计算机图形学-2.1用中点画线扫描转换算法，绘制任意斜率 的直线。可以通过调用此函数绘制图案（图案中包含各 种斜率）

（1）算法设计原理
计算k值不同情况下d的初值和d’与d的计算式


总结如下

（2）程序关键代码

//直线垂直于y轴,斜率不存在
if(x2==x1)
	{
		y1=y1<y2?y1:y2;
		y2=y1<y2?y2:y1;
		while(y1<=y2)
			putpixel(x1,y1++,color);
	}

	//计算斜率
k=(float)(y2-y1)/(float)(x2-x1);  
//根据斜率选择相应的函数
（	if(k>=0&&k<=1)
		MidPointLine1(x1,y1,x2,y2,color);
	else if(k>1)
		MidPointLine2(x1,y1,x2,y2,color);
	else if(k>=-1&&k<=0)
		MidPointLine3(x1,y1,x2,y2,color);
	else
		MidPointLine4(x1,y1,x2,y2,color);
）
//0<k<1 
void MidPointLine1(int x0,int y0,int x1,int y1,int color)
{
	x=x0<x1?x0:x1;
	x1=x1>x0?x1:x0;
	y=x0<x1?y0:y1;
	y1=x1>x0?y1:y0;  //使(x,y),(x1,y1)的关系满足x<x1
	a=y-y1;
	b=x1-x;
	d=2*a+b;
	delta1=2*(a+b);
	delta2=2*a;
	putpixel(x,y,color);
	while(x<x1)
	{
		if(d<0)
		{
			x++;y++;
			d+=delta1;
		}
		else
		{
			x++;
			d+=delta2;
		}
		putpixel(x,y,color);
	}
}
//k>1
void MidPointLine2(int x0,int y0,int x1,int y1,int color)
{

	int a,b,delta1,delta2,d,x,y;
	x=y0<y1?x0:x1;
	x1=y1>y0?x1:x0;
	y=y0<y1?y0:y1;
	y1=y1>y0?y1:y0;  //使(x,y),(x1,y1)的关系满足y<y1
	a=y-y1;
	b=x1-x;
	d=a+2*b;
	delta1=2*b;
	delta2=2*(a+b);
	putpixel(x,y,color);
	while(y<y1)
	{
		if(d<0)
		{
			y++;
			d+=delta1;
		}
		else
		{
			y++;x++;
			d+=delta2;
		}	
		putpixel(x,y,color);
	}
}
//-1<k<0
void MidPointLine3(int x0,int y0,int x1,int y1,int color)
{

	int a,b,delta1,delta2,d,x,y;
	x=x0<x1?x0:x1;
	x1=x1>x0?x1:x0;
	y=x0<x1?y0:y1;
	y1=x1>x0?y1:y0;  //使(x,y),(x1,y1)的关系满足x<x1
	a=y-y1;
	b=x1-x;
	d=2*a-b;
	delta1=2*a;
	delta2=2*a-2*b;
	putpixel(x,y,color);
	while(x<x1)
	{
		if(d<0)
		{
			x++;
			d+=delta1;
		}
		else
		{
			x++;y--;
			d+=delta2;
		}
		putpixel(x,y,color);
	}
}
//k<-1
void MidPointLine4(int x0,int y0,int x1,int y1,int color)
{
	int a,b,delta1,delta2,d,x,y;
	x=y0>y1?x0:x1;
	x1=y0>y1?x1:x0;
	y=y0>y1?y0:y1;
	y1=y0>y1?y1:y0;  //使(x,y),(x1,y1)的关系满足y>y1
	a=y-y1;
	b=x1-x;
	d=a-2*b;
	delta1=2*(a-b);
	delta2=-2*b;
	putpixel(x,y,color);
	while(y>y1)
	{
		if(d<0)
		{
			x++;y--;
			d+=delta1;
		}
		else
		{
			y--;
			d+=delta2;
		}
		putpixel(x,y,color);
	}	
}

