GRAPHICS

This TS2068 BASIC program presents a looping graphics demonstration featuring ten distinct visual sequences, each accompanied by BEEP-generated sound effects. Sequences include sine-wave plots combined with DRAW crosses, rolling CIRCLE chains, colored horizontal scan-line fills using INVERSE, spiral inward rectangles, radial line fans, random noise patterns, an outlined “US ARMY” shield graphic with arc-based DRAW calls using the three-argument (arc) form, and a polygon-based star/box pattern. The program concludes with a separate spiral section (credited in a REM to “2068 SPIRAL”) that computes polygon vertices using trigonometry stored in DIM arrays, drawing all connecting chords between vertex pairs. The variable zo is initialized to 0 at line 20 and used pervasively as a zero placeholder throughout, a common memory-saving idiom. After completing all sequences, the program restarts itself with RUN for continuous looping, and line 1650 provides a SAVE with autostart.

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Program Structure

The program is organized as a linear sequence of self-contained display segments, each concluded by a PAUSE 240 and CLS to give the viewer time before the next segment. After all segments complete, line 1630 executes RUN, restarting the demonstration from the beginning. Lines 1640–1650 are unreachable dead code providing a CLEAR and SAVE with autostart at line 10.

Lines	Segment Description
10–50	Title screen: “GRAPHICS” centered, brief PAUSE
60–120	Sine wave (PLOT) combined with DRAW crosses at each point
130–170	Rolling chain of CIRCLE calls, INK 2
180–330	Outlined shape (lines 180–230), then horizontal scan-fill with INVERSE wipe
340–470	Colored horizontal stripes (STEP 5) then OVER 1 vertical lines
480–530	Radial fan lines from two fixed points
540–670	Random small box-and-diamond shapes scattered across screen
680–770	Inward-spiraling rectangle using alternating DRAW calls
780–820	Concentric circles with random INK colors, centered at (127,87)
830–920	Parallelogram-like shape built from accumulated DRAW calls
930–1100	US ARMY shield: fan lines, arc DRAW calls, OVER 1 text label
1110–1240	Nested square spiral with diagonal connectors, random INK
1250–1310	Star burst: random DRAW from screen center (127,87)
1320–1390	Expanding concentric squares from center
1400–1420	“SPIRALS” title card
1430–1630	Polygon chord spiral: n-gons from n=1 to 20, all chords drawn

The zo Zero Variable Idiom

Line 20 sets LET zo=0. This variable is then used throughout the program wherever a literal 0 would appear — in DRAW arguments, FOR loop starts, INK and INVERSE values, and PAPER settings. On the ZX Spectrum, storing a frequently-used small integer as a variable and referencing it by name is slightly shorter in tokenized form than repeating the full floating-point literal 0, saving a few bytes per occurrence across a long program.

DRAW Arc Syntax

Several segments use the three-argument form of DRAW, e.g. DRAW -10,zo,-2 (line 190) and DRAW -30,-7,.77 (line 1030). The third argument is the arc angle in radians; a value of 0 gives a straight line, positive/negative values curve the line clockwise or counterclockwise. The US ARMY shield segment (lines 930–1060) makes deliberate use of this to form curved top and bottom edges of the shield outline.

Polygon Chord Spiral (Lines 1430–1630)

This segment, credited in a REM to “2068 SPIRAL”, computes the vertices of a regular n-gon for n from 1 to 20 and draws all chords between every pair of vertices — producing the visual effect of increasingly complex complete graphs. Key implementation details:

• DIM x(25) and DIM y(25) store computed vertex coordinates.
• Vertex angle: a = i * (PI / (n/2)), equivalent to 2*PI*i/n, giving evenly spaced points on a circle of radius 87 pixels.
• Chord drawing uses a double nested loop (j from 1 to n, k from j to n) for the upper triangle of the adjacency matrix, avoiding duplicate lines.
• Line 1490 contains IF n<3 THEN NEXT n, skipping the inner FOR i loop body for n=1 and n=2 by jumping directly to the next n.
• Line 1610 checks INKEY$="z" to trigger a COPY (printer dump) while the image is displayed.
• POKE 23609,20 at line 1430 sets the printer speed system variable (CDFLAG area), relevant to the COPY command behavior on this system.

Sound Generation

Sound is produced throughout using BEEP with very short durations (typically 0.005 or 0.01 seconds) tied to the loop variable A. The pitch formula a/10+10 maps the loop counter to a rising musical pitch, producing a continuous ascending tone as graphics are drawn. The condition a/10 - INT(a/10) < 0.05 (or similar thresholds) is used in some loops to reduce BEEP frequency, only sounding a note every 10 iterations approximately, preventing the audio from slowing down rendering too much.

OVER and INVERSE Usage

Lines 280–320 demonstrate a scan-line fill and wipe technique: first a FOR loop draws horizontal lines bottom-to-top using DRAW 255,zo (INK 1, normal video), then a second pass uses INVERSE 1 to erase those lines top-to-bottom, effectively creating a wipe animation. Lines 390–450 use OVER 1 (XOR drawing mode) to overlay vertical colored lines on top of the previously drawn horizontal stripes without erasing them, producing a color grid.

Potential Anomalies

• Line 1490: IF n<3 THEN NEXT n — this exits the FOR i loop prematurely for small n by jumping to the outer NEXT n, which is valid but non-obvious control flow.
• Lines 370 and 410: LET i=i+1: IF i=8 THEN LET i=2: INK i — the INK i call only executes when i wraps to 2, so the INK attribute set by PLOT INK i; in the following line is the operative color-setting mechanism for other values of i.
• Lines 1640–1650 are unreachable since line 1630 executes RUN, restarting the program. These lines exist solely for the SAVE with autostart and would need to be run manually from the editor.
• The segment at lines 830–920 accumulates DRAW calls without resetting position via PLOT between many of them, so the figure drawn is highly dependent on the exact turtle-graphics position left by prior DRAW calls — this appears intentional for the abstract shape produced.