计算机图形学笔记一——绘制直线的算法

绘制直线的算法

数值微分法

数值微分法（digital differential analyzer DDA）使用直线的增量方程来计算直线的下一个迭代点像素的方法。直线的微分方程：

\[\frac{dy}{dx}=\frac{\Delta y}{\Delta x}=\frac{y_1-y_0}{x_1-x_0}=k \]

得到迭代公式：

\[x_{i+1}=x_i+\beta \Delta x \]

\[y_{i+1}=y_i +\beta \Delta y \]

其中$\beta=\frac{1}{max(|\Delta x|,|\Delta y|)}$
也就是说，当斜率的绝对值大于1的时候，取y的步长为1，当斜率的绝对值小于1时，取x的步长为1。然后对非步长点进行近似，得到这样的代码

double x, y;
double k;			//斜率
k = (static_cast<float>(endy - starty) / static_cast<float>(endx - startx));
x = (double)startx, y = (double)starty;
if (abs(k) < 1.0) {
	for (int i = 0; i < abs(endx - startx); i++) {
		if (endx > startx) {
			x += 1;
			y += k;
		}
		else {
			x -= 1;
			y -= k;
		}
		glVertex2d(static_cast<int>(x), static_cast<int>(y + 0.5));		//y四舍五入
	}
}
else if (abs(k) > 1.0) {
	for (int i = 0; i < abs(endy - starty); i++) {
		if (endy > starty) {
			y += 1;
			x += 1.0 / k;
		}
		else {
			y -= 1;
			x -= 1.0 / k;
		}
		glVertex2d(static_cast<int>(x + 0.5), static_cast<int>(y));		//x四舍五入
	}
}

再考虑斜率为0和斜率不存在的情况（垂直x轴和平行x轴），我们可以写出这样的代码，这便是DAA方法的实现。

DAA完整代码

void DDA_Line(int startx, int starty, int endx, int endy) {
	glBegin(GL_POINTS);
	if (startx != endx && starty != endy)
	{
		double x, y;
		double k;			//斜率
		k = (static_cast<float>(endy - starty) /static_cast<float>(endx - startx));
		x = (double)startx, y = (double)starty;
		if (abs(k)<1.0) {
			for (int i = 0; i < abs(endx - startx); i++) {
				if (endx > startx) {
					x += 1;
					y += k;
				}
				else {
					x -= 1;
					y -= k;
				}
				glVertex2d(static_cast<int>(x), static_cast<int>(y+0.5));		//y四舍五入
			}
		}
		else if (abs(k) > 1.0) {
			for (int i = 0; i < abs(endy - starty); i++) {
				if (endy > starty) {
					y += 1;
					x += 1.0/k;
				}
				else {
					y -= 1;
					x -= 1.0/k;
				}
				glVertex2d(static_cast<int>(x + 0.5), static_cast<int>(y));		//x四舍五入
			}
		}
	}
	else if (startx == endx) {//垂直画线
		if (starty < endy) {
			for (int i = starty; i < endy; i++) {
				glVertex2d(startx, i);
			}
		}
		else if (starty > endy) {
			for (int i = endy; i < starty; i++) {
				glVertex2d(startx, i);
			}
		}
	}
	else if (starty == endy) {//水平画线
		if (startx < endx) {
			for (int i = startx; i < endx; i++) {
				glVertex2d(i, starty);
			}
		}
		else if (startx > endx) {
			for (int i = endx; i < startx; i++) {
				glVertex2d(i, starty);
			}
		}
	}
	glFlush();
	glEnd();
}

中点画线算法

不进行除法运算，减少计算量，使用一种近似的思想进行取样。下面两幅图中说明了这种算法的实施方法：

当曲线在一格的中间偏上时，使用上面的点作为采样点，当在中间偏下时，使用下面的一格。如此计算，便能绕靠斜率的除法运算。
实际实现方法：从左边的点开始，每次往右移动一格，计算这个点的中点处y与函数值的差，如果大于零，则y不变；反之y向上（下）移动一格。
那么有两个要求：
一、计算y移动的方向是上还是下
二、开始的点必须在左边
分析一下这个差值的计算：

\[F(x,y)=ax+by+c=0 \]

\[a=y_0-y_1 \]

\[b=x_1-x_0 \]

\[c=x_0y_1-x_1y_0 \]

\[d=F(x_i+1,y_i+0.5) \]

