endpoints
- As a grid of pixel values in a frame buffer; the beam sweeps every row, pixel by pixel
- As geometric shapes only; the beam draws circles
- It does not store the picture at all
Show the answer
As a grid of pixel values in a frame buffer; the beam sweeps every row, pixel by pixel
A raster-scan display stores the picture as a grid of pixel values in a frame buffer, and the beam sweeps the screen row by row, covering every pixel, reading the buffer to colour each one. Option A describes a RANDOM-scan (vector) display, which stores a list of line commands and moves the beam directly between line endpoints, the opposite approach. Option C is not how raster scanning works; it paints pixels from the buffer, not shapes directly. Option D is wrong: raster displays absolutely store the picture, in the frame buffer, which is exactly what is refreshed onto the screen. Raster = every pixel from a frame buffer; random scan = only lines from a display list.
Think first
Why did raster scan become dominant over random scan?
Random scan drew beautifully smooth lines. Why did raster-scan displays win out? Then tap.
Show the answer
Because raster scan can display FILLED, SHADED, PHOTOREALISTIC images, not just line drawings, and as memory became cheap, its pixel-grid approach proved far more versatile and scalable for the images people actually wanted. A random-scan (vector) display is excellent at one thing: crisp lines. It draws the strokes of a picture directly, so wireframes and line art look wonderfully smooth. But it fundamentally draws LINES, not regions, so it cannot easily fill areas with colour or produce the smooth shading, textures, and photographic detail of real scenes; its picture is a set of strokes, not a full field of colour. A raster display, by contrast, stores and controls EVERY pixel independently in a frame buffer, so it can paint any image at all, solid fills, gradients, photographs, video, arbitrary colour per pixel, which is exactly what television, photographs, games, and rich user interfaces demand. The historical catch was memory: a frame buffer holding every pixel needs a lot of it, which was expensive early on (favouring vector displays that only stored a line list). But as memory got dramatically cheaper and faster, that barrier fell, and raster's ability to show ANY image, combined with cheap memory and mass-produced raster hardware (TVs, then monitors), made it the universal choice. The price raster pays is that fine diagonal lines can look jagged (aliasing), because they must be approximated on a pixel grid, which is precisely why line-drawing algorithms like Bresenham exist. So raster won on VERSATILITY once memory was affordable: it can show everything, not just lines. Full-image capability plus cheap memory beat beautiful-lines-only.
Summary
Key takeaways
- Displays draw in two ways: raster scan (sweep every pixel) or random scan (draw only lines).
- Raster scan sweeps the beam row by row over every pixel, reading the picture from a frame buffer (grid of pixel values).
- Raster handles filled areas, shading, and realistic images well; it dominates modern screens.
- Random scan (vector/stroke) moves the beam directly between line endpoints, drawing strokes stored as a list of line commands (display file).
- Random scan gives smooth high-resolution lines but cannot easily fill or shade areas.
- Raster won because it can display any image (fills, shading, photos), especially once memory became cheap.
- Memory hook: raster = every pixel from a frame buffer; random scan = only lines from a display list.