（3）运行结果截屏（对数据输入的说明）

完整代码：

#include <stdio.h>
#include <graphics.h>
#include <conio.h>

//0<k<1 
void MidPointLine1(int x0,int y0,int x1,int y1,int color)
{
	int a,b,delta1,delta2,d,x,y;
	x=x0<x1?x0:x1;
	x1=x1>x0?x1:x0;
	y=x0<x1?y0:y1;
	y1=x1>x0?y1:y0;
	a=y-y1;
	b=x1-x;
	d=2*a+b;
	delta1=2*(a+b);
	delta2=2*a;
	putpixel(x,y,color);
	while(x<x1)
	{
		if(d<0)
		{
			x++;y++;
			d+=delta1;
		}
		else
		{
			x++;
			d+=delta2;
		}
		putpixel(x,y,color);
	}

}

//k>1
void MidPointLine2(int x0,int y0,int x1,int y1,int color)
{

	int a,b,delta1,delta2,d,x,y;
	x=y0<y1?x0:x1;
	x1=y1>y0?x1:x0;
	y=y0<y1?y0:y1;
	y1=y1>y0?y1:y0;
	a=y-y1;
	b=x1-x;
	d=a+2*b;
	delta1=2*b;
	delta2=2*(a+b);
	putpixel(x,y,color);
	while(y<y1)
	{
		if(d<0)
		{
			y++;
			d+=delta1;
		}
		else
		{
			y++;x++;
			d+=delta2;
		}	
		putpixel(x,y,color);
	}

}

//-1<k<0
void MidPointLine3(int x0,int y0,int x1,int y1,int color)
{

	int a,b,delta1,delta2,d,x,y;
	x=x0<x1?x0:x1;
	x1=x1>x0?x1:x0;
	y=x0<x1?y0:y1;
	y1=x1>x0?y1:y0;
	a=y-y1;
	b=x1-x;
	d=2*a-b;
	delta1=2*a;
	delta2=2*a-2*b;
	putpixel(x,y,color);
	while(x<x1)
	{
		if(d<0)
		{
			x++;
			d+=delta1;
		}
		else
		{
			x++;y--;
			d+=delta2;
		}
		putpixel(x,y,color);
	}

}

//k<-1
void MidPointLine4(int x0,int y0,int x1,int y1,int color)
{
	int a,b,delta1,delta2,d,x,y;
	x=y0>y1?x0:x1;
	x1=y0>y1?x1:x0;
	y=y0>y1?y0:y1;
	y1=y0>y1?y1:y0;
	a=y-y1;
	b=x1-x;
	d=a-2*b;
	delta1=2*(a-b);
	delta2=-2*b;
	putpixel(x,y,color);
	while(y>y1)
	{
		if(d<0)
		{
			x++;y--;
			d+=delta1;
		}
		else
		{
			y--;
			d+=delta2;
		}
		putpixel(x,y,color);
	}	
}
void MidPointLine(int x1,int y1,int x2,int y2,int color)
{
	float k;
	k=(float)(y2-y1)/(float)(x2-x1);
	if(x2==x1)
	{
		y1=y1<y2?y1:y2;
		y2=y1<y2?y2:y1;
		while(y1<=y2)
			putpixel(x1,y1++,color);
	}
	else if(k>=0&&k<=1)
		MidPointLine1(x1,y1,x2,y2,color);
	else if(k>1)
		MidPointLine2(x1,y1,x2,y2,color);
	else if(k>=-1&&k<=0)
		MidPointLine3(x1,y1,x2,y2,color);
	else 
		MidPointLine4(x1,y1,x2,y2,color);
}
void main()
{	
	initgraph(640, 480);

    MidPointLine(400,100,0,0,0xAA0000);  //斜率为0~1，蓝色
	MidPointLine(100,400,0,0,0x00AA00);  //斜率大于1，绿色
	MidPointLine(0,100,400,0,0x55FFFF);   //斜率为-1~0，黄色
	MidPointLine(0,400,100,0,0xFF55FF);   //斜率小于-1，亮紫
    MidPointLine(0,240,640,240,0xFFFFFF);   //垂直于y轴，白色
	MidPointLine(320,0,320,480,0xFFFF55);   //垂直于x轴，亮青

		_getch();
	closegraph();
}