Sem 4‎ > ‎CG LAB‎ > ‎

P8: WAP to perform Line CLipping

posted Jan 25, 2012, 3:03 AM by Neil Mathew   [ updated Nov 3, 2012, 8:41 PM ]

What I learnt:
  • This algo fails where there is a vertical Line ( x=c ). I've made provisions for the Slope to be 1, rather than having the program crash due to division by zero. Even by specifying it as 1, the clipping is All Wrong. I get some strange diagonal within the clipping window when I should be getting a straight vertical line. 

    > Realized that making slope = 1 was a bad decision. It seemed rational at the time, but I see now that zero was needed. When the intersection is made on a Y axis based side, according to the algo, the difference between Y and Yaxis is multiplied by inverse of slope, in this case, zero.
    (Don't calculate Slope separately, write the expression as part of the statement. 
    Problem Solved)

  • This program implements Array Deletion, Tertiary operators and a manipulated Palindrome algo. This is there only because I chose to represent the 4-bit indicators as a long datatype rather than an int array. This was only possible because the 4-bit indicator is of the order 4321.


    Each Point indicated by four bit code: [ 4 3 2 1 ]
    (where the 4,3,2,1 will be either 0 or 1)



Correction:
(Later I realize I'm wrong. I've messed with convention.
What is explained as 4 3 2 1 should have been 1 2 3 4 .
That is, 1 if extends over top edge, 2 if it goes over bottom edge, etc)

SOURCE CODE:


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#include<graphics.h>
#include<iostream.h>
#include<conio.h>
 
int n; //No of Lines
 
int ClipWindow[2][2]; //Clipping Window Coordinates
/* For 2 vertices A and C, each having x and y coordinates*/
 
int Line[20][4];  //Line Coordinates
/* The 20 represents total number of lines possible
 4 represents the 4 coordinates required to draw ONE line */
 
void Draw()
{
clrscr();
 
int gdriver=DETECT,gmode;
initgraph(&gdriver, &gmode, "C:/TC/BGI");
 
setbkcolor(BLACK);
setcolor(WHITE);
for(int i=0; i<n; i++)
line(Line[i][0],Line[i][1],Line[i][2],Line[i][3]);
 
setcolor(CYAN);
rectangle(ClipWindow[0][0],ClipWindow[0][1]
                ,ClipWindow[1][0],ClipWindow[1][1]);
 
outtextxy(10,10, "Press any key to continue");
getch(); closegraph();
}
 
void UnClipped_Input()
{
cout<<"\n Enter the number of lines to draw : ";
cin>>n; cout<<endl;
 
for(int i=0; i<n; i++)
{
cout<<" LINE #"<<(i+1);
cout<<"\n Enter the Initial position (x,y) : ";
cin>>Line[i][0];
cin>>Line[i][1];
cout<<" Enter the Final position (x,y)   : ";
cin>>Line[i][2];
cin>>Line[i][3];
cout<<"\n";
}
cout<<"\n >> Enter the coordinates of the Clipping Window:";
cout<<"\n >> Vertex A (x,y) : ";
cin>>ClipWindow[0][0];
cin>>ClipWindow[0][1];
cout<<" >> Vertex C (x,y) : ";
cin>>ClipWindow[1][0];
cin>>ClipWindow[1][1];
}
long CheckBit(long x,long y)
{
long b=5; /* The 5 here is any random non zero number. 
This ensures 0000 doesn't become 0 */
int Xmax= 
ClipWindow[0][0] > ClipWindow[1][0] 
? ClipWindow[0][0] : ClipWindow[1][0];
 
int Xmin= 
ClipWindow[0][0] < ClipWindow[1][0] 
? ClipWindow[0][0] : ClipWindow[1][0];
int Ymax= 
ClipWindow[0][1] > ClipWindow[1][1] 
? ClipWindow[0][1] : ClipWindow[1][1];
 
int Ymin= 
ClipWindow[0][1] < ClipWindow[1][1] 
? ClipWindow[0][1] : ClipWindow[1][1];
 
//Case 4:
if(x < Xmin)
b=b*10 + 1;
else
b=b*10 + 0;
 