其中d也就是中点和直线的竖直距离，我们可以通过判断d的正负来进行采样。进一步分析
当d>=0时，下一个构造点为F(x_i+2,y_i)，那么

\[d_1=a(x_i+1+1)+b(y_i+0.5)+c=d+a \]

同理，当$d<0$时d_1=F(x_i+2,y_i+1.5)，即：

\[d=a(x_i+2)+b(y_i+1.5)+c=d+a+b \]

这样我们就得到了迭代方程，我们可以写出这样的代码：

if (startx != endx && starty != endy)
{
	bool kFlag = 0;		//斜率是否大于1
	int sFlag = 1;			//斜率的正负
	if (startx > endx) {		//因为算法是x递增的，所以要保持起点x在终点左边
		int tempx = startx, tempy = starty;
		startx = endx, starty = endy;
		endx = tempx, endy = tempy;
	}
	if (abs(starty - endy) > abs(startx - endx)) {
		kFlag = true;
	}
	if (starty > endy) {		//标记斜率小于零
		sFlag = -1;
	}
	int a, b, TA, TAB, d, x, y;
	if (sFlag == -1)
		endy = starty + (starty - endy);

	a = starty - endy;
	b = endx - startx;
	TA = 2 * a;					//twoA
	TAB = 2 * (a + b);		//twoAB
	d = 2 * a + b;
	x = startx, y = starty;
	if (!kFlag) {
		for (int i = 0; i < (endx - startx); i++) {
			if (d >= 0) {
				glVertex2i(++x, y);
				d += TA;
			}
			else {
				glVertex2i(++x, y += sFlag);
				d += TAB;
			}
		}
	}
	else if (kFlag) {
		if (kFlag == 1) {
			TA = 2 * b;
			d = 2 * b + a;
		}
		for (int i = 0; i < abs(endy - starty); i++) {
			if (d >= 0) {
				glVertex2i(++x, y += sFlag);
				d += TAB;
			}
			else {
				glVertex2i(x, y += sFlag);
				d += TA;
			}
		}
	}
}

你可以在这里找到所有代码

TMP算法完整代码

void TMP_Line(int startx, int starty, int endx, int endy) {
	glBegin(GL_POINTS);
	if (startx != endx && starty != endy)
	{
		bool kFlag = 0;		//斜率是否大于1
		int sFlag = 1;			//斜率的正负
		if (startx > endx) {		//因为算法是x递增的，所以要保持起点x在终点左边
			int tempx = startx, tempy = starty;
			startx = endx, starty = endy;
			endx = tempx, endy = tempy;
		}
		if (abs(starty - endy) > abs(startx - endx)) {
			kFlag = true;
		}
		if (starty > endy) {		//标记斜率小于零
			sFlag = -1;
		}
		int a, b, TA, TAB, d, x, y;
		if (sFlag == -1)
			endy = starty + (starty - endy);

		a = starty - endy;
		b = endx - startx;
		TA = 2 * a;					//twoA
		TAB = 2 * (a + b);		//twoAB
		d = 2 * a + b;
		x = startx, y = starty;
		if (!kFlag) {
			for (int i = 0; i < (endx - startx); i++) {
				if (d >= 0) {
					glVertex2i(++x, y);
					d += TA;
				}
				else {
					glVertex2i(++x , y +=sFlag);
					d += TAB;
				}
			}
		}
		else if (kFlag) {
			if (kFlag == 1) {
				TA = 2 * b;
				d = 2 * b + a;
			}
			for (int i = 0; i < abs(endy - starty); i++) {
				if (d >= 0) {
					glVertex2i(++x , y +=sFlag);
					d += TAB;
				}
				else {
					glVertex2i(x, y+=sFlag);
					d += TA;
				}
			}
		}
	}
	else if (startx == endx) {//垂直画线
		if (starty < endy) {
			for (int i = starty; i < endy; i++) {
				glVertex2i(startx, i);
			}
		}
		else if (starty > endy) {
			for (int i = endy; i < starty; i++) {
				glVertex2i(startx, i);
			}
		}
	}
	else if (starty == endy) {//水平画线
		if (startx < endx) {
			for (int i = startx; i < endx; i++) {
				glVertex2i(i, starty);
			}
		}
		else if (startx > endx) {
			for (int i = endx; i < startx; i++) {
				glVertex2i(i, starty);
			}
		}
	}
	glFlush();
	glEnd();
}

