在本学期的所学的裁剪算法中，有一个叫做Nicholl-Lee-Nicholl的算法。
在实践中，我遇到了一个小问题，因为教程中所讲的只有起点在剪裁框内、左侧和左上的情况，并说其余情况均可以通过点的平移或旋转变换等方式变换到上述3个区域内。
但考虑到裁剪框并不都是正方形，所以上方区域和左侧区域也不都是完全映射的，所以并不能简单的旋转平移点，而是整个屏幕都旋转，这样的话还不如为上方区域单独做算法。
最终实现代码如下：
1 /* Nicholl-Lee-Nicholl Line Clipping Algorithm */
2 void NichollLeeNichollClip()
3 {
4 // 变量
5 clipped_line_num = 0;
6 float m, k;
7 float mLT, mLB, mRB, mRT; // 生成起点到四个剪裁窗口顶点的斜率
8 // 针对每条线
9 for (int i = 0; i < line_num; i++)
10 {
11 // 生成剪裁线
12 line clip_line = l[i];
13 // 区域码赋值
14 getRegionCode(clip_line.original);
15 getRegionCode(clip_line.terminal);
16 // 判断框内线条（两端点区域码都为00000）
17 if ((clip_line.original.code | clip_line.terminal.code) == 0x0)
18 {
19 clipped_l[clipped_line_num] = clip_line;
20 clipped_line_num++;
21 continue;
22 }
23 // 判断框外线条（两端点区域码按为与为真）
24 if (clip_line.original.code & clip_line.terminal.code)
25 {
26 continue;
27 }
28 // 判断起点是否在已有算法区域的对称区域
29 int before_code; // 变换之前的区域码
30 before_code = clip_line.original.code;
31 // 右侧变换
32 if (clip_line.original.code == winRightBitCode)
33 {
34 clip_line.original.x = clip_left + clip_right - clip_line.original.x;
35 clip_line.terminal.x = clip_left + clip_right - clip_line.terminal.x;
36 }
37 // 右上侧变换
38 if (clip_line.original.code == ( winRightBitCode | winTopBitCode ) )
39 {
40 clip_line.original.x = clip_left + clip_right - clip_line.original.x;
41 clip_line.terminal.x = clip_left + clip_right - clip_line.terminal.x;
42 }
43 // 左下侧变换
44 if (clip_line.original.code == ( winLeftBitCode | winBottomBitCode ) )
45 {
46 clip_line.original.y = clip_top + clip_bottom - clip_line.original.y;
47 clip_line.terminal.y = clip_top + clip_bottom - clip_line.terminal.y;
48 }
49 // 右下侧变换
50 if (clip_line.original.code == ( winRightBitCode | winBottomBitCode ) )
51 {
52 clip_line.original.x = clip_left + clip_right - clip_line.original.x;
53 clip_line.terminal.x = clip_left + clip_right - clip_line.terminal.x;
54 clip_line.original.y = clip_top + clip_bottom - clip_line.original.y;
55 clip_line.terminal.y = clip_top + clip_bottom - clip_line.terminal.y;
56 }
57 // 下侧变换
58 if (clip_line.original.code == winBottomBitCode)
59 {
60 clip_line.original.y = clip_top + clip_bottom - clip_line.original.y;
61 clip_line.terminal.y = clip_top + clip_bottom - clip_line.terminal.y;
62 }
63 // 重新加载区域码
64 getRegionCode(clip_line.original);
65 getRegionCode(clip_line.terminal);
66 // 以下情况均要用到斜率
67 mLT=(float)(clip_top-clip_line.original.y)/(clip_left-clip_line.original.x);
68 mLB=(float)(clip_bottom-clip_line.original.y)/(clip_left-clip_line.original.x);
69 mRB=(float)(clip_bottom-clip_line.original.y)/(clip_right-clip_line.original.x);
70 mRT=(float)(clip_top-clip_line.original.y)/(clip_right-clip_line.original.x);
71 m=(float)(clip_line.terminal.y-clip_line.original.y)/(clip_line.terminal.x-clip_line.original.x);
72 k = 1 / m;
73
74 // 判断起点位置
75 // 情况一：在框内
76 if (clip_line.original.code == 0x0)
77 {
78 // left
79 if ((mLT <= m && m < mLB) && clip_line.terminal.code & winLeftBitCode)
80 {
81 clip_line.terminal.y = clip_line.terminal.y + round(m * (clip_left - clip_line.terminal.x));
82 clip_line.terminal.x = clip_left;
83 }
84 // bottom
85 if ((m >= mLB || m < mRB) && clip_line.terminal.code & winBottomBitCode)
86 {
