剪切线算法

int Clip_Line(int &x1,int &y1,int &x2, int &y2)

{

// this function clips the sent line using the globally defined clipping

// region

 

// internal clipping codes

#define CLIP_CODE_C  0x0000

#define CLIP_CODE_N  0x0008

#define CLIP_CODE_S  0x0004

#define CLIP_CODE_E  0x0002

#define CLIP_CODE_W  0x0001

 

#define CLIP_CODE_NE 0x000a

#define CLIP_CODE_SE 0x0006

#define CLIP_CODE_NW 0x0009 

#define CLIP_CODE_SW 0x0005

 

int xc1=x1, 

    yc1=y1, 

xc2=x2, 

yc2=y2;

 

int p1_code=0, 

    p2_code=0;

 

// determine codes for p1 and p2

if (y1 < min_clip_y)

p1_code|=CLIP_CODE_N;

else

if (y1 > max_clip_y)

p1_code|=CLIP_CODE_S;

 

if (x1 < min_clip_x)

p1_code|=CLIP_CODE_W;

else

if (x1 > max_clip_x)

p1_code|=CLIP_CODE_E;

 

if (y2 < min_clip_y)

p2_code|=CLIP_CODE_N;

else

if (y2 > max_clip_y)

p2_code|=CLIP_CODE_S;

 

if (x2 < min_clip_x)

p2_code|=CLIP_CODE_W;

else

if (x2 > max_clip_x)

p2_code|=CLIP_CODE_E;

 

// try and trivially reject

if ((p1_code & p2_code)) 

return(0);

 

// test for totally visible, if so leave points untouched

if (p1_code==0 && p2_code==0)

return(1);

 

// determine end clip point for p1

switch(p1_code)

 {

 case CLIP_CODE_C: break;

 

 case CLIP_CODE_N:

  {

  yc1 = min_clip_y;

  xc1 = x1 + 0.5+(min_clip_y-y1)*(x2-x1)/(y2-y1);

  } break;

 case CLIP_CODE_S:

  {

  yc1 = max_clip_y;

  xc1 = x1 + 0.5+(max_clip_y-y1)*(x2-x1)/(y2-y1);

  } break;

 

 case CLIP_CODE_W:

  {

  xc1 = min_clip_x;

  yc1 = y1 + 0.5+(min_clip_x-x1)*(y2-y1)/(x2-x1);

  } break;

 case CLIP_CODE_E:

  {

  xc1 = max_clip_x;

  yc1 = y1 + 0.5+(max_clip_x-x1)*(y2-y1)/(x2-x1);

  } break;

 

// these cases are more complex, must compute 2 intersections

 case CLIP_CODE_NE:

  {

  // north hline intersection

  yc1 = min_clip_y;

  xc1 = x1 + 0.5+(min_clip_y-y1)*(x2-x1)/(y2-y1);

 

  // test if intersection is valid, of so then done, else compute next

if (xc1 < min_clip_x || xc1 > max_clip_x)

{

// east vline intersection

xc1 = max_clip_x;

yc1 = y1 + 0.5+(max_clip_x-x1)*(y2-y1)/(x2-x1);

} // end if

 

  } break;

 

 case CLIP_CODE_SE:

         {

  // south hline intersection

  yc1 = max_clip_y;

  xc1 = x1 + 0.5+(max_clip_y-y1)*(x2-x1)/(y2-y1);

 

  // test if intersection is valid, of so then done, else compute next

  if (xc1 < min_clip_x || xc1 > max_clip_x)

     {

 // east vline intersection

 xc1 = max_clip_x;

 yc1 = y1 + 0.5+(max_clip_x-x1)*(y2-y1)/(x2-x1);

 } // end if

 

  } break;

   

 case CLIP_CODE_NW: 

         {

  // north hline intersection

  yc1 = min_clip_y;

  xc1 = x1 + 0.5+(min_clip_y-y1)*(x2-x1)/(y2-y1);

  

  // test if intersection is valid, of so then done, else compute next

  if (xc1 < min_clip_x || xc1 > max_clip_x)

     {

 xc1 = min_clip_x;

     yc1 = y1 + 0.5+(min_clip_x-x1)*(y2-y1)/(x2-x1);

