Auto Draw

This program draws a three-dimensional box wireframe on screen using PLOT and DRAW commands, accepting user-supplied height, width, and length values. After rendering the box, it decorates the screen border with a repeating diagonal zigzag pattern drawn along all four edges using an ON ERR loop technique to detect when the drawing reaches the screen boundary. A separate, unreachable routine (lines 580–660) demonstrates an INK-colored horizontal fill pass through the interior of the box using POINT to locate a starting position.

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Program Analysis

Program Structure

The program is organized into three distinct phases, with a fourth unreachable section appended as a work-in-progress:

1. Box drawing (lines 300–360): Accepts three dimensions via INPUT and renders a wireframe isometric box using a sequence of PLOT/DRAW commands, then prints dimension labels.
2. Border decoration (lines 370–550): Draws a repeating diagonal zigzag along each of the four screen edges, using ON ERR GO TO to exit each edge’s loop when the drawing goes out of bounds.
3. Unreachable color fill (lines 570–660): A commented stub that would flood-fill the box interior with a colored horizontal stripe pattern; it is never reached because line 560 STOPs the program.
4. Save/Verify (line 9000): A standalone tape utility line not called from the main flow.

Box Drawing Technique

The isometric box is constructed from a fixed anchor point PLOT 75,95 using only relative DRAW commands. The front face is a rectangle drawn with vertical and horizontal segments, while depth lines are drawn diagonally using equal X and Y offsets (the l parameter), which gives an approximate 45-degree isometric projection. The back face and connecting edges are then completed to close the shape.

The dimension labels are printed at fixed screen positions (rows 18–20) using PRINT AT, which may overlap the border zigzag decoration drawn immediately afterward.

Error-Trapping Border Loop Idiom

Each of the four border zigzag sections uses the same pattern:

• PLOT to a corner of the screen.
• Two DRAW calls producing a V- or W-shaped zigzag step.
• ON ERR GO TO exit_line — sets an error handler to jump forward when the next draw goes out of bounds.
• An unconditional GO TO back to the DRAW pair, repeating indefinitely until the out-of-bounds error fires.

This is an elegant boundary-detection idiom: no coordinate arithmetic is needed to know when an edge is reached. The four corners used are (0,0), (0,175), (255,175), and (250,0), tracing bottom-left → top-left → top-right → bottom-right. The starting X for the bottom-right corner is 250 rather than 255, which may cause a slight gap at the final corner where the two bottom zigzags meet.

Unreachable Color Fill Routine

Lines 570–660 contain an unfinished fill routine that is dead code — line 560 halts execution before it can be reached. The routine attempts to find the left edge of the box’s top face using a POINT probe scan along row 100, then draws 48 horizontal lines with DRAW INK c;47,0 stepping upward. The color is set via LET c=4 (green in Spectrum/TS2068 color indexing). The loop variable n is reused across both FOR loops, with the second loop starting from the final value of n after the first loop exits — this is intentional, carrying the detected X position into the fill phase.

Notable Techniques and Anomalies

• ON ERR as a loop terminator: Using error trapping rather than bounds-checking arithmetic is a compact and effective TS2068 technique.
• ON ERR RESET at line 550: Clears the error handler before STOP, which is good practice to avoid stale error handlers interfering with subsequent user interaction.
• Dead code block (lines 570–660): The fill routine appears to be an experimental addition never integrated into the main flow. It could be activated by removing or commenting out line 560.
• Fixed anchor point: The box is always drawn anchored at PLOT 75,95 regardless of the entered dimensions, so very large values of h, w, or l will cause out-of-bounds errors during the box draw itself, before the border decoration phase.
• Isometric approximation: Using equal X and Y for the depth parameter l produces 45-degree depth lines rather than true isometric (which would use a 2:1 ratio), giving a cabinet oblique projection appearance.

Variable Summary

Variable	Purpose
h	Height of the box (vertical extent)
w	Width of the box (horizontal extent)
l	Depth/length of the box (diagonal offset)
c	INK color index for fill (set to 4, green)
n	Loop counter; reused as edge-scan X position and fill row index