87 clip_line.terminal.x = clip_line.terminal.x + round(k * (clip_bottom - clip_line.terminal.y));
88 clip_line.terminal.y = clip_bottom;
89 }
90 // right
91 if ((mRB <= m && m < mRT) && clip_line.terminal.code & winRightBitCode)
92 {
93 clip_line.terminal.y = clip_line.terminal.y + round(m * (clip_right - clip_line.terminal.x));
94 clip_line.terminal.x = clip_right;
95 }
96 // top
97 if ((m >= mRT || m < mLT) && clip_line.terminal.code & winTopBitCode)
98 {
99 clip_line.terminal.x = clip_line.terminal.x + round(k * (clip_top - clip_line.terminal.y));
100 clip_line.terminal.y = clip_top;
101 }
102 }
103 // 情况二：在框左侧
104 elseif (clip_line.original.code == winLeftBitCode)
105 {
106 if (m < mLB || m >= mLT) // 区域外
107 continue;
108 else
109 {
110 // 起点剪裁 L
111 clip_line.original.y = clip_line.original.y + round(m * (clip_left - clip_line.original.x);
112 clip_line.original.x = clip_left;
113 if (clip_line.terminal.code != 0x0)
114 {
115 if (mLB <= m && m < mRB) // LB
116 {
117 clip_line.terminal.x = clip_line.terminal.x + round(k * (clip_bottom - clip_line.terminal.y));
118 clip_line.terminal.y = clip_bottom;
119 }
120 elseif (mRB <= m && m < mRT) // LR
121 {
122 clip_line.terminal.y = clip_line.terminal.y + round(m * (clip_right - clip_line.terminal.x));
123 clip_line.terminal.x = clip_right;
124 }
125 elseif (mRT <= m && m < mLT) // LT
126 {
127 clip_line.terminal.x = clip_line.terminal.x + round(k * (clip_top - clip_line.terminal.y));
128 clip_line.terminal.y = clip_top;
129 }
130 }
131 }
132 }
133 // 情况三：在框左上侧
134 elseif (clip_line.original.code == ( winLeftBitCode | winTopBitCode ) )
135 {
136 if (m < mLB || m >= mRT) // 区域外
137 continue;
138 else
139 {
140 // 起点靠近左裁剪线
141 if (clip_left - clip_line.original.x < clip_line.original.y - clip_top)
142 {
143 if (mLB <= m && m < mLT)
144 {
145 // L
146 clip_line.original.y = clip_line.original.y + round(m * (clip_left - clip_line.original.x));
147 clip_line.original.x = clip_left;
148 if (clip_line.terminal.code != 0x0) // LB
149 {
150 clip_line.terminal.x = clip_line.terminal.x + round(k * (clip_bottom - clip_line.terminal.y));
151 clip_line.terminal.y = clip_bottom;
152 }
153 }
154 elseif (mLT <= m && m < mRB)
155 {
156 // T
157 clip_line.original.x = clip_line.original.x + round(k * (clip_top - clip_line.original.y));
158 clip_line.original.y = clip_top;
159 if (clip_line.terminal.code != 0x0) // TB
160 {
161 clip_line.terminal.x = clip_line.terminal.x + round(k * (clip_bottom - clip_line.terminal.y));
162 clip_line.terminal.y = clip_bottom;
163 }
164 }
165 elseif (mRB <= m && m < mRT)
166 {
167 // T
168 clip_line.original.x = clip_line.original.x + round(k * (clip_top - clip_line.original.y));
169 clip_line.original.y = clip_top;
170 if (clip_line.terminal.code != 0x0) // TR
171 {
172 clip_line.terminal.y = clip_line.terminal.y + round(m * (clip_right - clip_line.terminal.x));
173 clip_line.terminal.x = clip_right;
174 }
175 }
176 }
177 // 起点靠近上裁剪线
178 else
179 {
180 if (mLB <= m && m < mRB)
181 {
182 // L
183 clip_line.original.y = clip_line.original.y + round(m * (clip_left - clip_line.original.x));
184 clip_line.original.x = clip_left;
185 if (clip_line.terminal.code != 0x0) // LB
186 {
187 clip_line.terminal.x = clip_line.terminal.x + round(k * (clip_bottom - clip_line.terminal.y));
