改进的有效边表算法，多边形的扫描转换

这里不仔细讲原理，只是把我写的算法发出来，跟大家分享下，如果有错误的话，还请大家告诉我，如果写的不好，也请指出来，一起讨论进步。

边表构造的算法:

(1) 首先构造一个纵向链表，链表的长度为多边形所占有的最大扫描线数，链表的每个结点，称为一个桶，则对应多边形覆盖的每一条扫描线。

(2) 将每条边的信息链入与该边最小y坐标相对的桶处。也就是说，若某条边的较低点为ymin，则该边就放在相应的扫描线中。

(3) 每条边的数据形成一个结点，内容包括:该扫描线与该的初始交点x(即较低端点的x值),1/k，以及该边的最大y值ymax如下：

x|ymin　　ymax　　1/k 　　next

(4) 同一桶中若干条边按x|ymin由小到在大排序，若x|ymin相等，则按照1/k由小到大排序。

改进的有效边表填充算法步骤如下:

(1) 初始化： 构造边表， AET表置空.

(2) 将第一个不空的ET表中的边与AET表合并。

(3) 删除AET表中y = ymax的边后再填充，按“下闭上升”的原则进行填充，此时就无需缩短任何边。然后进行填充，填充时设置一个布尔量b(初值为假)，令指针从有效边表中第一个结点到最后一个结点进行遍历一次。每访问一个结点，把b取反一次，若b为真，则把从当前结点的x值开始下一结点的x值结束的区间用多边形色填充。(填充时需要对x坐标进行四舍五入处理)。

(4) y_{i+1 =} y_i + 1,根据x_i+1 = x_i + 1 / k计算并修改AET表，同时合并ET表中y = y_{i+1桶的边，按次序插入到AET表中，形成新的AET表。}

(5) AET表不空则转(3)，否则结束。

/*
*	Date: 11/23/2010
*/
#include <GL/freeglut.h>
#include <iostream>
#include <vector>
#include <fstream>
std::ifstream cin ("polypoints.txt");
using std::vector;
using std::endl;
typedef struct _EdgeItem
{
	float	x;
	int		yMax;
	float	reverseK;					// 1/k
	_EdgeItem * next;
}EdgeItem;
vector<EdgeItem *> g_pItemVector;
typedef struct _Point
{
	int x;
	int y;
}Point;

typedef struct _EdgePtr
{
	int		nScanLine;					// Current scan line
	EdgeItem * pItem;					// Pointer to edge item
}EdgePtr;

typedef struct _PolyPoints
{
	Point * pPoint;						// Pointer to points
	int		n;							// Number of points
	int		yMax;						// Max y of all points
	int		yMin;						// Min y of all points
}PolyPoints;

EdgePtr * g_pEdgeList;					// Edge list
EdgePtr * g_pActiveEdgeList;				// Active edge list
PolyPoints g_polyPoints;				// Storage for points of polygons

void inputPoints (void)
{
	int n;
	cin>>n;
	if (n < 3)
	{
		std::cout<<"number of points can not be less than 3"<<endl;
		exit (0);
	}
	g_polyPoints.n = n;
	g_polyPoints.pPoint = new Point[n];
	g_polyPoints.yMax = INT_MIN;
	g_polyPoints.yMin = INT_MAX;
	int x, y;
	for (int i = 0; i < n; ++i)
	{
		cin>>x>>y;
		g_polyPoints.pPoint[i].x = x;
		g_polyPoints.pPoint[i].y = y;
		if (g_polyPoints.yMax < y)
		{
			g_polyPoints.yMax = y;
		}
		if (g_polyPoints.yMin > y)
		{
			g_polyPoints.yMin = y;
		}
	}


}
// Calculate the reverse of k
float calculateReverseK (const Point & p1, const Point & p2)
{
	return (float)(p2.x - p1.x) / (float)(p2.y - p1.y);
}

// Sort one scan line's list, first sort by x, if x is equal then sort by 1/k
void sortOneScanLineEdgeList (EdgePtr & edgePtr)
{
	// Sort by x (select sort)
	EdgeItem * pEdgeItem = edgePtr.pItem;
	EdgeItem * pNext;
	EdgeItem * pTmp;
	while (pEdgeItem)
	{
		pNext = pEdgeItem->next;
		pTmp = pEdgeItem;
		while (pNext)
		{
			if (pNext->x < pTmp->x)
			{
				pTmp = pNext;
			}
			pNext = pNext->next;
		}
		if (pTmp != pEdgeItem)
		{
			// Swap x
			float fTmp = pTmp->x;
			pTmp->x = pEdgeItem->x;
			pEdgeItem->x = fTmp;
			// Swap yMax
			
