Random Fan

This program draws an animated rotating fan pattern on screen using line-drawing subroutines. It cycles a point around the perimeter of the screen — along the bottom, right side, top, and left side in sequence — and from each perimeter position draws a straight line to the diagonally opposite point, creating a continuously evolving geometric pattern. The line-drawing routine (lines 410–510) manually implements Bresenham-style interpolation using unit vectors and a parametric step loop rather than the built-in DRAW command, allowing lines to be drawn between arbitrary pixel coordinates. INK colour and step interval q are randomised periodically, producing colour variation in the fan effect. A border rectangle is drawn at startup via the subroutine at line 610.

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Program Structure

The program is divided into four logical blocks:

1. Initialisation (lines 40–130): Clears screen, sets paper colour, draws border, initialises variables.
2. Perimeter loop (lines 140–220): Cycles a point around all four edges of the screen and calls the line-dispatch subroutine at line 300 for each position.
3. Line-dispatch subroutine (lines 310–380, labelled at line 300 by GO SUB): Computes opposite endpoint, calls the vector line-drawing routine, and periodically randomises colour and step size.
4. Vector line-drawing subroutine (lines 410–510): Draws a line between two arbitrary pixel points using unit-vector interpolation.
5. Border subroutine (lines 610–660): Draws the screen border rectangle with four DRAW calls.

Perimeter Traversal

The main loop in lines 140–220 moves a point around the screen perimeter in four legs using FOR loops with a step of 4 pixels:

• Lines 140–150: bottom edge, x from 5 to 250 (y implicit at 0 via initial value)
• Lines 160–170: right edge, y from 5 to 170
• Lines 180–190: top edge, x from 250 back to 5
• Lines 200–210: left edge, y from 170 back to 5

After all four legs, GO TO 140 repeats the animation indefinitely.

Opposite-Point Calculation

The dispatch subroutine at line 310 computes the diagonally opposite perimeter point as xb = 250 - x, yb = 170 - y. This maps each perimeter position to its geometric opposite within the 255×175 screen area, which is what creates the fan/string-art effect. The start point (xe, ye) is simply set to the current (x, y) at line 320 before the line routine is called.

Vector Line-Drawing Routine (Lines 410–510)

Rather than using the built-in DRAW command (which uses relative coordinates and would require offset arithmetic), the program implements its own parametric line plotter:

• a and b are the x and y spans from (xb,yb) to (xe,ye).
• The Euclidean length is computed: q = SQR(a*a + b*b).
• Unit vector components ux = a/q, uy = b/q are derived.
• A FOR l = 0 TO q STEP 4 loop steps along the line, plotting PLOT xb + l*ux, yb + l*uy at each step.

Using a step of 4 in the parametric loop means only every 4th pixel is plotted, which speeds up drawing at the cost of a dotted appearance. This is consistent with the perimeter step of 4.

Colour Randomisation

The variable q serves double duty: it is used as the line length in the drawing routine but is also separately used as a countdown threshold in the dispatch subroutine (lines 340–370). Every call to the dispatch routine increments z; when z >= q, z resets and both q (a new random value 0–49) and c (a random INK colour 0–6) are chosen. This creates irregular colour-change intervals.

Variable Summary

Variable	Role
x, y	Current perimeter point coordinates
xb, yb	Opposite endpoint (line start)
xe, ye	Current point copy (line end)
a, b	X and Y spans of line
q	Dual use: colour-change interval / line length
ux, uy	Unit vector components along line
l	Parametric distance along line
z	Counter for colour-change trigger
c	Random INK colour value