path: root/gs/src/gxpflat.c

/* Copyright (C) 1997, 1998 Aladdin Enterprises.  All rights reserved.

   This file is part of Aladdin Ghostscript.

   Aladdin Ghostscript is distributed with NO WARRANTY OF ANY KIND.  No author
   or distributor accepts any responsibility for the consequences of using it,
   or for whether it serves any particular purpose or works at all, unless he
   or she says so in writing.  Refer to the Aladdin Ghostscript Free Public
   License (the "License") for full details.

   Every copy of Aladdin Ghostscript must include a copy of the License,
   normally in a plain ASCII text file named PUBLIC.  The License grants you
   the right to copy, modify and redistribute Aladdin Ghostscript, but only
   under certain conditions described in the License.  Among other things, the
   License requires that the copyright notice and this notice be preserved on
   all copies.
 */

/*$Id$ */
/* Path flattening algorithms */
#include "gx.h"
#include "gxarith.h"
#include "gxfixed.h"
#include "gzpath.h"

/* Define whether to merge nearly collinear line segments when flattening */
/* curves.  This is very good for performance, but we feel a little */
/* uneasy about its effects on character appearance. */
#define MERGE_COLLINEAR_SEGMENTS 1

/* ---------------- Curve flattening ---------------- */

#define x1 pc->p1.x
#define y1 pc->p1.y
#define x2 pc->p2.x
#define y2 pc->p2.y
#define x3 pc->pt.x
#define y3 pc->pt.y

/*
 * To calculate how many points to sample along a path in order to
 * approximate it to the desired degree of flatness, we define
 *      dist((x,y)) = abs(x) + abs(y);
 * then the number of points we need is
 *      N = 1 + sqrt(3/4 * D / flatness),
 * where
 *      D = max(dist(p0 - 2*p1 + p2), dist(p1 - 2*p2 + p3)).
 * Since we are going to use a power of 2 for the number of intervals,
 * we can avoid the square root by letting
 *      N = 1 + 2^(ceiling(log2(3/4 * D / flatness) / 2)).
 * (Reference: DEC Paris Research Laboratory report #1, May 1989.)
 *
 * We treat two cases specially.  First, if the curve is very
 * short, we halve the flatness, to avoid turning short shallow curves
 * into short straight lines.  Second, if the curve forms part of a
 * character (indicated by flatness = 0), we let
 *      N = 1 + 2 * max(abs(x3-x0), abs(y3-y0)).
 * This is probably too conservative, but it produces good results.
 */
int
gx_curve_log2_samples(fixed x0, fixed y0, const curve_segment * pc,
		      fixed fixed_flat)
{
    fixed
	x03 = x3 - x0,
	y03 = y3 - y0;
    int k;

    if (x03 < 0)
	x03 = -x03;
    if (y03 < 0)
	y03 = -y03;
    if ((x03 | y03) < int2fixed(16))
	fixed_flat >>= 1;
    if (fixed_flat == 0) {	/* Use the conservative method. */
	fixed m = max(x03, y03);

	for (k = 1; m > fixed_1;)
	    k++, m >>= 1;
    } else {
	const fixed
	      x12 = x1 - x2, y12 = y1 - y2, dx0 = x0 - x1 - x12, dy0 = y0 - y1 - y12,
	      dx1 = x12 - x2 + x3, dy1 = y12 - y2 + y3, adx0 = any_abs(dx0),
	      ady0 = any_abs(dy0), adx1 = any_abs(dx1), ady1 = any_abs(dy1);

	fixed
	    d = max(adx0, adx1) + max(ady0, ady1);
	/*
	 * The following statement is split up to work around a
	 * bug in the gcc 2.7.2 optimizer on H-P RISC systems.
	 */
	uint qtmp = d - (d >> 2) /* 3/4 * D */ +fixed_flat - 1;
	uint q = qtmp / fixed_flat;

	if_debug6('2', "[2]d01=%g,%g d12=%g,%g d23=%g,%g\n",
		  fixed2float(x1 - x0), fixed2float(y1 - y0),
		  fixed2float(-x12), fixed2float(-y12),
		  fixed2float(x3 - x2), fixed2float(y3 - y2));
	if_debug2('2', "     D=%f, flat=%f,",
		  fixed2float(d), fixed2float(fixed_flat));
	/* Now we want to set k = ceiling(log2(q) / 2). */
	for (k = 0; q > 1;)
	    k++, q = (q + 3) >> 2;
	if_debug1('2', " k=%d\n", k);
    }
    return k;
}

/*
 * Define the maximum number of points for sampling if we want accurate
 * rasterizing.  2^(k_sample_max*3)-1 must fit into a uint with a bit
 * to spare; also, we must be able to compute 1/2^(3*k) by table lookup.
 */
#define k_sample_max min((size_of(int) * 8 - 1) / 3, 10)

