Vector (Random-Scan) Displays

Theory of Computation & Computer Graphics — Introduction of Computer Graphics, NEC licence examination syllabus (Nepal Engineering Council).

Vector (Random-Scan) Displays

The technology that came before raster-scan dominated everything — still occasionally relevant for specific use cases.

A random-scan (vector) display draws an image by directing the electron beam DIRECTLY along the lines that make up the picture — like an Etch-A-Sketch tracing outlines, rather than filling in a grid of pixels. This made vector displays excellent for crisp line drawings (like old radar screens and oscilloscopes), but essentially useless for solid, filled color regions.

💡 One-liner: "Give one classic real-world use of a vector/random-scan display." → Radar screens and oscilloscopes — applications needing crisp lines, not filled colour regions.

Why it could not survive, and what it was good at

The vector display's defining constraint is that it draws only what is there. There is no grid, and therefore no way to fill an area: to make a solid rectangle the beam would have to trace hundreds of adjacent lines, and it would flicker badly doing it.

But that same property was its advantage. Because it stores a display list of line endpoints rather than a pixel for every position, a screen showing a few hundred lines needed very little memory — at a time when memory was the most expensive component in the machine. And the lines were perfectly smooth, with none of the staircase edges that raster displays produce.

⚡ Vector versus raster, as examiners frame it

StorageVector stores line endpoints; raster stores every pixel whether or not anything is drawn there.

Filled areasVector cannot practically produce them; raster does so naturally.

Line qualityVector lines are smooth; raster lines are approximated by pixels and show jagged edges.

Refresh costVector refresh time grows with picture COMPLEXITY; raster refresh is constant regardless of content.

💡 That last row is the subtle one and a common exam question. A vector display redrawing a complicated picture flickers because each refresh takes longer, while a raster display sweeps the same fixed grid at the same rate no matter what is on it. Complexity costs vector time; it costs raster nothing.

Syllabus points

Random-scan architecture

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