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3 Copyright 1987, 1998 The Open Group
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26 Copyright 1987 by Digital Equipment Corporation, Maynard, Massachusetts.
30 Permission to use, copy, modify, and distribute this software and its
31 documentation for any purpose and without fee is hereby granted,
32 provided that the above copyright notice appear in all copies and that
33 both that copyright notice and this permission notice appear in
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49 * This file contains a few macros to help track
50 * the edge of a filled object. The object is assumed
51 * to be filled in scanline order, and thus the
52 * algorithm used is an extension of Bresenham's line
53 * drawing algorithm which assumes that y is always the
55 * Since these pieces of code are the same for any filled shape,
56 * it is more convenient to gather the library in one
57 * place, but since these pieces of code are also in
58 * the inner loops of output primitives, procedure call
59 * overhead is out of the question.
60 * See the author for a derivation if needed.
65 * In scan converting polygons, we want to choose those pixels
66 * which are inside the polygon. Thus, we add .5 to the starting
67 * x coordinate for both left and right edges. Now we choose the
68 * first pixel which is inside the pgon for the left edge and the
69 * first pixel which is outside the pgon for the right edge.
70 * Draw the left pixel, but not the right.
72 * How to add .5 to the starting x coordinate:
73 * If the edge is moving to the right, then subtract dy from the
74 * error term from the general form of the algorithm.
75 * If the edge is moving to the left, then add dy to the error term.
77 * The reason for the difference between edges moving to the left
78 * and edges moving to the right is simple: If an edge is moving
79 * to the right, then we want the algorithm to flip immediately.
80 * If it is moving to the left, then we don't want it to flip until
81 * we traverse an entire pixel.
83 #define BRESINITPGON(dy, x1, x2, xStart, d, m, m1, incr1, incr2) { \
84 int dx; /* local storage */ \
87 * if the edge is horizontal, then it is ignored \
88 * and assumed not to be processed. Otherwise, do this stuff. \
96 incr1 = -2 * dx + 2 * (dy) * m1; \
97 incr2 = -2 * dx + 2 * (dy) * m; \
98 d = 2 * m * (dy) - 2 * dx - 2 * (dy); \
102 incr1 = 2 * dx - 2 * (dy) * m1; \
103 incr2 = 2 * dx - 2 * (dy) * m; \
104 d = -2 * m * (dy) + 2 * dx; \
109 #define BRESINCRPGON(d, minval, m, m1, incr1, incr2) { \
133 * This structure contains all of the information needed
134 * to run the bresenham algorithm.
135 * The variables may be hardcoded into the declarations
136 * instead of using this structure to make use of
137 * register declarations.
140 int minor_axis; /* minor axis */
141 int d; /* decision variable */
142 int m, m1; /* slope and slope+1 */
143 int incr1, incr2; /* error increments */
147 #define BRESINITPGONSTRUCT(dmaj, min1, min2, bres) \
148 BRESINITPGON(dmaj, min1, min2, bres.minor_axis, bres.d, \
149 bres.m, bres.m1, bres.incr1, bres.incr2)
151 #define BRESINCRPGONSTRUCT(bres) \
152 BRESINCRPGON(bres.d, bres.minor_axis, bres.m, bres.m1, bres.incr1, bres.incr2)
157 * These are the data structures needed to scan
158 * convert regions. Two different scan conversion
159 * methods are available -- the even-odd method, and
160 * the winding number method.
161 * The even-odd rule states that a point is inside
162 * the polygon if a ray drawn from that point in any
163 * direction will pass through an odd number of
165 * By the winding number rule, a point is decided
166 * to be inside the polygon if a ray drawn from that
167 * point in any direction passes through a different
168 * number of clockwise and counter-clockwise path
171 * These data structures are adapted somewhat from
172 * the algorithm in (Foley/Van Dam) for scan converting
174 * The basic algorithm is to start at the top (smallest y)
175 * of the polygon, stepping down to the bottom of
176 * the polygon by incrementing the y coordinate. We
177 * keep a list of edges which the current scanline crosses,
178 * sorted by x. This list is called the Active Edge Table (AET)
179 * As we change the y-coordinate, we update each entry in
180 * in the active edge table to reflect the edges new xcoord.
181 * This list must be sorted at each scanline in case
182 * two edges intersect.
183 * We also keep a data structure known as the Edge Table (ET),
184 * which keeps track of all the edges which the current
185 * scanline has not yet reached. The ET is basically a
186 * list of ScanLineList structures containing a list of
187 * edges which are entered at a given scanline. There is one
188 * ScanLineList per scanline at which an edge is entered.
189 * When we enter a new edge, we move it from the ET to the AET.
191 * From the AET, we can implement the even-odd rule as in
193 * The winding number rule is a little trickier. We also
194 * keep the EdgeTableEntries in the AET linked by the
195 * nextWETE (winding EdgeTableEntry) link. This allows
196 * the edges to be linked just as before for updating
197 * purposes, but only uses the edges linked by the nextWETE
198 * link as edges representing spans of the polygon to
199 * drawn (as with the even-odd rule).
203 * for the winding number rule
206 #define COUNTERCLOCKWISE -1
208 typedef struct _EdgeTableEntry {
209 int ymax; /* ycoord at which we exit this edge. */
210 BRESINFO bres; /* Bresenham info to run the edge */
211 struct _EdgeTableEntry *next; /* next in the list */
212 struct _EdgeTableEntry *back; /* for insertion sort */
213 struct _EdgeTableEntry *nextWETE; /* for winding num rule */
214 int ClockWise; /* flag for winding number rule */
218 typedef struct _ScanLineList{
219 int scanline; /* the scanline represented */
220 EdgeTableEntry *edgelist; /* header node */
221 struct _ScanLineList *next; /* next in the list */
226 int ymax; /* ymax for the polygon */
227 int ymin; /* ymin for the polygon */
228 ScanLineList scanlines; /* header node */
233 * Here is a struct to help with storage allocation
234 * so we can allocate a big chunk at a time, and then take
235 * pieces from this heap when we need to.
237 #define SLLSPERBLOCK 25
239 typedef struct _ScanLineListBlock {
240 ScanLineList SLLs[SLLSPERBLOCK];
241 struct _ScanLineListBlock *next;
248 * a few macros for the inner loops of the fill code where
249 * performance considerations don't allow a procedure call.
251 * Evaluate the given edge at the given scanline.
252 * If the edge has expired, then we leave it and fix up
253 * the active edge table; otherwise, we increment the
254 * x value to be ready for the next scanline.
255 * The winding number rule is in effect, so we must notify
256 * the caller when the edge has been removed so he
257 * can reorder the Winding Active Edge Table.
259 #define EVALUATEEDGEWINDING(pAET, pPrevAET, y, fixWAET) { \
260 if (pAET->ymax == y) { /* leaving this edge */ \
261 pPrevAET->next = pAET->next; \
262 pAET = pPrevAET->next; \
265 pAET->back = pPrevAET; \
268 BRESINCRPGONSTRUCT(pAET->bres); \
276 * Evaluate the given edge at the given scanline.
277 * If the edge has expired, then we leave it and fix up
278 * the active edge table; otherwise, we increment the
279 * x value to be ready for the next scanline.
280 * The even-odd rule is in effect.
282 #define EVALUATEEDGEEVENODD(pAET, pPrevAET, y) { \
283 if (pAET->ymax == y) { /* leaving this edge */ \
284 pPrevAET->next = pAET->next; \
285 pAET = pPrevAET->next; \
287 pAET->back = pPrevAET; \
290 BRESINCRPGONSTRUCT(pAET->bres); \