---
title: "Contour"
id: 70771
type: "computer_media"
slug: "contour"
url: "http://localhost/computer_media/contour/"
markdown_url: "http://localhost/computer_media/contour.md"
published_at: "2026-08-23T15:08:14+00:00"
modified_at: "2026-08-23T17:18:27+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2022/04/20230809-041332.jpg"
excerpt: "Draw two terrain profiles by hand or let a sine-wave generator do it, then watch the program project a contoured 3D landscape in three different orientations."
category:
  - name: "Archived Media"
    slug: "archived-media"
    taxonomy: "category"
    url: "http://localhost/category/archived-media/"
post_tag:
  - name: "Downloadable"
    slug: "downloadable"
    taxonomy: "post_tag"
    url: "http://localhost/tag/downloadable/"
  - name: "TS 2068"
    slug: "ts2068"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts2068/"
model:
  - name: "Timex/Sinclair 2068"
    slug: "ts-2068"
    taxonomy: "model"
    url: "http://localhost/model/ts-2068/"
genre:
  - name: "Graphics"
    slug: "graphics"
    taxonomy: "genre"
    url: "http://localhost/type/graphics/"
media_type: "Program"
download_url: "https://archive.org/download/timex-sinclair-software-archive/Contour%20%28198x%29%28-%29%28TS2068%29%28US%29%28Program%29.zip"
mediadate: "198x"
media_type_tags: "Graphics"
---

# Contour

Contour is an interactive 3D terrain visualization program that lets the user draw two elevation profiles — a lower (south) curve and an upper (north) curve — by pressing keys to adjust height in real time as a cursor advances across the screen. The two profiles are stored in arrays L() and H() indexed by horizontal position. From these profiles, the program can render a south-to-north cross-section using DRAW with a radian-angle argument, a west-to-east cross-section by interpolating 20 intermediate points per column, or both combined. A built-in sine-wave terrain generator (lines 1000–1060) auto-populates the L() array with a sinusoidal landscape for users who prefer not to draw manually. The program reuses the same two 127-element arrays for both drawing passes, with the second pass offset by D=127 so the cursor plots into the right half of the display.

***

### Program Structure

The program is organized into three functional phases:

1. **Profile drawing (lines 10–130):** The user interactively draws two elevation curves — a lower (L) and an upper (H) — using key presses, or jumps to the sine-wave generator at line 1000.
2. **Rendering (lines 140–290):** The user chooses a projection direction and the stored profiles are rendered as a contour plot in one or both orientations.
3. **Sine-wave generator (lines 1000–1060):** Auto-populates the L() array with a sinusoidal curve before falling through to the rendering phase.

### Interactive Profile Drawing

Lines 40–130 run a `FOR T=1 TO 2` outer loop that collects two profiles. In pass 1 (`T=1`), values are stored in `L(G)`; in pass 2 (`T=2`), in `H(G)`. The variable `D` is used as a horizontal offset: `D=-1` in the first pass (so `G+D` starts at 0) and `D=127` in the second pass, shifting the second curve to the right half of the display. The variable `A` tracks the current height, modified by key presses:

- `"9"` — hold height (no change)
- `"0"` — increment A by 1
- `"5"` — decrement A by 1
- `"7"` — increment A by 0.5
- `"6"` — decrement A by 0.5
- `"1"` — jump to sine-wave auto-generator

Line 95 loops back to line 60, so the inner `FOR G` loop only advances when a valid key is pressed and a `PLOT` is made — effectively pausing until user input.

### Rendering: South-to-North Projection (Lines 150–192)

For option 1, the program steps through `A=0 TO 126 STEP 4` and for each column plots a point at `L(A+1)` then uses `DRAW 128, delta, D` where `D` is a user-supplied radian angle and `delta = (H(A+1)-88) - L(A+1)`. This uses the three-argument form of DRAW (x, y, angle) to draw arcs, giving the contour lines a curved perspective appearance. The midpoint 88 is used as a vertical reference baseline.

### Rendering: West-to-East Projection (Lines 200–260)

For option 2, the program iterates over all 127 columns and for each interpolates 20 intermediate points between the lower and upper elevations. Each step advances `A` (horizontal) by 6.35 and `B` (vertical) by 4.4 plus a proportional share of `(H(T+1)-88) - L(T+1)` divided by 20. The constants 6.35 and 4.4 scale the 127-pixel-wide profile into a diagonal perspective raster across the screen.