/*
 * Split a curve segment into two pieces at the (parametric) midpoint.
 * Algorithm is from "The Beta2-split: A special case of the Beta-spline
 * Curve and Surface Representation," B. A. Barsky and A. D. DeRose, IEEE,
 * 1985, courtesy of Crispin Goswell.
 */
private void
split_curve_midpoint(fixed x0, fixed y0, const curve_segment * pc,
		     curve_segment * pc1, curve_segment * pc2)
{				/*
				 * We have to define midpoint carefully to avoid overflow.
				 * (If it overflows, something really pathological is going
				 * on, but we could get infinite recursion that way....)
				 */
#define midpoint(a,b)\
  (arith_rshift_1(a) + arith_rshift_1(b) + ((a) & (b) & 1) + 1)
    fixed x12 = midpoint(x1, x2);
    fixed y12 = midpoint(y1, y2);

    /*
     * pc1 or pc2 may be the same as pc, so we must be a little careful
     * about the order in which we store the results.
     */
    pc1->p1.x = midpoint(x0, x1);
    pc1->p1.y = midpoint(y0, y1);
    pc2->p2.x = midpoint(x2, x3);
    pc2->p2.y = midpoint(y2, y3);
    pc1->p2.x = midpoint(pc1->p1.x, x12);
    pc1->p2.y = midpoint(pc1->p1.y, y12);
    pc2->p1.x = midpoint(x12, pc2->p2.x);
    pc2->p1.y = midpoint(y12, pc2->p2.y);
    if (pc2 != pc)
	pc2->pt.x = pc->pt.x,
	    pc2->pt.y = pc->pt.y;
    pc1->pt.x = midpoint(pc1->p2.x, pc2->p1.x);
    pc1->pt.y = midpoint(pc1->p2.y, pc2->p1.y);
#undef midpoint
}

/*
 * Flatten a segment of the path by repeated sampling.
 * 2^k is the number of lines to produce (i.e., the number of points - 1,
 * including the endpoints); we require k >= 1.
 * If k or any of the coefficient values are too large,
 * use recursive subdivision to whittle them down.
 */
int
gx_flatten_sample(gx_path * ppath, int k, curve_segment * pc,
		  segment_notes notes)
{
    fixed x0, y0;

    /* x1 ... y3 were defined above */
    fixed cx, bx, ax, cy, by, ay;
    fixed ptx, pty;
    fixed x, y;

    /*
     * We can compute successive values by finite differences,
     * using the formulas:
     x(t) =
     a*t^3 + b*t^2 + c*t + d =>
     dx(t) = x(t+e)-x(t) =
     a*(3*t^2*e + 3*t*e^2 + e^3) + b*(2*t*e + e^2) + c*e =
     (3*a*e)*t^2 + (3*a*e^2 + 2*b*e)*t + (a*e^3 + b*e^2 + c*e) =>
     d2x(t) = dx(t+e)-dx(t) =
     (3*a*e)*(2*t*e + e^2) + (3*a*e^2 + 2*b*e)*e =
     (6*a*e^2)*t + (6*a*e^3 + 2*b*e^2) =>
     d3x(t) = d2x(t+e)-d2x(t) =
     6*a*e^3;
     x(0) = d, dx(0) = (a*e^3 + b*e^2 + c*e),
     d2x(0) = 6*a*e^3 + 2*b*e^2;
     * In these formulas, e = 1/2^k; of course, there are separate
     * computations for the x and y values.
     *
     * There is a tradeoff in doing the above computation in fixed
     * point.  If we separate out the constant term (d) and require that
     * all the other values fit in a long, then on a 32-bit machine with
     * 12 bits of fraction in a fixed, k = 4 implies a maximum curve
     * size of 128 pixels; anything larger requires subdividing the
     * curve.  On the other hand, doing the computations in explicit
     * double precision slows down the loop by a factor of 3 or so.  We
     * found to our surprise that the latter is actually faster, because
     * the additional subdivisions cost more than the slower loop.
     *
     * We represent each quantity as I+R/M, where I is an "integer" and
     * the "remainder" R lies in the range 0 <= R < M=2^(3*k).  Note
     * that R may temporarily exceed M; for this reason, we require that
     * M have at least one free high-order bit.  To reduce the number of
     * variables, we don't actually compute M, only M-1 (rmask).  */
    uint i;
    uint rmask;			/* M-1 */
    fixed idx, idy, id2x, id2y, id3x, id3y;	/* I */
    uint rx, ry, rdx, rdy, rd2x, rd2y, rd3x, rd3y;	/* R */
    gs_fixed_point *ppt;

