[OpenGL]课后案例15：Bresenham画线算法

//A.15  Bresenham画线算法
#include<GL/glut.h>
#include<stdio.h>
#define BLACK 0
void draw_pixel(int ix,int iy,int value)
{
    glBegin(GL_POINTS);
    glVertex2i(ix,iy);
    glEnd();
}
void bres(int x1,int y1,int x2,int y2)
{
    int dx,dy,i,e;
    int incx,incy,inc1,inc2;
    int x,y;
    dx=x2-x1;
    dy=y2-y1;
    if(dx<0) dx=-dx;
    if(dy<0) dy=-dy;
    incx=1;
    if(x2<x1) incx=-1;
    incy=1;
    if(y2<y1) incy=-1;
    x=x1;
    y=y1;
    if(dx>dy)
    {
        draw_pixel(x,y,BLACK);
        e=2*dy-dx;
        inc1 = 2*(dy-dx);
        inc2 = 2*dy;
        for(i=0;i<dx;i++)
        {
            if(e>=0)
            {
                y+=incy;
                e+=inc1;
            }
            else
                e+=inc2;
            x+=incx;
            draw_pixel(x,y,BLACK);
        }
    }
    else
    {
        draw_pixel(x,y,BLACK);
        e=2*dx-dy;
        inc1=2*(dx-dy);
        inc2=2*dx;
        for(i=0;i<dy;i++)
        {
            if(e>=0)
            {
                x+=incx;
                e+=inc1;
            }
            else
                e+=inc2;
            y+=incy;
            draw_pixel(x,y,BLACK);
        }
    }
}
void display()
{

    glClear(GL_COLOR_BUFFER_BIT);
    bres(200,200,100,50);
    glFlush();
}
void myinit()
{
    glClearColor(1.0,1.0,1.0,1.0);
    glColor3f(1.0,0.0,0.0);
    glPointSize(1.0);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0.0,499.0,0.0,499.0);
}
void main(int argc,char **argv)
{
    /* standard GLUT initialization  */
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB );/*default,not needed*/
    glutInitWindowSize(500,500); /* 500×500 pixel window */
    glutInitWindowPosition(0,0);/* place window top left on display */
    glutCreateWindow("Bresenham's algorithm ");/* window title */
    glutDisplayFunc(display); /* display callback invoked when window
                              opened
                              */
    myinit(); /*set attributes */
    glutMainLoop(); /* enter event loop */
}