Moire

This program is a multi-section abstract graphics demonstration that cycles through several distinct drawing routines. The first section (lines 10–100) uses PLOT and DRAW with OVER 1 (XOR drawing mode) to create a pair of overlapping line-sweep patterns across the full screen. The second section (lines 200–380) draws a continuously mutating four-sided figure by PEEKing the current pen position from system variables at addresses 23677–23678 (COORDS X and Y) to chain successive DRAW commands into a closed shape, while bouncing the origin point off screen boundaries. The third section (lines 400–500) uses arrays to animate two triangles with INVERSE toggling, and the fourth section (lines 600–800) draws a spinning or rotating line figure built from four symmetric FOR loops converging on a central region, toggling INVERSE each full cycle. A final section (lines 810–880) generates random-step line fans from the screen center and edges before clearing and repeating.

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Program Analysis

Program Structure

The program is organized as five self-contained graphical routines that run sequentially, though only one contains an explicit loop back to restart it. There is no menu or user input — execution simply falls through from one section to the next.

Lines	Section	Description
10–100	Line Sweep (OVER 1)	Two-pass XOR line fan across full screen
200–380	Bouncing Quad	Chained DRAW using COORDS PEEKs, bouncing origin
400–500	Triangle Animation	Array-driven dual triangles with INVERSE toggling
600–800	Rotating Star	Four-loop radial line figure with INVERSE cycling
810–880	Random Fan	Random-step line fans from center and edges

Note the significant line number gaps (100→200, 380→400, 500→600, 800→810): these are deliberate spacing choices to keep sections visually separated and to allow future expansion.

Section 1: XOR Line Fan (Lines 10–100)

OVER 1 at line 10 enables XOR pixel drawing throughout. Lines 20–50 sweep t from 0 to 100 in steps of 0.753, drawing pairs of lines from the bottom and top of the screen that converge toward the center, creating a moiré-like pattern. Lines 70–100 repeat with t running from −50 to 150, effectively shifting the fan and producing interference with the first pass due to XOR mode. The step value 0.753 is carefully chosen to produce a dense but non-repeating fan within the screen’s 256-pixel width. The PAUSE 500 at line 60 holds the completed first pass for about 10 seconds before clearing.

Section 2: Bouncing Quad with COORDS PEEK (Lines 200–380)

This section exploits the ZX Spectrum system variables COORDS at addresses 23677 (X) and 23678 (Y), which hold the coordinates of the last PLOTted or DRAWn point. After a PLOT x+10,y, each subsequent DRAW at lines 260–290 reads the current pen position via PEEK 23677 and PEEK 23678 to compute relative displacements, forming a closed four-segment figure. This chains four lines into a quadrilateral without needing to track position in BASIC variables.

The origin point (x,y) bounces off screen edges: lines 320–350 clamp values to [0,175], and lines 360–370 negate the step variables e and m on boundary contact. There is a minor bug at line 370: the upper boundary check uses y=190 instead of y=175, meaning the top boundary is never actually triggered and m only negates on y=0.

Section 3: Dual Triangle Animation (Lines 400–500)

Two 1D arrays x(6) and y(6) define the vertices of two triangles. Lines 430–440 initialize the six vertices at corners and midpoints of the screen. At line 450, INVERSE RND*1 randomly selects normal or inverse drawing mode, since RND*1 produces a value in [0,1) which, when passed to INVERSE, is truncated to either 0 or 1. Lines 460–470 draw two closed triangles by plotting and chaining three DRAW calls per triangle. Line 480 updates all six vertices in a single compound LET statement, shrinking and shifting them. The loop terminates at line 500 when x(6)>=128, halting after roughly 128 iterations.

Section 4: Rotating Star Figure (Lines 600–800)

Four FOR loops (lines 630–650, 660–690, 700–730, 740–770) each draw a set of lines radiating from a common region near screen center. The loops step in opposite directions on alternating axes to produce rotational symmetry. After all four loops complete, a is toggled with NOT a at line 780, and INVERSE a at line 790 flips the drawing attribute before looping back to line 620 indefinitely. This produces an alternating normal/inverse pulsing animation as lines accumulate on screen without clearing.

Section 5: Random Line Fan (Lines 810–880)

Lines 810–880 generate two sets of line fans: one sweeping horizontally from center point (128,175) and (128,0), and one sweeping vertically from (255,88) and (0,88). The step sizes a and b are randomized each cycle via RND*10+1, giving variable density. After both fans are drawn, PAUSE 100 holds the display for about 5 seconds before CLS and GO TO 810 restart the loop. Note the inconsistent capitalization NEXT W (uppercase) at line 870 versus w (lowercase) used throughout — this is accepted by the interpreter as variable names are case-insensitive in this context.

Notable Techniques

• OVER 1 / XOR drawing: Used in section 1 to create moiré interference patterns between two line-sweep passes.
• PEEK 23677/23678 (COORDS): Reads the hardware-maintained current drawing position to chain DRAW commands without manual coordinate tracking.
• INVERSE RND*1: Compact idiom for randomly toggling inverse video mode by exploiting integer truncation of a [0,1) random value.
• NOT a for toggling: Uses the logical NOT function to flip a binary flag between 0 and 1 each cycle.
• Compound LET: Line 480 packs ten assignments into one statement, saving memory at the cost of readability.
• Non-integer STEP values: Steps of 0.753 and 7.5 produce dense, non-repeating patterns suited to the screen’s pixel resolution.

Bugs and Anomalies

• Line 370: boundary check y=190 should likely be y=175 to match the screen height; as written, the upper Y bounce never fires.
• Line 380: GO TO 260 skips the initial PLOT at line 250 on subsequent iterations, meaning the chained DRAW calls depend on the previous draw endpoint rather than a freshly PLOTted origin — this may be intentional to create continuous curves.
• Line 870: NEXT W uses uppercase while the loop variable is declared lowercase w; functionally harmless but inconsistent.
• Lines 100 and 500 fall through to the next section with no explicit GO TO, relying on sequential execution — if a section is jumped into directly, initialization from prior lines may be missing.