PlotLine

To run the demo: Under SMSQ/E type:

EX <path>PlotLine_bas.

Under Qdos type:

AUTO 100: LOAD flp1_PlotLine_bas<ENTER> <BREAK>

and then type:

RUN: PAUSE.

Note: In mode 0/4 you wont see the horizontal line as its black on black, so just alter the
colour codes to taste.

rem + ------------------------------------------------------------------------ +
rem |<                              Plot a Line                               >|
rem + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +
rem | Draw a straight line using only integer math and using pixel coordinates |
rem |                                                                          |
rem | Note: Unlike S*BASIC LINE, if given coordinates are outside the screen's |
rem | range the routine errors. So you either have to check this before call-  |
rem | ing or insert some error trapping code of your own. (Which I did here.)  |
rem + ------------------------------------------------------------------------ +
rem | V0.01, pjw, 2022 Dec 23                                                  |
rem + ------------------------------------------------------------------------ +
:
:
rem     Harness
cw = FOPEN("con_512x256a0x0"): CLS#cw
:
limX% = 510:     rem Max x
limY% = 254:     rem Max y
:
plotLine#cw; 0, 250, 510, 0, 6
plotLine#cw; 0, 0, 510, 254, 2
plotLine#cw; 0, 127, 510, 127, 1
plotLine#cw; 254, 0, 254, 254, 7
PAUSE#cw: CLOSE#cw
:
:
rem + ------------------------------------------------------------------------ +
rem |<                                 plotLine                               >|
rem + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +
rem |               Draw a straight line using only integer math               |
rem |                                                                          |
rem | Bresenham's line algorithm. From Wikipedia, 2019 Jul 14                  |
rem |                                                                          |
rem | By switching the x and y axis an implementation for positive or          |
rem | negative steep gradients can be written.                                 |
rem | The complete solution detects whether x1 > x0 or y1 > y0 and reverses    |
rem | the input coordinates before drawing.                                    |
rem |                                                                          |
rem | plotLine is the calling procedure, plLo and plHi are dependent           |
rem | subroutines; they have no life of their own!                             |
rem |                                                                          |
rem | For SMSQ/E all variables could be changed to integer!                    |
rem |                                                                          |
rem | Dependencies: OutR and GLOBal limX%, limY%                               |
rem + ------------------------------------------------------------------------ +
rem | V0.00, pjw, 2019 Jul 14                                                  |
rem | V0.01, pjw, 2022 Dec 23                                                  |
rem + ------------------------------------------------------------------------ +
:
DEFine PROCedure plotLine(ch, x0,y0, x1,y1, c)
LOCal D, dx, dy
  IF OutR(x0, limX%) OR OutR(x1, limX%) OR OutR(y0, limY%) OR OutR(y1, limY%): RETurn
  IF x0 = x1 OR y0 = y1
    IF x0 > x1: dx = x0: ELSE : dx = x1
    IF x0 = x1
      IF y1 > y0: dy = y0: ELSE : dy = y1
      IF OutR(ABS(y1 - y0) + dy, limY%): RETurn
      BLOCK#ch; 1, ABS(y1 - y0), x1, dy, c
    ELSE
      IF x1 > x0: dx = x0: ELSE : dx = x1
      IF OutR(ABS(x1 - x0) + dx, limX%): RETurn
      BLOCK#ch; ABS(x1 - x0), 1, dx, y1, c
    END IF
    RETurn
  END IF
  :
  IF ABS(y1 - y0) < ABS(x1 - x0)
    IF x0 > x1
      plLo x1, y1, x0, y0
    ELSE
      plLo x0, y0, x1, y1
    END IF
  ELSE
    IF y0 > y1
      plHi x1, y1, x0, y0
    ELSE
      plHi x0, y0, x1, y1
    END IF
  END IF
END DEFine plotLine
:
DEFine PROCedure plLo(x0,y0, x1,y1)
LOCal x, y, yi
  dx = x1 - x0
  dy = y1 - y0
  yi = 1
  IF dy < 0
    yi = -1
    dy = -dy
  END IF
  D = dy + dy - dx
  y = y0
  :
  FOR x = x0 TO x1
    BLOCK#ch; 1, 1, x, y, c
    IF D > 0
       y = y + yi
       D = D - dx - dx
    END IF
    D = D + dy + dy
  END FOR x
END DEFine plLo
:
DEFine PROCedure plHi(x0,y0, x1,y1)
LOCal y, x, xi
  dx = x1 - x0
  dy = y1 - y0
  xi = 1
  IF dx < 0
    xi = -1
    dx = -dx
  END IF
  D = dx + dx - dy
  x = x0
  :
  FOR y = y0 TO y1
    BLOCK#ch; 1, 1, x, y, c
    IF D > 0
       x = x + xi
       D = D - dy - dy
    END IF
    D = D + dx + dx
  END FOR y
END DEFine plHi
:
:
DEFine FuNction OutR(n, limN)
IF n < 0 OR n > limN: RETurn 1
RETurn 0
END DEFine OutR
:
:

  

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