Architecture of Raster-Scan Displays

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

Architecture of Raster-Scan Displays

The technology behind virtually every screen you own today.

bytes = width × height × bits-per-pixel ÷ 8 With w = 1024, h = 768, bpp = 24, KB = 2304, MB/s = 135.

1024 × 768 at 24 bpp needs 2304 KB — about 2.25 MB. Set it to 1920 × 1080 and watch both numbers climb.

A raster-scan display draws an image by scanning across the screen row by row (like reading a book line by line), lighting up each individual pixel according to data stored in a frame buffer — a chunk of memory holding the color value for every single pixel on screen.

🖥️ Key Terms

Refresh rateHow many times per second the entire screen is redrawn (e.g. 60Hz, 144Hz) — higher feels smoother, especially for fast motion.

ResolutionThe number of pixels across and down (e.g. 1920×1080) — more pixels means finer detail.

Aspect ratioThe proportional relationship between width and height (e.g. 16:9).

💡 One-liner: "Why can raster-scan displays show filled, shaded regions easily, while vector displays cannot?" → Raster scan lights up EVERY pixel in a region individually via the frame buffer, so solid fills are natural; vector displays can only trace outlines/lines.

Sizing the frame buffer — the standard numerical

The frame buffer holds one colour value per pixel, so its size follows directly from the resolution and the colour depth:

bytes = width × height × bits-per-pixel ÷ 8 Worked example: 1024 × 768 at 24 bits per pixel bits = 1024 × 768 × 24 = 18,874,368 bits bytes = 18,874,368 ÷ 8 = 2,359,296 bytes = 2,359,296 ÷ 1024 ÷ 1024 ≈ 2.25 MB

Note what happens when resolution doubles in both directions: the pixel count quadruples, and so does the memory. Going from 1024×768 to 2048×1536 takes the frame buffer from about 2.25 MB to about 9 MB. Resolution costs are quadratic, not linear, which is the point the question is usually testing.

💡 Bits per pixel determines the number of available colours as 2^bpp: 1 bit gives 2 colours, 8 bits gives 256, and 24 bits gives about 16.7 million — a byte each for red, green and blue. Early systems used a palette precisely to avoid storing three full bytes per pixel when memory was scarce.

Why refresh needs bandwidth as well as capacity

The buffer is not merely stored, it is read out completely for every refresh — 60 or more times per second. A 1920×1080 screen at 24 bpp is about 5.93 MB per frame, so at 60 Hz the display alone streams roughly 0.37 GB every second, continuously, whether or not the picture changes.

That sustained bandwidth requirement, rather than the capacity, is why display memory has historically been separate and faster than main memory.

Syllabus points

Raster scan, frame buffer, refresh rate
Aspect ratio, resolution

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