Doorway

This program draws a perspective doorway scene using a series of PLOT and DRAW commands on the Spectrum’s high-resolution graphics display. It constructs the floor, ceiling, archway, and door frame through four distinct FOR loops that calculate vanishing-point coordinates geometrically. The door opening is filled with horizontal ink stripes, and an elliptical arch is drawn using DRAW’s optional angle parameter. A final animation loop flashes random PAPER and INK colours over the doorway area to simulate a flickering light effect, using OVER 1 to XOR the colours without permanently overwriting the underlying graphics.

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Program Analysis

Program Structure

The program is divided into clearly sequential phases, each handled by one or two numbered lines:

1. Setup (line 10): Sets INK 1 (blue), PAPER 6 (yellow), BORDER 3 (magenta), BRIGHT 1, and clears the screen.
2. Door frame sides (line 20): Draws the two vertical uprights of the door frame with a simple PLOT/DRAW pair.
3. Floor perspective (line 30): Iterates f from 100 to 1 in steps of −2, plotting horizontal lines that narrow and rise to simulate a receding floor.
4. Ceiling perspective (line 40): Similar loop going upward to form the ceiling above the doorway.
5. Arch top (line 50): Draws the top of the arch area with narrowing horizontal lines converging to a vanishing point.
6. Door surround and arch (line 60): Two iterations (f=0 and f=10) draw the rectangular door frame sides and top, plus a semi-elliptical arch using DRAW …,-PI.
7. Door fill (line 70): Fills the door opening with INK 0 (black) horizontal stripes every 2 pixels.
8. Arch highlight (line 80): Uses OVER 1 to XOR semi-ellipses in INK 2 (red) over the arch, creating a decorative inlay effect without destroying the background.
9. Centre divider (line 90): Draws a vertical line down the middle of the door.
10. Caption (line 95): Prints a description to the lower screen channel #1.
11. Flash animation (line 100): Triple-nested loop cycles through random PAPER, INK, and BORDER colours with FLASH 1 over the door area for an animated light-flicker effect.

Geometric Techniques

The perspective effect in lines 30–50 is achieved by a simple linear projection. For the floor (line 30), each horizontal stripe at depth f is positioned at x = 128−f/2 and y = 100−f, with width f. This makes lines converge toward the point (128, 100) — the notional vanishing point at the centre of the doorway. The ceiling uses a similar formula but projects upward (100+f/2).

Line 60 exploits the three-argument form of DRAW x,y,angle. Passing -PI as the angle argument draws a semi-ellipse, enabling the curved arch top without any trigonometric calculation in BASIC.

Colour and Display Attributes

Lines 70 and 80 make effective use of attribute modifiers within DRAW statements. INK 0 in line 70 fills the door interior with black stripes, while OVER 1; INK 2 in line 80 XOR-draws the arch highlight. Because OVER 1 XORs pixels rather than setting them, the highlight appears only where the arch outline already exists, avoiding colour clashes in the attribute grid.

Animation Loop (Line 100)

The animation in line 100 is a triple FOR loop: the outer loop (f, 50 iterations) controls duration; the middle loop (g, stepping by 2 through rows 9–20) selects screen rows; the inner loop (h, 0–1) prints two consecutive rows per iteration. Each PRINT uses PAPER RND*8; OVER 1; FLASH 1 with a ten-space string to paint random colour attributes with flashing enabled over the doorway region. BORDER RND*7 is also called each inner iteration, adding border-colour variation to the effect.

Notable Idioms and Details

• PRINT #1; in line 95 directs output to the lower screen (channel 1) rather than the main display area, preserving the graphics.
• The STEP -2 and STEP 2 increments in the perspective loops reduce the number of iterations by half while still producing a solid visual fill at the resolution used.
• The two-value iteration in line 60 (STEP 10 over 0 and 10) is a compact way to draw both a thin inner and thicker outer door surround in a single loop.
• SAVE "doorway" LINE 10 at line 9998 saves the program with an auto-start at line 10, bypassing the REM header lines.

Bugs and Anomalies

The source REM at line 3 contains a typo — “Specyrum” instead of “Spectrum” — which appears to be a transcription error from the original book listing. There are no logical bugs in the drawing or animation routines. The INK RND*8 expression in line 100 could theoretically yield INK 8 (transparent), which on the TS2068 would leave the ink colour unchanged rather than setting a new one, potentially reducing the randomness of the effect in some iterations.