---
title: "Straight Line Subroutine"
type: "article"
slug: "straight-line-subroutine"
url: "http://localhost/article/straight-line-subroutine/"
markdown_url: "http://localhost/article/straight-line-subroutine.md"
published_at: "2025-08-09T13:12:52+00:00"
modified_at: "2026-06-21T23:26:15+00:00"
featured_image:
  url: "http://localhost/wp-content/uploads/2025/08/SyncWare-News-v1n1-6_0077-1.jpg"
excerpt: "This subroutine draws an unbroken line segment between pixel (X1,Y1) and (X2,Y2). It is based on the equation for a straight line, Y=M*X+B where M is the slope and B is the Y-intercept. First the slope is evaluated in line 1020, after making sure that (X2-X1) is non-zero (division by zero is a no-no). If…"
category:
  - name: "SyncWare News"
    slug: "syncware-news"
    taxonomy: "category"
    url: "http://localhost/category/periodicals/syncware-news/"
post_tag:
  - name: "Full Text"
    slug: "fulltext"
    taxonomy: "post_tag"
    url: "http://localhost/tag/fulltext/"
  - name: "TS 1000"
    slug: "ts1000"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts1000/"
  - name: "Type-in program"
    slug: "type-in-program"
    taxonomy: "post_tag"
    url: "http://localhost/tag/type-in-program/"
model:
  - name: "Timex/Sinclair 1000"
    slug: "ts-1000"
    taxonomy: "model"
    url: "http://localhost/model/ts-1000/"
indiv:
  - name: "Fred Nachbaur"
    slug: "fred-nachbaur"
    taxonomy: "indiv"
    url: "http://localhost/indiv/fred-nachbaur/"
publication_r:
  id: 10245
  title: "SyncWare News"
  type: "periodical"
  url: "http://localhost/periodical/syncware-news/"
authors_r:
  - name: "Fred Nachbaur"
    slug: "fred-nachbaur"
    taxonomy: "indiv"
    url: "http://localhost/indiv/fred-nachbaur/"
volume: "1"
issues_articles:
  - id: 38991
    title: "SyncWare News v1"
    type: "issue"
    url: "http://localhost/issue/syncware-v1/"
pages: "76"
pubdate: "1984"
archive_link: false
---

# Straight Line Subroutine

This subroutine draws an unbroken line segment between pixel (X1,Y1) and (X2,Y2). It is based on the equation for a straight line, Y=M*X+B where M is the slope and B is the Y-intercept.

First the slope is evaluated in line 1020, after making sure that (X2-X1) is non-zero (division by zero is a no-no). If the slope is less than one, the line is drawn by incrementing X, otherwise the line is drawn by incrementing Y, resulting in an unbroken line in either case.

```
100 INPUT X1
110 INPUT Y1
120 INPUT X2
130 INPUT Y2
140 GOSUB 1000
150 RUN
1000 IF X2-XT=NOT PI THEN LET X2=X2+.1
1005 LET M=(Y2-Y2)/(X2-X1)
1010 IF ABS M>1 THEN GOTO 1100
1020 LET I=Y1-M*X1
1030 FOR B=X1 TO X2 STEP SGN (X2-X1)
1040 LET C=INT (M*B+I+.5)
1050 PLOT B,C
1060 NEXT B
1070 RETURN
1100 FOR C=Y1 TO Y2 STEP SGN (Y2-Y1)
1110 LET B=INT(C+M*X1-Y1)/M
1115 PLOT B,C
1120 NEXT C
1130 RETURN
```

Enter RUN, then the values of X1, Y1, X2, and Y2. The program draws lines until you fill the screen too far and run out of memory.

If you have a 2K machine (or greater) you can use the subroutine to graph the vectors obtained in the [PLR-CART program](http://localhost/article/polar-rectangular-conversion/). Enter or LOAD PLR-CART, and add the subroutine. Now enter the following lines:

```
295 IF INKEY$="G" THEN GOTO 500
500 CLS
505 FOR B=1 TO 31
510 PRINT AT 11,B;","
520 IF B<22 THEN PRINT AT B,15;"."
530 NEXT B
540 PRINT AT 0,0;"2";TAB 15;"+Y";TAB 31;"1";AT 11,0;"-";TAB 30;"+X";AT 21,0;"3";TAB 15;"-";TAB 31;"4"
550 LET X1=31
560 LET X2=X+X1
570 LET Y1=20
580 LET Y2=Y+Y1
590 GOSUB 1000
595 GOTO 295
```

After results have been printed, press “G” to draw graph of vector/phasor. Line 540 – display looks best if quadrant markers 1 – 4 are in inverse video.

Beware of vectors that go off the-screen; if you go off to the right or top of the screen, the program stops – enter RETURN to get back to the program. If you go off to the left or the bottom, the vector reflects back – an interesting oddity on the ZX81/TS1000 (PLOT ignores the signs of the argument.) Maximum X value is +/-30, max. Y value is +/-20.

When the display is complete, press any key but BREAK for another problem.