Bresenham画线算法

这是目前计算机图形学使用的最多的直线生成算法。在计算最佳逼近中使用的全部都是整数运算，可以大幅度的提升计算速度。
与TMP不同的一点就是，TMP是计算中点与直线上的点，而Bresenham是计算上下格子与直线竖直距离的差。也就是下面图像中的d2-d1

推导过程如下：
直线方程可以表示为（不管两种特殊情况）

\[y=kx+b \]

\[k=\frac{y_1-y_0}{x_1-x_0}=\frac{dy}{dx},b=y_0-kx_0 \]

当 $ 0<k<1$时直线向正方向前进一个像素,对应的最佳逼近的 $x_{i+1}=x_i+1$, $y_{i+1}=y_i+1 $or $y_i$

\[y=k(x_i+1)+b \]

\[d_1=y-y_i \]

\[d_2=y_i+1-y \]

令

\[d_1-d_2=2y-2y_i-1=2(k(x_i+1)+b)-2y_i-1 \]

当 $d_1-d_2>0$时

\[y_{i+1}=y_i+1 \]

当 $d_1-d_2<=0$时

\[y_{i+1}=y_i \]

魔法:当dx>0时,把 $d_10d_2$乘上$dx$,不影响整个式子的正负得到:

\[p_i=dx(d_1-d_2) \]

又

\[k=\frac{dy}{dx} \]

所以

\[p_i=2x_0dy-2y_idx+2dy+(2b-1)dx \]

当i=0时,得到迭代起点:

\[p_0=2dy-dx \]

当 $p_i>0时$

\[p_{i+1}=p_i+2dy-2dx \]

当 $p_i<=0时$

\[p_{i+1}=p_i+2dy \]

推导到此结束,下面是实现,这个实现和TMP十分相似,但是计算量比TMP稍微小一点。

if (startx != endx && starty != endy) {
	bool kFlag = 0;		//斜率是否大于1
	int sFlag = 1;			//斜率的正负
	if (startx > endx) {		//因为算法是x递增的，所以要保持起点x在终点左边
		int tempx = startx, tempy = starty;
		startx = endx, starty = endy;
		endx = tempx, endy = tempy;
	}
	if (abs(starty - endy) > abs(startx - endx)) {
		kFlag = true;
	}
	if (starty > endy) {		//标记斜率小于零
		sFlag = -1;
	}
	int x, y;
	int dx, dy, p;
	if (sFlag == -1)
		endy = starty + (starty - endy);
	dx = endx - startx;
	dy = endy - starty;
	x = startx, y = starty;
	if (!kFlag) {
		p = 2 * dy - dx;
		for (int i = 0; i < (endx - startx); i++) {
			if (p <= 0) {
				glVertex2i(++x, y);
				p += 2 * dy;
			}
			else {
				glVertex2i(++x, y += sFlag);
				p += 2 * dy - 2 * dx;
			}
		}
	}
	else {
		p = 2 * dx - dy;
		for (int i = 0; i < (endy - starty); i++) {
			if (p <= 0) {
				glVertex2i(x, y += sFlag);
				p += 2 * dx;
			}
			else {
				glVertex2i(++x, y += sFlag);
				p += 2 * dx - 2 * dy;
			}
		}
	}
}