#define max_points 50		/* arbitrary */
    gs_fixed_point points[max_points + 1];

  top:x0 = ppath->position.x;
    y0 = ppath->position.y;
#ifdef DEBUG
    if (gs_debug_c('3')) {
	dlprintf4("[3]x0=%f y0=%f x1=%f y1=%f\n",
		  fixed2float(x0), fixed2float(y0),
		  fixed2float(x1), fixed2float(y1));
	dlprintf5("   x2=%f y2=%f x3=%f y3=%f  k=%d\n",
		  fixed2float(x2), fixed2float(y2),
		  fixed2float(x3), fixed2float(y3), k);
    }
#endif
    {
	fixed x01, x12, y01, y12;

	curve_points_to_coefficients(x0, x1, x2, x3, ax, bx, cx,
				     x01, x12);
	curve_points_to_coefficients(y0, y1, y2, y3, ay, by, cy,
				     y01, y12);
    }

    if_debug6('3', "[3]ax=%f bx=%f cx=%f\n   ay=%f by=%f cy=%f\n",
	      fixed2float(ax), fixed2float(bx), fixed2float(cx),
	      fixed2float(ay), fixed2float(by), fixed2float(cy));
#define max_fast (max_fixed / 6)
#define min_fast (-max_fast)
#define in_range(v) (v < max_fast && v > min_fast)
    if (k == 0) {		/* The curve is very short, or anomalous in some way. */
	/* Just add a line and exit. */
	return gx_path_add_line_notes(ppath, x3, y3, notes);
    }
    if (k <= k_sample_max &&
	in_range(ax) && in_range(ay) &&
	in_range(bx) && in_range(by) &&
	in_range(cx) && in_range(cy)
	) {
	x = x0, y = y0;
	rx = ry = 0;
	ppt = points;
	/* Fast check for n == 3, a common special case */
	/* for small characters. */
	if (k == 1) {
#define poly2(a,b,c)\
  arith_rshift_1(arith_rshift_1(arith_rshift_1(a) + b) + c)
	    x += poly2(ax, bx, cx);
	    y += poly2(ay, by, cy);
#undef poly2
	    if_debug2('3', "[3]dx=%f, dy=%f\n",
		      fixed2float(x - x0), fixed2float(y - y0));
	    if_debug3('3', "[3]%s x=%g, y=%g\n",
		      (((x ^ x0) | (y ^ y0)) & float2fixed(-0.5) ?
		       "add" : "skip"),
		      fixed2float(x), fixed2float(y));
	    if (((x ^ x0) | (y ^ y0)) & float2fixed(-0.5))
		ppt->x = ptx = x,
		    ppt->y = pty = y,
		    ppt++;
	    goto last;
	} else {
	    fixed bx2 = bx << 1, by2 = by << 1;
	    fixed ax6 = ((ax << 1) + ax) << 1, ay6 = ((ay << 1) + ay) << 1;

#define adjust_rem(r, q)\
  if ( r > rmask ) q ++, r &= rmask
	    const int k2 = k << 1;
	    const int k3 = k2 + k;

	    rmask = (1 << k3) - 1;
	    /* We can compute all the remainders as ints, */
	    /* because we know they don't exceed M. */
	    /* cx/y terms */
	    idx = arith_rshift(cx, k),
		idy = arith_rshift(cy, k);
	    rdx = ((uint) cx << k2) & rmask,
		rdy = ((uint) cy << k2) & rmask;
	    /* bx/y terms */
	    id2x = arith_rshift(bx2, k2),
		id2y = arith_rshift(by2, k2);
	    rd2x = ((uint) bx2 << k) & rmask,
		rd2y = ((uint) by2 << k) & rmask;
	    idx += arith_rshift_1(id2x),
		idy += arith_rshift_1(id2y);
	    rdx += ((uint) bx << k) & rmask,
		rdy += ((uint) by << k) & rmask;
	    adjust_rem(rdx, idx);
	    adjust_rem(rdy, idy);
	    /* ax/y terms */
	    idx += arith_rshift(ax, k3),
		idy += arith_rshift(ay, k3);
	    rdx += (uint) ax & rmask,
		rdy += (uint) ay & rmask;
	    adjust_rem(rdx, idx);
	    adjust_rem(rdy, idy);
	    id2x += id3x = arith_rshift(ax6, k3),
		id2y += id3y = arith_rshift(ay6, k3);
	    rd2x += rd3x = (uint) ax6 & rmask,
		rd2y += rd3y = (uint) ay6 & rmask;
	    adjust_rem(rd2x, id2x);
	    adjust_rem(rd2y, id2y);
#undef adjust_rem
	}
    } else {			/*
				 * Curve is too long.  Break into two pieces and recur.
				 */
	curve_segment cseg;
	int code;

	k--;
	split_curve_midpoint(x0, y0, pc, &cseg, pc);
	code = gx_flatten_sample(ppath, k, &cseg, notes);
	if (code < 0)
	    return code;
	notes |= sn_not_first;
	goto top;
    }
    if_debug1('2', "[2]sampling k=%d\n", k);
    ptx = x0, pty = y0;
    for (i = (1 << k) - 1;;) {
	int code;

