source: buchla-68k/vlib/lseg.c@ 4b63bf3

/*
   ===========================================================================
        LSEG.C -- Generalized line drawing function  (Integer DDA algorithm)
        Version 6 -- 1987-08-04 -- D.N. Lynx Crowe
        Copyright 1985, 1986, 1987 -- D.N. Lynx Crowe
   ===========================================================================
*/

#include "ram.h"

void   (*point)(int16_t x, int16_t y, int16_t pen);

#define ABS(x)  ((x) < 0 ? (-(x)) : (x))
#define SIGN(x) ((x) < 0 ? (-1) : ((x) ? 1 : 0))

#define LE      <=

/*
   =============================================================================
        lseg(x1, y1, x2, y2, t) -- draw a line from ('x1', 'y1') to ('x2', 'y2')
        in drawing mode 't'.

        All coordinates are short integers in whatever plotting units the
        'point' function expects.  No clipping or scaling is done, as this is
        the lowest level 'line' primitive in the drawing code, and coordinates
        are expected to have already been checked and found to be on screen. 

        The 'point' variable must be initialized to point at the
        pixel plotting function before this function is called.  The pixel
        plotting function is passed 3 arguments:  the x coordinate, the y
        coordinate, and the plotting mode.  It is declared here as type 'short',
        but any value it returns will be ignored.

        The register variable assignments were chosen based on the Alcyon C
        compiler for the Atari 1040ST and reflect the capabilities of that
        compiler.  Other compilers may require different optimizations based
        on their use of register variables.
   =============================================================================
*/

void lseg(int16_t x1, int16_t y1, int16_t x2, int16_t y2, int16_t t)
{
        register int16_t dx, dy, ptx, pty, p;
        int16_t i, px, py;

        p = x2 - (ptx = x1);
        dx = SIGN(p);
        py = ABS(p);

        p = y2 - (pty = y1);
        dy = SIGN(p);
        px = ABS(p);

        (*point)(ptx, pty, t);

        if (py > px) {

                p = py >> 1;

                for (i = 1; i < py; i++) {

                        ptx += dx;

                        if ( (p -= px) < 0) {

                                pty += dy;
                                p += py;
                        }

                        (*point)(ptx, pty, t);
                }

        } else {

                p = px >> 1;

                for (i = 1; i LE px; i++) {

                        pty += dy;

                        if ( (p -= py) < 0) {

                                ptx += dx;
                                p += px;
                        }

                        (*point)(ptx, pty, t);
                }
        }
}