同样的，你可以在这里找到完整代码：

BRESENHAM算法完整代码

void BRESENHAM_Line(int startx, int starty, int endx, int endy) {
	glBegin(GL_POINTS);
	if (startx != endx && starty != endy) {
		bool kFlag = 0;		//斜率是否大于1
		int sFlag = 1;			//斜率的正负
		if (startx > endx) {		//因为算法是x递增的，所以要保持起点x在终点左边
			int tempx = startx, tempy = starty;
			startx = endx, starty = endy;
			endx = tempx, endy = tempy;
		}
		if (abs(starty - endy) > abs(startx - endx)) {
			kFlag = true;
		}
		if (starty > endy) {		//标记斜率小于零
			sFlag = -1;
		}
		int x, y;
		int dx, dy, p;
		if (sFlag == -1)
			endy = starty + (starty - endy);
		dx = endx - startx;
		dy = endy - starty;
		x = startx, y = starty;
		if (!kFlag) {
			p = 2 * dy - dx;
			for (int i = 0; i < (endx - startx); i++) {
				if (p <= 0) {
					glVertex2i(++x, y);
					p += 2 * dy;
				}
				else {
					glVertex2i(++x, y += sFlag);
					p += 2 * dy - 2 * dx;
				}
			}
		}
		else {
			p = 2 * dx - dy;
			for (int i = 0;i < (endy - starty); i++) {
				if (p <= 0) {
					glVertex2i(x, y += sFlag);
					p += 2 * dx;
				}
				else {
					glVertex2i(++x, y += sFlag);
					p += 2 * dx - 2 * dy;
				}
			}
		}
	}
	else if (startx == endx) {//垂直画线
		if (starty < endy) {
			for (int i = starty; i < endy; i++) {
				glVertex2i(startx, i);
			}
		}
		else if (starty > endy) {
			for (int i = endy; i < starty; i++) {
				glVertex2i(startx, i);
			}
		}
	}
	else if (starty == endy) {//水平画线
		if (startx < endx) {
			for (int i = startx; i < endx; i++) {
				glVertex2i(i, starty);
			}
		}
		else if (startx > endx) {
			for (int i = endx; i < startx; i++) {
				glVertex2i(i, starty);
			}
		}
	}
	glFlush();
	glEnd();
}

代码汇总

DrawLine.hpp代码

//DrawLine.hpp
#pragma once
#include <cmath>
#include <gl/glut.h>

/*
数值微分算法实现
*/
/// <summary>
/// digitial differential analyzer 
/// called DAA
/// </summary>
/// <param name="startx">起始位置x</param>
/// <param name="starty"></param>
/// <param name="endx">终止位置x</param>
/// <param name="endy"></param>
void DDA_Line(int startx, int starty, int endx, int endy) {
	glBegin(GL_POINTS);
	if (startx != endx && starty != endy)
	{
		double x, y;
		double k;			//斜率
		k = (static_cast<float>(endy - starty) /static_cast<float>(endx - startx));
		x = (double)startx, y = (double)starty;
		if (abs(k)<1.0) {
			for (int i = 0; i < abs(endx - startx); i++) {
				if (endx > startx) {
					x += 1;
					y += k;
				}
				else {
					x -= 1;
					y -= k;
				}
				glVertex2i(static_cast<int>(x), static_cast<int>(y+0.5));		//y四舍五入
			}
		}
		else if (abs(k) > 1.0) {
			for (int i = 0; i < abs(endy - starty); i++) {
				if (endy > starty) {
					y += 1;
					x += 1.0/k;
				}
				else {
					y -= 1;
					x -= 1.0/k;
				}
				glVertex2i(static_cast<int>(x + 0.5), static_cast<int>(y));		//x四舍五入
			}
		}
	}
	else if (startx == endx) {//垂直画线
		if (starty < endy) {
			for (int i = starty; i < endy; i++) {
				glVertex2i(startx, i);
			}
		}
		else if (starty > endy) {
			for (int i = endy; i < starty; i++) {
				glVertex2i(startx, i);
			}
		}
	}
	else if (starty == endy) {//水平画线
		if (startx < endx) {
			for (int i = startx; i < endx; i++) {
				glVertex2i(i, starty);
			}
		}
		else if (startx > endx) {
			for (int i = endx; i < startx; i++) {
				glVertex2i(i, starty);
			}
		}
	}
	glFlush();
	glEnd();
}