#ifdef DEBUG
	if (gs_debug_c('3')) {
	    dlprintf4("[3]dx=%f+%d, dy=%f+%d\n",
		      fixed2float(idx), rdx,
		      fixed2float(idy), rdy);
	    dlprintf4("   d2x=%f+%d, d2y=%f+%d\n",
		      fixed2float(id2x), rd2x,
		      fixed2float(id2y), rd2y);
	    dlprintf4("   d3x=%f+%d, d3y=%f+%d\n",
		      fixed2float(id3x), rd3x,
		      fixed2float(id3y), rd3y);
	}
#endif
#define accum(i, r, di, dr)\
  if ( (r += dr) > rmask ) r &= rmask, i += di + 1;\
  else i += di
	accum(x, rx, idx, rdx);
	accum(y, ry, idy, rdy);
	if_debug3('3', "[3]%s x=%g, y=%g\n",
		  (((x ^ ptx) | (y ^ pty)) & float2fixed(-0.5) ?
		   "add" : "skip"),
		  fixed2float(x), fixed2float(y));
	/*
	 * Skip very short segments -- those that lie entirely within
	 * a square half-pixel.  Also merge nearly collinear
	 * segments -- those where one coordinate of all three points
	 * (the two endpoints and the midpoint) lie within the same
	 * half-pixel and both coordinates are monotonic.
	 * Note that ptx/y, the midpoint, is the same as ppt[-1].x/y;
	 * the previous point is ppt[-2].x/y.
	 */
#define coord_near(v, ptv)\
  (!( ((v) ^ (ptv)) & float2fixed(-0.5) ))
#define coords_in_order(v0, v1, v2)\
  ( (((v1) - (v0)) ^ ((v2) - (v1))) >= 0 )
	if (coord_near(x, ptx)) {	/* X coordinates are within a half-pixel. */
	    if (coord_near(y, pty))
		goto skip;	/* short segment */
#if MERGE_COLLINEAR_SEGMENTS
	    /* Check for collinear segments. */
	    if (ppt > points + 1 && coord_near(x, ppt[-2].x) &&
		coords_in_order(ppt[-2].x, ptx, x) &&
		coords_in_order(ppt[-2].y, pty, y)
		)
		--ppt;		/* remove middle point */
#endif
	} else if (coord_near(y, pty)) {	/* Y coordinates are within a half-pixel. */
#if MERGE_COLLINEAR_SEGMENTS
	    /* Check for collinear segments. */
	    if (ppt > points + 1 && coord_near(y, ppt[-2].y) &&
		coords_in_order(ppt[-2].x, ptx, x) &&
		coords_in_order(ppt[-2].y, pty, y)
		)
		--ppt;		/* remove middle point */
#endif
	}
#undef coord_near
#undef coords_in_order
	/* Add a line. */
	if (ppt == &points[max_points]) {
	    if (notes & sn_not_first)
		code = gx_path_add_lines_notes(ppath, points, max_points,
					       notes);
	    else {
		code = gx_path_add_line_notes(ppath, points[0].x,
					      points[0].y, notes);
		if (code < 0)
		    return code;
		code = gx_path_add_lines_notes(ppath, points,
				      max_points - 1, notes | sn_not_first);
	    }
	    if (code < 0)
		return code;
	    ppt = points;
	    notes |= sn_not_first;
	}
	ppt->x = ptx = x;
	ppt->y = pty = y;
	ppt++;
      skip:if (--i == 0)
	    break;		/* don't bother with last accum */
	accum(idx, rdx, id2x, rd2x);
	accum(id2x, rd2x, id3x, rd3x);
	accum(idy, rdy, id2y, rd2y);
	accum(id2y, rd2y, id3y, rd3y);
#undef accum
    }
  last:if_debug2('3', "[3]last x=%g, y=%g\n",
	      fixed2float(x3), fixed2float(y3));
    if (ppt > points) {
	int count = ppt + 1 - points;
	gs_fixed_point *pts = points;

	if (!(notes & sn_not_first)) {
	    int code = gx_path_add_line_notes(ppath,
					      points[0].x, points[0].y,
					      notes);

	    if (code < 0)
		return code;
	    ++pts, --count;
	    notes |= sn_not_first;
	}
	ppt->x = x3, ppt->y = y3;
	return gx_path_add_lines_notes(ppath, pts, count, notes);
    }
    return gx_path_add_line_notes(ppath, x3, y3, notes);
}

#undef x1
#undef y1
#undef x2
#undef y2
#undef x3
#undef y3