Graphics

This program is a multi-section graphics demonstration that cycles through six different mathematical drawing routines using the Spectrum’s PLOT, DRAW, and CIRCLE commands. Section 100 draws user-defined spirograph-style arcs using the angular form of DRAW with a calculated arc angle based on input X multiplied by PI. Section 200 plots a parametric spiral using X·sin(X) and X·cos(X) coordinates. Sections 300–600 explore further arc patterns, sine-wave compositions with relative DRAW offsets, vertical arc fans, and a circle-of-circles arrangement. The program uses OVER 1 (XOR drawing mode) in sections 100 and 300 so that repeated drawing erases itself, and ends with a colour-cycling text banner and a thank-you message before auto-saving.

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Program Analysis

Program Structure

The program is divided into numbered sections, each handling one graphical effect followed by a pause-for-keypress prompt. The layout is consistent throughout:

1. Lines 100–110: User-controlled spirograph arcs using angular DRAW
2. Lines 200–210: Parametric polar spiral (X·SIN X, X·COS X)
3. Lines 300–310: Arc fan with user-supplied odd number, drawn with OVER 1
4. Lines 400–410: Sine wave with relative DRAW decorations
5. Lines 500–510: Rotating arc fans from a central point, stepping J from 49 to 99
6. Lines 600–610: Circle-of-circles arrangement using trigonometric positioning
7. Lines 700–800–900: Colour-cycling text banner followed by a two-line thank-you message
8. Line 9999: Auto-saving SAVE with LINE 1

Each graphical section sets its own PAPER and INK colours and calls CLS, giving each effect a distinct colour scheme.

Wait-for-Keypress Idiom

Lines 110, 210, 310, 410, 510, and 610 all use PRINT #1;TAB 4;"PRESS ENTER TO CONTINUE" followed by PAUSE 0. Printing to stream #1 sends text to the lower screen (the two-line input area), keeping the graphics display in the upper screen intact. PAUSE 0 waits indefinitely until any key is pressed, which on this platform is satisfied by the Enter key being held.

Angular DRAW and OVER 1

The three-argument form DRAW dx,dy,angle draws a circular arc. Section 100 calculates the arc angle as (X*10+9)*3*PI, which for X in the range 0–46 produces a wide variety of arc lengths, creating spirograph-like patterns. The loop counter Y (entered by the user) is applied with OVER 1 active implicitly because DRAW OVER 1; sets XOR mode for that draw call — even numbers of repetitions restore the screen to blank, as the prompt text explains.

Section 300 similarly uses OVER 1 set globally before a two-iteration loop (FOR N=0 TO 1), so the arc is drawn and then erased, leaving the screen blank — which appears to be intentional given the instruction to choose an odd number to produce a visible multi-petal arc.

Parametric Spiral (Section 200)

The loop FOR X=0 TO 65 STEP .1 plots each point at coordinates (2*X*SIN X + 125, X*COS X + 88). This is a variant of an Archimedean or involute spiral expressed in Cartesian form using X as both the parameter and the radius scaling factor. The step of 0.1 means 651 PLOT calls, which is slow but produces a smooth curve.

Sine Wave with Relative DRAW (Section 400)

Section 400 plots a sine wave baseline with PLOT X, 40*SIN(X/10)+88 and then adds decorative elements at each point using two successive relative DRAW calls: DRAW 10,10 followed by DRAW -10,5. This creates a jagged zigzag overlay on top of the sine wave, producing a distinctive saw-tooth visual texture.

Circle-of-Circles (Section 600)

Section 600 steps N from 0 to 360 in steps of 8 degrees, converting to radians via N*PI/180. For each angle it computes a point on a circle of radius 50 centred near the middle of the screen, then draws a CIRCLE of radius 30 at that point. With OVER 1 not set, overlapping circles accumulate, but because the Spectrum’s CIRCLE command uses XOR at the pixel level when circles overlap, some arcs erase each other, creating interference patterns.

Colour-Cycling Banner (Section 700)

Lines 700 uses nested loops — FOR M=1 TO 5 outer, FOR N=0 TO 7 inner — to PRINT the string "L.I.S.T. USR GR " repeatedly while cycling PAPER N through all eight colours. INK 9 selects contrasting ink using the “bright” attribute value. Because CLS is not called within the loop, the text fills the screen progressively with a rainbow-striped effect.

Thank-You Message Centering

Lines 800 and 900 centre their respective strings using the formula INT((33 - LEN A$) / 2). The value 33 (rather than the standard 32) as the screen width divisor is slightly off — the Spectrum screen is 32 columns wide — so the centring will be shifted one column to the right compared to a perfectly centred calculation. This is a minor cosmetic anomaly.

Variable Usage Summary

Variable	Used in	Purpose
X	100, 200, 300, 400	User input or loop counter / coordinate
Y	100	User-entered repeat count for arc loop
N	100, 300, 600, 700	Loop counter (multi-use across sections)
J	500	Arc angle parameter for fan effect
M	700	Outer repeat count for colour banner
A$	800	“THANK YOU” message string
B$	900	“FOR YOUR INTEREST !” message string