 } // end if

 

  } break;

   

 case CLIP_CODE_SW:

  {

  // south hline intersection

  yc1 = max_clip_y;

  xc1 = x1 + 0.5+(max_clip_y-y1)*(x2-x1)/(y2-y1);

  

  // test if intersection is valid, of so then done, else compute next

  if (xc1 < min_clip_x || xc1 > max_clip_x)

     {

 xc1 = min_clip_x;

     yc1 = y1 + 0.5+(min_clip_x-x1)*(y2-y1)/(x2-x1);

 } // end if

 

  } break;

 

 default:break;

 

 } // end switch

 

// determine clip point for p2

switch(p2_code)

 {

 case CLIP_CODE_C: break;

 

 case CLIP_CODE_N:

  {

  yc2 = min_clip_y;

  xc2 = x2 + (min_clip_y-y2)*(x1-x2)/(y1-y2);

  } break;

 

 case CLIP_CODE_S:

  {

  yc2 = max_clip_y;

  xc2 = x2 + (max_clip_y-y2)*(x1-x2)/(y1-y2);

  } break;

 

 case CLIP_CODE_W:

  {

  xc2 = min_clip_x;

  yc2 = y2 + (min_clip_x-x2)*(y1-y2)/(x1-x2);

  } break;

 case CLIP_CODE_E:

  {

  xc2 = max_clip_x;

  yc2 = y2 + (max_clip_x-x2)*(y1-y2)/(x1-x2);

  } break;

 

// these cases are more complex, must compute 2 intersections

 case CLIP_CODE_NE:

  {

  // north hline intersection

  yc2 = min_clip_y;

  xc2 = x2 + 0.5+(min_clip_y-y2)*(x1-x2)/(y1-y2);

 

  // test if intersection is valid, of so then done, else compute next

if (xc2 < min_clip_x || xc2 > max_clip_x)

{

// east vline intersection

xc2 = max_clip_x;

yc2 = y2 + 0.5+(max_clip_x-x2)*(y1-y2)/(x1-x2);

} // end if

 

  } break;

 

 case CLIP_CODE_SE:

         {

  // south hline intersection

  yc2 = max_clip_y;

  xc2 = x2 + 0.5+(max_clip_y-y2)*(x1-x2)/(y1-y2);

 

  // test if intersection is valid, of so then done, else compute next

  if (xc2 < min_clip_x || xc2 > max_clip_x)

     {

 // east vline intersection

 xc2 = max_clip_x;

 yc2 = y2 + 0.5+(max_clip_x-x2)*(y1-y2)/(x1-x2);

 } // end if

 

  } break;

   

 case CLIP_CODE_NW: 

         {

  // north hline intersection

  yc2 = min_clip_y;

  xc2 = x2 + 0.5+(min_clip_y-y2)*(x1-x2)/(y1-y2);

  

  // test if intersection is valid, of so then done, else compute next

  if (xc2 < min_clip_x || xc2 > max_clip_x)

     {

 xc2 = min_clip_x;

     yc2 = y2 + 0.5+(min_clip_x-x2)*(y1-y2)/(x1-x2);

 } // end if

 

  } break;

   

 case CLIP_CODE_SW:

  {

  // south hline intersection

  yc2 = max_clip_y;

  xc2 = x2 + 0.5+(max_clip_y-y2)*(x1-x2)/(y1-y2);

  

  // test if intersection is valid, of so then done, else compute next

  if (xc2 < min_clip_x || xc2 > max_clip_x)

     {

 xc2 = min_clip_x;

     yc2 = y2 + 0.5+(min_clip_x-x2)*(y1-y2)/(x1-x2);

 } // end if

 

  } break;

 default:break;

 

 } // end switch

 

// do bounds check

if ((xc1 < min_clip_x) || (xc1 > max_clip_x) ||

(yc1 < min_clip_y) || (yc1 > max_clip_y) ||

(xc2 < min_clip_x) || (xc2 > max_clip_x) ||

(yc2 < min_clip_y) || (yc2 > max_clip_y) )

{

return(0);

} // end if

 

// store vars back

x1 = xc1;

y1 = yc1;

x2 = xc2;

y2 = yc2;

 

return(1);

 

} // end Clip_Line