REM  try 20 segments and click a square to get the idea!
      REM 
      ON BREAK GOSUB 5000
      ON ERROR GOTO 5000
      SCREEN 1,640,200,4,2
      WINDOW 2,"Triangles",(0,0)-(600,180),7,1
    1 col% = 2
    5 DIM xx(500),yy(500)
   10 INPUT"Divide segments by how many [0 to EXIT]";space
   20 PRINT"Click your points. When done, click top left."
      IF space=0 THEN 5000
   40 GOSUB 1000
   50 c%=0
   60 xx(c%)=x%:yy(c%)=y%
   65 CLS
   70 PSET (x%,y%)
   80 c%=c%+1
   90 GOSUB 1000
  100 IF (x%<10) AND (y%<10) THEN GOTO 200
  110 xx(c%)=x%:yy(c%)=y%
  120 LINE -( x%,y%)
  125 FOR j%=0 TO 500:NEXT
  130 IF c% < 499 GOTO 80
  200 LINE -(xx(0),yy(0))
  210 '                **********
  220 FOR i% = 0 TO c%-1
  230 xx(i%)=xx(i%) + ((xx(i%+1)-xx(i%))/space)
  240 yy(i%)=yy(i%) + ((yy(i%+1)-yy(i%))/space)
  245 xx(c%)=xx(0):yy(c%)=yy(0)
  250 NEXT i%
  260 xx(c%)=xx(0)
  265 yy(c%)=yy(0)
  270 PSET (xx(0),yy(0))
  280 FOR i%=0 TO c%
  290 LINE -(xx(i%),yy(i%))
  300 NEXT i%:IF MOUSE(0)<0 THEN COLOR 1:CLS:GOTO 10
  305 GOSUB 2000
  310 GOTO 220
 1000 b%= MOUSE(0):x%=MOUSE(1):y%=MOUSE(2)
 1010 IF b% >= 0 THEN GOTO 1000
 1020 RETURN
 2000 col% = col% + 1
 2010 col% = col% MOD 16
 2020 COLOR col%,col%  
 2030 RETURN
 5000 SCREEN CLOSE 1
 5010 STOP