			int iTmp = pTmp->yMax;
			pTmp->yMax = pEdgeItem->yMax;
			pEdgeItem->yMax = iTmp;
			// Swap 1/k
			float kTmp = pTmp->reverseK;
			pTmp->reverseK = pEdgeItem->reverseK;
			pEdgeItem->reverseK = kTmp;
		}
		pEdgeItem = pEdgeItem->next;
	}
	// When the x is the same, then sort by 1/k
	pEdgeItem = edgePtr.pItem;
	EdgeItem * pStart = NULL;
	EdgeItem * pEnd = NULL;
	while (pEdgeItem)
	{
		// Find the start pointer and end pointer with the same x, then sort them
		pEnd = pStart = pEdgeItem;
		pNext = pStart->next;
		while (pNext && (pNext->x == pStart->x))
		{
			pEnd = pNext;
			pNext = pNext->next;
		}
		// Sort the edge list from pStart to pEnd by 1/k (select sort)
		while (pStart != pEnd)
		{
			pTmp = pStart;
			pNext = pTmp->next;
			while (pNext != pEnd)
			{
				if (pNext->reverseK < pTmp->reverseK)
				{
					pTmp = pNext;
				}
				pNext = pNext->next;
			}
			// Swap values
			if (pTmp != pStart)
			{
				// Swap x
				float fTmp = pTmp->x;
				pTmp->x = pStart->x;
				pStart->x = fTmp;
				// Swap yMax
				int iTmp = pTmp->yMax;
				pTmp->yMax = pStart->yMax;
				pStart->yMax = iTmp;
				// Swap 1/k
				float kTmp = pTmp->reverseK;
				pTmp->reverseK = pStart->reverseK;
				pStart->reverseK = kTmp;
				
			}
			pStart = pStart->next;
		} // while (pStart != pEnd)
		pEdgeItem = pEnd->next;
	}
}
// Construct the edge list
void constructEdgeList (void)
{
	// Construct the edge list
	int nScanLines = g_polyPoints.yMax - g_polyPoints.yMin + 1;
	g_pEdgeList = new EdgePtr[nScanLines];
	memset (g_pEdgeList, 0, sizeof (EdgePtr) * nScanLines);
	Point * pPoint = g_polyPoints.pPoint;
	int nScanLine = g_polyPoints.yMin;
	
	EdgeItem * pEdgeItem = NULL;
	for (int i = 0; i < nScanLines; ++i, ++ nScanLine)
	{
		g_pEdgeList[i].nScanLine = nScanLine;
		for (int j = 0; j < g_polyPoints.n; ++j)
		{
			if (pPoint[j].y == nScanLine)
			{
				int j1 = (j+g_polyPoints.n-1) % g_polyPoints.n;
				int j2 = (j+1) % g_polyPoints.n;
				// if point j1's y > nScanLine then add this edge to the current scanline's list
				if (pPoint[j1].y > nScanLine)
				{
					pEdgeItem = new EdgeItem;
					pEdgeItem->reverseK = calculateReverseK (pPoint[j], pPoint[j1]);
					pEdgeItem->x = (float)pPoint[j].x;
					pEdgeItem->yMax = pPoint[j1].y;
					// Add pEdgeItem to the scanline's list
					pEdgeItem->next = g_pEdgeList[i].pItem;
					g_pEdgeList[i].pItem = pEdgeItem;

				}
				// if point j2's y > nScanLine then add this edge to the curretn scanline's list
				if (pPoint[j2].y > nScanLine)
				{
					pEdgeItem = new EdgeItem;
					pEdgeItem->reverseK = calculateReverseK (pPoint[j], pPoint[j2]);
					pEdgeItem->x = (float)pPoint[j].x;
					pEdgeItem->yMax = pPoint[j2].y;
					// Add pEdgeItem to the scanline's list
					pEdgeItem->next = g_pEdgeList[i].pItem;
					g_pEdgeList[i].pItem = pEdgeItem;
				}
			} // if (pPoints[j].y == nScanLine)
		} // for (int j = 0; j < g_polyPoints.n; ++j)
		sortOneScanLineEdgeList (g_pEdgeList[i]);
	}
	// Init the active edge list
	g_pActiveEdgeList = new EdgePtr[nScanLines];
}
// free the memory
void destroy (void)
{
	if (g_pActiveEdgeList)
	{
		delete g_pActiveEdgeList;
	}
	int nScanLines = g_polyPoints.yMax - g_polyPoints.yMin + 1;
	EdgeItem * pItem, * pNext;
	if (g_pEdgeList)
	{
		for (int i = 0; i < nScanLines; ++i)
		{
			if (g_pEdgeList[i].pItem)
			{
				pItem = g_pEdgeList[i].pItem;
				pNext = pItem;
				while (pItem)
				{
					pNext = pItem->next;
					delete pItem;
					pItem = pNext;
				}
			}
		}
	}
}
void init (void)
{
	glClearColor (0.0f, 0.0f, 0.0f, 1.0f);
}