/*
中点画线算法实现
*/
/// <summary>
/// The Middle Point
/// Called TMP
/// </summary>
/// <param name="startx">起始位置x</param>
/// <param name="starty"></param>
/// <param name="endx">终止位置x</param>
/// <param name="endy"></param>
void TMP_Line(int startx, int starty, int endx, int endy) {
	glBegin(GL_POINTS);
	if (startx != endx && starty != endy)
	{
		bool kFlag = 0;		//斜率是否大于1
		int sFlag = 1;			//斜率的正负
		if (startx > endx) {		//因为算法是x递增的，所以要保持起点x在终点左边
			int tempx = startx, tempy = starty;
			startx = endx, starty = endy;
			endx = tempx, endy = tempy;
		}
		if (abs(starty - endy) > abs(startx - endx)) {
			kFlag = true;
		}
		if (starty > endy) {		//标记斜率小于零
			sFlag = -1;
		}
		int a, b, TA, TAB, d, x, y;
		if (sFlag == -1)
			endy = starty + (starty - endy);

		a = starty - endy;
		b = endx - startx;
		TA = 2 * a;					//twoA
		TAB = 2 * (a + b);		//twoAB
		d = 2 * a + b;
		x = startx, y = starty;
		if (!kFlag) {
			for (int i = 0; i < (endx - startx); i++) {
				if (d >= 0) {
					glVertex2i(++x, y);
					d += TA;
				}
				else {
					glVertex2i(++x , y +=sFlag);
					d += TAB;
				}
			}
		}
		else if (kFlag) {
			if (kFlag == 1) {
				TA = 2 * b;
				d = 2 * b + a;
			}
			for (int i = 0; i < abs(endy - starty); i++) {
				if (d >= 0) {
					glVertex2i(++x , y +=sFlag);
					d += TAB;
				}
				else {
					glVertex2i(x, y+=sFlag);
					d += TA;
				}
			}
		}
	}
	else if (startx == endx) {//垂直画线
		if (starty < endy) {
			for (int i = starty; i < endy; i++) {
				glVertex2i(startx, i);
			}
		}
		else if (starty > endy) {
			for (int i = endy; i < starty; i++) {
				glVertex2i(startx, i);
			}
		}
	}
	else if (starty == endy) {//水平画线
		if (startx < endx) {
			for (int i = startx; i < endx; i++) {
				glVertex2i(i, starty);
			}
		}
		else if (startx > endx) {
			for (int i = endx; i < startx; i++) {
				glVertex2i(i, starty);
			}
		}
	}
	glFlush();
	glEnd();
}

/*
Bresenham算法
这是图形学中用的最多的直线生成算法，全部是整数计算，加快了计算的速度
*/
/// <summary>
/// Bresenham
/// </summary>
/// <param name="startx"></param>
/// <param name="starty"></param>
/// <param name="endx"></param>
/// <param name="endy"></param>
void BRESENHAM_Line(int startx, int starty, int endx, int endy) {
	glBegin(GL_POINTS);
	if (startx != endx && starty != endy) {
		bool kFlag = 0;		//斜率是否大于1
		int sFlag = 1;			//斜率的正负
		if (startx > endx) {		//因为算法是x递增的，所以要保持起点x在终点左边
			int tempx = startx, tempy = starty;
			startx = endx, starty = endy;
			endx = tempx, endy = tempy;
		}
		if (abs(starty - endy) > abs(startx - endx)) {
			kFlag = true;
		}
		if (starty > endy) {		//标记斜率小于零
			sFlag = -1;
		}
		int x, y;
		int dx, dy, p;
		if (sFlag == -1)
			endy = starty + (starty - endy);
		dx = endx - startx;
		dy = endy - starty;
		x = startx, y = starty;
		if (!kFlag) {
			p = 2 * dy - dx;
			for (int i = 0; i < (endx - startx); i++) {
				if (p <= 0) {
					glVertex2i(++x, y);
					p += 2 * dy;
				}
				else {
					glVertex2i(++x, y += sFlag);
					p += 2 * dy - 2 * dx;
				}
			}
		}
		else {
			p = 2 * dx - dy;
			for (int i = 0;i < (endy - starty); i++) {
				if (p <= 0) {
					glVertex2i(x, y += sFlag);
					p += 2 * dx;
				}
				else {
					glVertex2i(++x, y += sFlag);
					p += 2 * dx - 2 * dy;
				}
			}
		}
	}
	else if (startx == endx) {//垂直画线
		if (starty < endy) {
			for (int i = starty; i < endy; i++) {
				glVertex2i(startx, i);
			}
		}
		else if (starty > endy) {
			for (int i = endy; i < starty; i++) {
				glVertex2i(startx, i);
			}
		}
	}
	else if (starty == endy) {//水平画线
		if (startx < endx) {
			for (int i = startx; i < endx; i++) {
				glVertex2i(i, starty);
			}
		}
		else if (startx > endx) {
			for (int i = endx; i < startx; i++) {
				glVertex2i(i, starty);
			}
		}
	}
	glFlush();
	glEnd();
}

