4 Patterns

This program is a collection of four graphical pattern generators, selectable from a menu at line 1500. The first two modules draw symmetrical random-color plots and overlapping line patterns using OVER 1 (XOR) drawing mode. The third module, converted from an IBM program by Ted Knyszek, draws three successive geometric figures using an iterative polygon-shrinking technique: each pass replaces vertex coordinates with weighted averages (RU and SU fractions) of adjacent vertices, producing spirograph-like converging polygons. A Tiny Turtle module at line 1000 implements a simple Logo-style turtle graphics engine using trigonometric functions, accepting user input for repeat count, turn angle, and step distance. The program uses RANDOMIZE USR 50450 between geometric sections, suggesting a machine code routine stored in memory for screen capture or other utility purposes.

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Program Analysis

Program Structure

The program is organized as four independent modules sharing a common menu entry point at line 1500. Execution begins via RUN 1500 at line 5. Each module runs autonomously and returns to the menu via GO TO 90 or RUN at the end. The modules are:

1. Pattern 1 (lines 10–90): Random symmetric four-point XOR plots.
2. Pattern 2 (lines 100–160): Overlapping diagonal line sweeps using subroutines.
3. Geometric Drawings (lines 300–740): Three iterated polygon-shrink figures, converted from IBM BASIC.
4. Tiny Turtle (lines 1000–1130): Interactive Logo-style turtle graphics.

Pattern 1 — Symmetric Random Plotting

Lines 40–80 pick random coordinates x (0–127) and y (0–87), then plot four symmetrically mirrored points using PLOT x,y, PLOT 255-x,y, PLOT x,175-y, and PLOT 255-x,175-y. This produces four-fold reflective symmetry across both screen axes. OVER 1 enables XOR plotting so points toggle rather than overwrite, giving a flickering layered effect. Random INK and BRIGHT are set each iteration. The loop continues until any key is pressed (INKEY$<>"").

Pattern 2 — Diagonal Line Sweeps

Two subroutines at lines 150 and 160 each sweep all 256 x-positions, drawing diagonal lines across the full screen in opposite directions. Line 140 calls them in sequence with color changes, creating a dense cross-hatch interference pattern under OVER 1. The color selection at line 130 avoids repeating the previous ink color by looping until i <> a.

Geometric Drawings — Iterated Polygon Shrinking

This section (lines 320–740), credited to Ted Knyszek as an IBM conversion, draws three distinct figures using the same algorithmic skeleton: an outer loop over grid positions, an inner loop of N iterations that connects current polygon vertices, then updates them by blending adjacent vertices using the formula:

• T(M) = RU*X(M) + SU*X(NJ)
• K(M) = RU*Y(M) + SU*Y(NJ)

where SU is a small step fraction and RU = 1 - SU. Each iteration moves every vertex slightly toward the next one, causing the polygon to spiral inward, producing converging nested polygon figures. The three sub-figures use quadrilateral (4-vertex), triangular (3-vertex), and hexagonal (6-vertex) base polygons respectively.

NOT PI and SGN PI Idioms

NOT PI evaluates to 0 (since PI is non-zero, NOT PI = 0 in Sinclair BASIC), and SGN PI evaluates to 1. These are used throughout lines 350–710 as loop start values, replacing the literals 0 and 1. This is a well-known Sinclair BASIC code-golf idiom that saves a few bytes in the tokenized program.

Machine Code Call — RANDOMIZE USR 50450

Lines 440, 580, and 720 each call RANDOMIZE USR 50450 between geometric figures. The address 50450 falls in the upper RAM area; this is a machine code routine presumably stored there (likely loaded separately or resident in the system). It is called between patterns, possibly to perform a screen snapshot, palette reset, or other utility function. Its exact behavior depends on externally loaded code not present in this listing.

Tiny Turtle Module

Lines 1000–1130 implement a Logo-inspired turtle graphics engine. A helper function FN R(X) converts degrees to radians (X*PI/180). The turtle maintains a cumulative ANGLE variable, which accumulates the user-supplied right-turn value each step. Movement is computed as:

• ACROSS = SIN(FN R(ANGLE)) * FORWARD
• DOWN = COS(FN R(ANGLE)) * FORWARD

and rendered with a single DRAW ACROSS, DOWN. Note that DRAW uses relative coordinates, so this is correct. The initial PLOT 127,87 centers the turtle. After each full repeat loop, the program loops back to line 1030 to accept new parameters without clearing the screen unless the user requests it, allowing overlapping patterns to accumulate.

Menu and Navigation

The menu at lines 1500–1570 uses RUN n to jump to each module. RUN without a line number (line 1530) restarts from line 5, which immediately re-runs the menu via RUN 1500. Line 90 uses INPUT ... LINE a$ followed by bare RUN to return to the menu after Pattern 1 or Pattern 2. The CLEAR at line 1500 resets the variable space on each menu entry.

Notable Anomalies

• Line 350 uses NOT PI as a loop start of 0, iterating I and J from 0 to 3 (4 values). This means indices 1–4 of the X() and Y() arrays are used, matching the DIM sizes declared at line 330.
• Line 410 computes NJ = INT(M - 4*INT(M/4)) + 1, which is modulo-4 wrap (giving 1–4 cycling), used to link the last vertex back to the first.
• Line 540 contains DRAW (X2-X1),((Y2-30)-(Y1-30)), which simplifies to DRAW X2-X1, Y2-Y1; the subtraction of 30 from both operands cancels out and has no effect.
• The INK 9 at line 1500 is a transparent ink attribute (value 9 is not a standard ink color; on the TS2068, values above 7 may produce transparent or undefined ink behavior).