void activEdgeListFillPolygon (void)
{
	int nScanLines = g_polyPoints.yMax - g_polyPoints.yMin + 1;
	memset (g_pActiveEdgeList, 0, sizeof (EdgePtr) * nScanLines);
	int nScanLine = g_polyPoints.yMin;
	glBegin (GL_POINTS);
	int i = 0;
	for (;i < nScanLines;  ++ nScanLine, ++ i)
	{
		if (g_pEdgeList[i].pItem)
		{
			g_pActiveEdgeList[i].pItem = g_pEdgeList[i].pItem;
			break;
		}
	}
	for (int j = i; j < nScanLines; ++j, ++ nScanLine)
	{
		if (g_pActiveEdgeList[j].pItem)
		{
			// Delete the edge where yMax = nScanLine
			EdgeItem * pPre = NULL;
			EdgeItem * pNext = g_pActiveEdgeList[j].pItem;
			bool bEven = true;
			while (pNext)
			{
				if (pNext->yMax == nScanLine)
				{
					if (!pPre)
					{
						g_pActiveEdgeList[j].pItem = pNext->next;
						pNext = pNext->next;
					}
					else
					{
						pPre->next = pNext->next;
						pNext = pNext->next;
					}
				}
				else 
				{
					bEven = !bEven;
					pPre = pNext;
					pNext = pNext->next;
				}
			} // while (pNext)

			// Fill the scan line when bFill is true
			bool bFill = false;
			pNext = g_pActiveEdgeList[j].pItem;
			while (pNext && bEven)
			{
				bFill = !bFill;
				if (bFill)
				{
					int x1 = (int)(pNext->x + 0.5);
					int x2 = (int)(pNext->next->x + 0.5);
					//int x1 = pNext->x;
					//int x2 = pNext->next->x;
					for (int i = x1; i <= x2; ++i)
					{
						glVertex2i (i, nScanLine);
					}
				}
				pNext = pNext->next;
			}	// while (pNext)
			pNext = g_pActiveEdgeList[j].pItem;
			int kk = j + 1;
			if (kk < nScanLines)
			{
				while (pNext)
				{		
					EdgeItem * pItem = new EdgeItem;
					pItem->x = pNext->x + pNext->reverseK;
					pItem->reverseK = pNext->reverseK;
					pItem->yMax = pNext->yMax;
					pItem->next = g_pActiveEdgeList[kk].pItem;
					g_pActiveEdgeList[kk].pItem = pItem;
					pNext = pNext->next;
					g_pItemVector.push_back (pItem);
				} // while (pNext)
				// Add edge list to active edge list
				pNext = g_pEdgeList[kk].pItem;
				EdgeItem * pTemp = NULL;
				while (pNext)
				{
					pTemp = new EdgeItem;
					pTemp->reverseK = pNext->reverseK;
					pTemp->x = pNext->x;
					pTemp->yMax =pNext->yMax; 
					g_pItemVector.push_back (pTemp);
					pTemp->next = g_pActiveEdgeList[kk].pItem;
					g_pActiveEdgeList[kk].pItem = pTemp;
					pNext = pNext->next;
				}
				sortOneScanLineEdgeList (g_pActiveEdgeList[kk]);
			}


		} // 			if (g_pActiveEdgeList[j].pItem)
	}
	glEnd ();

　　　　// 这里为了简单所以把分配的内存放在vector容器中，方便删除。:-)
	vector<EdgeItem *>::iterator itr = g_pItemVector.begin (),
		endItr = g_pItemVector.end ();
	while (itr != endItr)
	{
		delete (*itr);
		++ itr;
	}
	g_pItemVector.clear ();
}

void display (void)
{
	glClear (GL_COLOR_BUFFER_BIT);
	glLoadIdentity ();
	glColor3f (1.0f, 0.0f, 0.0f);
	// Fill a polygon
	activEdgeListFillPolygon ();
	glutSwapBuffers ();
}

void reshape (int w, int h)
{
	glViewport (0, 0, (GLsizei) w, (GLsizei) h);
	glMatrixMode (GL_PROJECTION);
	glLoadIdentity ();
	if (w <= h)
	{
		gluOrtho2D (-600.0, 600.0, -600.0 * (GLfloat) h / (GLfloat) w, 600.0 * (GLfloat) h / (GLfloat) w);
	}
	else
	{
		gluOrtho2D (-600.0 * (GLfloat) w / (GLfloat) h,600.0 * (GLfloat) w / (GLfloat) h, -600.0, 600.0);
	}
	glMatrixMode (GL_MODELVIEW);
	glLoadIdentity ();
}
void keyboard (unsigned char key, int x, int y)
{
	switch (key)
	{
	case 27: // 'VK_ESCAPE'

		destroy ();

　　　　　　exit (0);
		break;
	default:
		break;
	}
}
int main (int argc, char ** argv)
{
	glutInit (&argc, argv);
	glutInitDisplayMode (GLUT_DOUBLE | GLUT_RGB);
	glutInitWindowSize (600, 600);
	glutCreateWindow ("Optimized active edge list");
	init ();
	inputPoints ();
	constructEdgeList ();

	glutReshapeFunc (reshape);
	glutDisplayFunc (display);
	glutKeyboardFunc (keyboard);
	glutMainLoop ();
	destroy ();   // 这里destroy并没有调用。
	return 0;
}

polypoints.txt

中的示例内容如下:

7

30 120

10 70

30 10

60 50

80 10

120 90

70 80