二维图形的裁剪

这是我们做试验时老师编的,发来大家看看给解释解释,俺看的不太明白,源代码如下:
#include<graphics.h>

typedef struct{  unsigned all;
               unsigned left,right,top,bottom;
          }outcode;
typedef struct{  float xmin,xmax,ymin,ymax;
          }Rectangle;
          Rectangle *rect;
void cohensutherlandlineclip(float x0,float y0,float x1,float y1,Rectangle *rect)
{
    void compoutcode(float,float,Rectangle *,outcode *);
    int accept,done;
    outcode outcode0,outcode1;


    outcode *outcodeout;
    float x,y;
    accept=0;
    done=0;
    compoutcode(x0,y0,rect,&outcode0);
    compoutcode(x1,y1,rect,&outcode1);
    do{
        if(outcode0.all==0&&outcode1.all==0)
        {
            accept=1;
            done=1;
        }
        else if(outcode0.all&outcode1.all!=0)
            done=1;
        else
        {
            if(outcode0.all!=0)
                outcodeout=&outcode0;
            else
                outcodeout=&outcode1;
            if(outcodeout->left)
            {
                y=y0+(y1-y0)*(rect->xmin-x0)/(x1-x0);
                x=(float)rect->xmin;
            }
            else if(outcodeout->top)
            {
                x=x0+(x1-x0)*(rect->ymax-y0)/(y1-y0);
                y=(float)rect->ymax;
            }
            else if(outcodeout->right)
            {
                y=y0+(y1-y0)*(rect->xmax-x0)/(x1-x0);
                x=(float)rect->xmax;
            }
            else if(outcodeout->bottom)
            {
                x=x0+(x1-x0)*(rect->ymin-x0)/(y1-y0);
                y=(float)rect->ymin;
            }
            if(outcodeout->all==outcode0.all)
            {
                x0=x;
                y0=y;
                compoutcode(x0,y0,rect,&outcode0);
            }
            else
            {
                x1=x;
                y1=y;
                compoutcode(x1,y1,rect,&outcode1);
            }
        }
    }while(! done);
    if(accept)
        line((int)x0,(int)y0,(int)x1,(int)y1);
}
void compoutcode(float x,float y,Rectangle *rect,outcode *outcode)
{
    outcode->all=0;
    outcode->top=outcode->bottom=0;
    if(y>(float)rect->ymax)
    {
        outcode->top=1;
        outcode->all+=1;
    }
    else if(y<(float)rect->ymin)
    {
        outcode->bottom=1;
        outcode->all+=1;
    }
    outcode->right=outcode->left=0;
    if(x>(float)rect->xmax)
    {
        outcode->right=1;
        outcode->all+=1;
    }
    else if(x<(float)rect->xmin)
    {
        outcode->left=1;
        outcode->all+=1;
    }
}

main()
{
int x0,y0,x1,y1;
int i;
int graphdriver=DETECT;
int graphmode=0;
initgraph(&graphdriver,&graphmode," ");
cleardevice();
x0=450;y0=0;x1=0;y1=450;

rect->xmin=100;
rect->xmax=300;
rect->ymin=100;
rect->ymax=300;

  setcolor(2);
  rectangle(rect->xmin,rect->ymin,rect->xmax,rect->ymax);
  line(x0,y0,x1,y1);
  outtextxy(100,400,"press any key to clip!");
  i=getch();
  clearviewport();
  rectangle(rect->xmin,rect->ymin,rect->xmax,rect->ymax);
  cohensutherlandlineclip(x0,y0,x1,y1,rect);
  outtextxy(100,400,"the result of the clip");
  getch();
  closegraph();
}
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