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curves.c

/* $XConsortium: curves.c,v 1.3 91/10/10 11:17:56 rws Exp $ */
/* Copyright International Business Machines,Corp. 1991              */
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/*
:h1.CURVES Module - Stepping Beziers
 
This module is responsible for "rasterizing"
third order curves.  That is, it changes the high level curve
specification into a list of pels that that curve travels
through.
 
:h3.Include Files
 
Include files needed:
*/
 
#include "types.h"
#include "objects.h"
#include "spaces.h"
#include "paths.h"
#include "regions.h"
#include "curves.h"
#include "lines.h"
#include "arith.h"
 
 
/*
:h3.Functions Provided to Other Modules
 
External entry points:
*/
/*SHARED LINE(S) ORIGINATED HERE*/
 
/*
Note that "stepping" and "flattening" are so similiar that they use the
same routine.  When the "region" parameter is NULL, that is a flag that
we are flattening instead of stepping.
*/
/*
:h2.Bezier Third Order Curves
*/
/*
:h3.The "bezierinfo" Structure
 
This structure is used to store information used when we subdivide
Bezier curves.
*/
 
struct bezierinfo {
       struct region *region;  /* the region being built or NULL             */
       struct fractpoint last;  /* not used yet; maybe could save some work  */
       struct fractpoint origin;  /* the origin of the bezier                */
} ;
 
/*
   Checking for termination of the subdivision process:
   This is the stupidest test in the world, just check if the coordinatewise
   distance from an end control point to the next control point is less than
   one half pel.   If so, we must be done.
   This returns 1 if the subdivision is terminated and 0 if you still need
   to subdivide.
*/
 
int BezierTerminationTest(xa,ya,xb,yb,xc,yc,xd,yd)
fractpel xa,ya,xb,yb,xc,yc,xd,yd;
{
  fractpel dmax;
  dmax =          TYPE1_ABS(xa - xb);
  dmax = TYPE1_MAX(dmax,TYPE1_ABS(ya - yb));
  dmax = TYPE1_MAX(dmax,TYPE1_ABS(xd - xc));
  dmax = TYPE1_MAX(dmax,TYPE1_ABS(yd - yc));
  if(dmax > FPHALF)
    return(0); /* not done yet */
  else
    return(1); /* done */
}
 
/*
:h3.StepBezierRecurse() - The Recursive Logic in StepBezier()
 
The recursion involves dividing the control polygon into two smaller
control polygons by finding the midpoints of the lines.  This idea is
described in any graphics text book and its simplicity is what caused
Bezier to define his curves as he did.  If the input region 'R' is NULL,
the result is a path that is the 'flattened' curve; otherwise StepBezier
returns nothing special.
*/
static struct segment *StepBezierRecurse(I,xA,yA,xB,yB,xC,yC,xD,yD)
       struct bezierinfo *I; /* Region under construction or NULL            */
       fractpel xA,yA;       /* A control point                              */
       fractpel xB,yB;       /* B control point                              */
       fractpel xC,yC;       /* C control point                              */
       fractpel xD,yD;       /* D control point                              */
 
{
 if (BezierTerminationTest(xA,yA,xB,yB,xC,yC,xD,yD))
 {
  if (I->region == NULL)
   return(PathSegment(LINETYPE, xD - xA, yD - yA));
  else
   StepLine(I->region, I->origin.x + xA, I->origin.y + yA,
                       I->origin.x + xD, I->origin.y + yD);
 }
 else
 {
  fractpel xAB,yAB;
  fractpel xBC,yBC;
  fractpel xCD,yCD;
  fractpel xABC,yABC;
  fractpel xBCD,yBCD;
  fractpel xABCD,yABCD;
 
  xAB = xA + xB;  yAB = yA + yB;
  xBC = xB + xC;  yBC = yB + yC;
  xCD = xC + xD;  yCD = yC + yD;
 
  xABC = xAB + xBC;  yABC = yAB + yBC;
  xBCD = xBC + xCD;  yBCD = yBC + yCD;
 
  xABCD = xABC + xBCD;  yABCD = yABC + yBCD;
 
  xAB >>= 1;   yAB >>= 1;
  xBC >>= 1;   yBC >>= 1;
  xCD >>= 1;   yCD >>= 1;
  xABC >>= 2;   yABC >>= 2;
  xBCD >>= 2;   yBCD >>= 2;
  xABCD >>= 3;   yABCD >>= 3;
 
  if (I->region == NULL)
  {
   return( Join(
    StepBezierRecurse(I, xA, yA, xAB, yAB, xABC, yABC, xABCD, yABCD),
    StepBezierRecurse(I, xABCD, yABCD, xBCD, yBCD, xCD, yCD, xD, yD)
                )
         );
  }
  else
  {
   StepBezierRecurse(I, xA, yA, xAB, yAB, xABC, yABC, xABCD, yABCD);
   StepBezierRecurse(I, xABCD, yABCD, xBCD, yBCD, xCD, yCD, xD, yD);
  }
 }
 /*NOTREACHED*/
 /* To make ANSI-C-comnpiler happy (RMz): */
 return(0);
}
 
/*
:h3.TOOBIG() - Macro to Test if a Coordinate is Too Big to Bezier SubDivide Normally
 
Intermediate values in the Bezier subdivision are 8 times bigger than
the starting values.  If this overflows, a 'long', we are in trouble:
*/
 
#define  BITS         (sizeof(LONG)*8)
#define  HIGHTEST(p)  (((p)>>(BITS-4)) != 0)  /* includes sign bit */
#define  TOOBIG(xy)   ((xy < 0) ? HIGHTEST(-xy) : HIGHTEST(xy))
 
/*
:h3.StepBezier() - Produce Run Ends for a Bezier Curve
 
This is the entry point called from outside the module.
*/
 
struct segment *StepBezier(R, xA, yA, xB, yB, xC, yC, xD, yD)
       struct region *R;     /* Region under construction or NULL            */
       fractpel xA,yA;       /* A control point                              */
       fractpel xB,yB;       /* B control point                              */
       fractpel xC,yC;       /* C control point                              */
       fractpel xD,yD;       /* D control point                              */
{
       struct bezierinfo Info;
 
       Info.region = R;
       Info.origin.x = xA;
       Info.origin.y = yA;
 
       xB -= xA;
       xC -= xA;
       xD -= xA;
       yB -= yA;
       yC -= yA;
       yD -= yA;
 
       if ( TOOBIG(xB) || TOOBIG(yB) || TOOBIG(xC) || TOOBIG(yC)
            || TOOBIG(xD) || TOOBIG(yD) )
               abort("Beziers this big not yet supported", 3);
 
       return(StepBezierRecurse(&Info,
                                (fractpel) 0, (fractpel) 0, xB, yB, xC, yC, xD, yD));
}
 

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