### Sine-Wave Auto-Generator (Lines 1000–1060)

Instead of drawing a profile manually, pressing `"1"` routes to line 1000, which synthesizes a sine-wave terrain into `L()`. A sine accumulator `A` (angle, starting at 1) advances by 0.04 per step, while `D` (horizontal position) advances by 0.5 and `B` (elevation, starting at 43) accumulates `SIN A`. The loop runs until `D >= 127`, storing each computed elevation via `L(INT D) = B`. After generation, execution falls through to line 130 to set up the second profile pass using `GO TO 130`.

### Notable Techniques and Idioms

- The `OVER 1` mode at line 140 is used to erase the axes drawn in line 10 by redrawing them in XOR mode, clearing them before the contour plot begins.
- The `FOR` loop variable `G` is reused as a horizontal pixel counter, with plotting done inside the loop body conditionally — the loop only advances when a key is pressed and a plot occurs.
- The program uses `RUN` (lines 192, 290) rather than `GO TO 10` to restart, which also clears all variables and arrays, giving a clean slate.
- The midpoint constant 88 appears to represent the vertical center of the drawing area (175/2 ≈ 87.5), used as a baseline reference for computing the arc height delta.

### Bugs and Anomalies

- The sine-wave generator only fills `L()` and then goes to line 130, which resets `A=131` and `D=127` and runs a second manual-draw pass for `H()` — so the user must still draw the upper profile by hand even when using the auto-generator.
- Arrays are dimensioned as `DIM L(127)` and `DIM H(127)` (1-indexed, 127 elements), but the loops reference indices from `G=1` to `127`, and line 205 uses `T+1` for `T=0 TO 126` — all accesses stay within bounds.

## Source Code

```
   10 PLOT 127,0: DRAW 0,175: PLOT 0,87: DRAW 255,0
   20 DIM L(127): DIM H(127)
   30 LET A=43
   35 LET D=-1
   40 FOR T=1 TO 2
   50 FOR G=1 TO 127
   60 IF INKEY$="9" THEN PLOT G+D,A: GO TO 100
   70 IF INKEY$="0" THEN LET A=A+1: PLOT G+D,A: GO TO 100
   75 IF INKEY$="5" THEN LET A=A-1: PLOT G+D,A: GO TO 100
   80 IF INKEY$="7" THEN LET A=A+.5: PLOT G+d,A: GO TO 100
   90 IF INKEY$="6" THEN LET A=A-.5: PLOT G+D,A: GO TO 100
   91 IF INKEY$="1" THEN GO TO 1000
   95 GO TO 60
  100 IF T=1 THEN LET L(G)=A: GO TO 115
  110 LET H(G)=A
  115 NEXT G
  130 LET A=131: LET D=127: NEXT T
  140 OVER 1: PLOT 127,0: DRAW 0,175: PLOT 0,87: DRAW 255,0: OVER 0
  145 PRINT "(1) SOUTH TO NORTH or           (2) WEST  TO EAST ?": INPUT O
  146 IF O=1 THEN INPUT "RADIANS ?";D: PRINT AT 0,0;"                                                   ": GO TO 150
  147 IF O=2 THEN GO TO 200
  148 GO TO 145
  150 FOR A=0 TO 126 STEP 4
  160 PLOT A,L(A+1)
  170 DRAW 128,88+((H(A+1)-88)-L(A+1)),D
  180 NEXT A
  190 INPUT "WEST TO EAST ASWELL ?";O$
  191 IF o$="Y" OR o$="y" THEN GO TO 200
  192 RUN 
  200 PRINT AT 0,0;"                                                   "
  205 FOR t=0 TO 126
  210 LET B=L(T+1): LET A=T
  220 FOR G=1 TO 20
  230 LET A=A+6.35: LET B=B+4.4
  240 LET B=B+((H(T+1)-88)-L(T+1))/20
  245 PLOT A,B
  250 NEXT G
  260 NEXT T
  270 INPUT "SOUTH TO EAST ASWELL ?";O$
  280 IF O$="Y" OR O$="y" THEN LET O=1: GO TO 146
  290 RUN 
 1000 LET B=43
 1010 LET A=1
 1015 LET D=1
 1020 LET L(INT D)=B
 1021 PLOT D,B
 1030 LET B=B+(SIN A)
 1040 LET A=A+.04: LET D=D+.5
 1045 IF D>=127 THEN GO TO 1060
 1050 GO TO 1020
 1060 GO TO 130
```