188 clip_line.terminal.y = clip_bottom;
189 }
190 }
191 elseif (mRB <= m && m < mLT)
192 {
193 // L
194 clip_line.original.y = clip_line.original.y + round(m * (clip_left - clip_line.original.x));
195 clip_line.original.x = clip_left;
196 if (clip_line.terminal.code != 0x0) // LR
197 {
198 clip_line.terminal.y = clip_line.terminal.y + round(m * (clip_right - clip_line.terminal.x));
199 clip_line.terminal.x = clip_right;
200 }
201 }
202 elseif (mLT <= m && m < mRT)
203 {
204 // T
205 clip_line.original.x = clip_line.original.x + round(k * (clip_top - clip_line.original.y));
206 clip_line.original.y = clip_top;
207 if (clip_line.terminal.code != 0x0) // TR
208 {
209 clip_line.terminal.y = clip_line.terminal.y + round(m * (clip_right - clip_line.terminal.x));
210 clip_line.terminal.x = clip_right;
211 }
212 }
213 }
214 }
215
216 }
217 // 情况四：在框上侧
218 elseif (clip_line.original.code == winTopBitCode)
219 {
220 if ((m > 0 && m < mLT) || (m < 0 && m >= mRT)) // 区域外
221 continue;
222 else
223 {
224 // T
225 clip_line.original.x = clip_line.original.x + round(k * (clip_top - clip_line.original.y));
226 clip_line.original.y = clip_top;
227 if (clip_line.terminal.code != 0x0)
228 {
229 if (mLT <= m && m < mLB) // TL
230 {
231 clip_line.terminal.y = clip_line.terminal.y + round(m * (clip_left - clip_line.terminal.x));
232 clip_line.terminal.x = clip_left;
233 }
234 elseif (m >= mLB || m <= mRB) // TB
235 {
236 clip_line.terminal.x = clip_line.terminal.x + round(k * (clip_bottom - clip_line.terminal.y));
237 clip_line.terminal.y = clip_bottom;
238 }
239 elseif (mRB <= m && m < mRT) // TR
240 {
241 clip_line.terminal.y = clip_line.terminal.y + round(m * (clip_right - clip_line.terminal.x));
242 clip_line.terminal.x = clip_right;
243 }
244 }
245 }
246 }
247 // 恢复对称区域的变化
248 // 右侧变换
249 if (before_code == winRightBitCode)
250 {
251 clip_line.original.x = clip_left + clip_right - clip_line.original.x;
252 clip_line.terminal.x = clip_left + clip_right - clip_line.terminal.x;
253 }
254 // 右上侧变换
255 if (before_code == ( winRightBitCode | winTopBitCode ) )
256 {
257 clip_line.original.x = clip_left + clip_right - clip_line.original.x;
258 clip_line.terminal.x = clip_left + clip_right - clip_line.terminal.x;
259 }
260 // 左下侧变换
261 if (before_code == ( winLeftBitCode | winBottomBitCode ) )
262 {
263 clip_line.original.y = clip_top + clip_bottom - clip_line.original.y;
264 clip_line.terminal.y = clip_top + clip_bottom - clip_line.terminal.y;
265 }
266 // 右下侧变换
267 if (before_code == ( winRightBitCode | winBottomBitCode ) )
268 {
269 clip_line.original.x = clip_left + clip_right - clip_line.original.x;
270 clip_line.terminal.x = clip_left + clip_right - clip_line.terminal.x;
271 clip_line.original.y = clip_top + clip_bottom - clip_line.original.y;
272 clip_line.terminal.y = clip_top + clip_bottom - clip_line.terminal.y;
273 }
274 // 下侧变换
275 if (before_code == winBottomBitCode)
276 {
277 clip_line.original.y = clip_top + clip_bottom - clip_line.original.y;
278 clip_line.terminal.y = clip_top + clip_bottom - clip_line.terminal.y;
279 }
280 // 将剪裁好的直线存入数组
281 clipped_l[clipped_line_num] = clip_line;
282 clipped_line_num++;
283 }
284 }
时间： 2024-08-23 23:45:52