用于测试的一个主函数

/*
你可以使用左键绘制顶点
你可以使用右键删除顶点
直线的绘制沿着顶点顺序
*/
#include <gl/glut.h>
#include <iostream>
#include <list>
#include "DrawLine.hpp"
using namespace std;
#define m_POINT_SIZE 10
#define m_LINE_SIZE 2
list<pair<int, int >>Vertex;
void onDisplay();
void onReshape(int w, int h);
void onMouse(int button, int state, int x, int y);
void onReshape(int w, int h)
{
	// 设置视口大小
	glViewport(0, 0, w, h);
	// 切换矩阵模式为投影矩阵
	glMatrixMode(GL_PROJECTION);
	// 载入单位矩阵
	glLoadIdentity();
	// 进行二维平行投影
	gluOrtho2D(0, w, h, 0);
	// 切换矩阵模式为模型矩阵
	glMatrixMode(GL_MODELVIEW);
	// 发送重绘
	glutPostRedisplay();
}
void onMouse(int button, int state, int x, int y){
	if (state == GLUT_DOWN) {
		if (button == GLUT_LEFT_BUTTON)Vertex.push_back(make_pair(x, y));
		else if (button == GLUT_RIGHT_BUTTON) {
			auto ibeg = Vertex.begin();
			while (ibeg != Vertex.end()) {
				if (((x - ibeg->first) * (x - ibeg->first)) + ((y - ibeg->second) * (y - ibeg->second)) < 400) {
					Vertex.erase(ibeg);
					break;
				}
				ibeg++;
			}
		}
	}
	onDisplay();
}
void onDisplay() {
	glClearColor(224 / 255.0, 237 / 255.0, 253 / 255.0,1.0);
	glClear(GL_COLOR_BUFFER_BIT);

	glColor3f(1.0f, 0, 0);
	auto ibeg = Vertex.begin(),jbeg=ibeg;
	bool flag = false;
	while (ibeg != Vertex.end()) {
		glPointSize(m_POINT_SIZE);
		glBegin(GL_POINTS);
		glVertex2i(ibeg->first, ibeg->second);
		glEnd();
		glPointSize(m_LINE_SIZE);
		if (!flag)
			flag = true;
		else {
			//这里可以选择直线绘制方式
			BRESENHAM_Line(ibeg->first, ibeg->second, jbeg->first, jbeg->second);
			//TMP_Line(ibeg->first, ibeg->second, jbeg->first, jbeg->second);
			//DDA_Line(ibeg->first, ibeg->second, jbeg->first, jbeg->second);
		}
		jbeg = ibeg;
		ibeg++;
	}
	glutSwapBuffers();
}

int main(int argc, char* argv[])
{
	// 初始化 glut
	glutInit(&argc, argv);
	// 设置 OpenGL 显示模式(双缓存, RGB 颜色模式)
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
	// 设置窗口初始尺寸
	glutInitWindowSize(1000,800);
	// 设置窗口初始位置
	glutInitWindowPosition(0, 0);
	// 设置窗口标题
	glutCreateWindow("Terix");
	glutReshapeFunc(onReshape);
	glutDisplayFunc(onDisplay);
	glutMouseFunc(onMouse);
	// 进入 glut 事件循环
	glutMainLoop();
	return 0;
}

## 下一篇 [下一篇->圆形的绘制](https://www.cnblogs.com/zhywyt/articles/17341756.html)

posted @ 2023-04-21 20:24 zhywyt 阅读(298) 评论(0) 编辑收藏举报

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