Consider a line AB with A = (0, 0) and B = (8, 4). Apply a simple DDA algorithm and compute the first four plots on this line.
[(0, 0), (1, 1), (2, 1), (3, 2)]
The Digital Differential Analyzer (DDA) steps along the axis of greater change one unit at a time and adds the slope to the other coordinate, rounding it to the nearest pixel.
Step 1 — \(\Delta x = 8,\ \Delta y = 4\), so the number of steps is \(\max(8, 4) = 8\).
Step 2 — Increments: \(x_{inc} = 8/8 = 1,\ y_{inc} = 4/8 = 0.5\).
Step 3 — Trace the first four points, rounding y (0.5 rounds up):
| k | x | y (exact) | plotted |
|---|---|---|---|
| 0 | 0 | 0 | (0, 0) |
| 1 | 1 | 0.5 | (1, 1) |
| 2 | 2 | 1.0 | (2, 1) |
| 3 | 3 | 1.5 | (3, 2) |
The lists containing 1.5 or 2.5 cannot be plots, because a pixel address is an integer; the list with (1, 2) would need y to rise by 2 for one unit of x, i.e. slope 2, whereas this line has slope 0.5.
[(0, 0), (1, 1), (2, 1), (3, 2)] option is correct.
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