//Case 3:
if(x > Xmax)
b=b*10 + 1;
else
b=b*10 + 0;
 
//Case 2:
if(y < Ymin)
b=b*10 + 1;
else
b=b*10 + 0;
 
//Case 1:
if(y > Ymax)
b=b*10 + 1;
else
b=b*10 + 0;
 
return b;
}
 
long AND(long a, long b)
{
//Using manipulated Palindrome Algo
long c;
int digitA;
int digitB;
int digitC[4];
 
for(int i=3; i>=0; i--)
{
digitA=a%10;
digitB=b%10;
 
if( digitA==1 && digitB==1)
digitC[i]=1;
else
digitC[i]=0;
 
a/=10;
b/=10;
}
c=5;
for(i=0; i<4; i++)
c=(long) c*10+digitC[i];
 
return c;
}                       //END of AND function
 
void Clip()
{
int Xmax= 
ClipWindow[0][0] > ClipWindow[1][0] 
? ClipWindow[0][0] : ClipWindow[1][0];
 
int Xmin= 
ClipWindow[0][0] < ClipWindow[1][0] 
? ClipWindow[0][0] : ClipWindow[1][0];
 
int Ymax= 
ClipWindow[0][1] > ClipWindow[1][1] 
? ClipWindow[0][1] : ClipWindow[1][1];
 
int Ymin= 
ClipWindow[0][1] < ClipWindow[1][1] 
? ClipWindow[0][1] : ClipWindow[1][1];
 
long b1, b2, b1ANDb2;
int i,j,k;
int digit_b1, digit_b2;
double Slope;
 
for(i=0; i<n; i++)
{  //FOR EACH LINE:
 
b1=CheckBit(Line[i][0], Line[i][1]);
b2=CheckBit(Line[i][2], Line[i][3]);
b1ANDb2=AND(b1,b2);
 
if( b1ANDb2 != 50000 ) //OUTSIDE
{
//Remove OUTSIDE Line Coordinates from Array
 
for(int k=i; k<n-1; k++)
{
Line[k][0]=Line[k+1][0];
Line[k][1]=Line[k+1][1];
Line[k][2]=Line[k+1][2];
Line[k][3]=Line[k+1][3];
}
n--;
i--;
 
}
 
else  //INSIDE
{
 
        //Completely Inside
        if( b1==50000 && b2==50000 )
        {
        //No Change to be made to Line Coordinates.
        }
 
 
        //Partially Inside Cases
        else if( b1!=50000 || b2!=50000 )
        {
 
        if( (Line[i][2] - Line[i][0]) == 0)
        Slope= 1.0;
        else
        Slope= 
        (double) ((Line[i][3]-Line[i][1])/(Line[i][2]-Line[i][0]));
 
 
         for(j=1; j<=4; j++)
         {
         digit_b1=b1%10;
         digit_b2=b2%10;
 
         switch(j)
         {
 
         case 1:
 
         if(digit_b1==1)
         {
         Line[i][0]= Line[i][0] + ( (Ymax - Line[i][1]) / Slope );
         Line[i][1]=Ymax;
         }
         else if(digit_b2==1)
         {
         Line[i][2]= Line[i][2] + ( (Ymax - Line[i][3] ) / Slope );
         Line[i][3]=Ymax;
         }
         break;
 
         case 2:
 
         if(digit_b1==1)
         {
         Line[i][0]= Line[i][0] + ( (Ymin - Line[i][1] ) / Slope );
         Line[i][1]=Ymin;
         }
         else if(digit_b2==1)
         {
         Line[i][2]= Line[i][2] + ( (Ymin - Line[i][3] ) / Slope );
         Line[i][3]=Ymin;
         }
         break;
 
         case 3:
 
         if(digit_b1==1)
         {
         Line[i][1]= Line[i][1] + ( ( Xmax - Line[i][0] ) * Slope) ;
         Line[i][0]=Xmax;
         }
         else if(digit_b2==1)
         {
         Line[i][3]= Line[i][3] + ( (Xmax - Line[i][2] ) * Slope) ;
         Line[i][2]=Xmax;
         }
         break;
 
         case 4:
 
         if(digit_b1==1)
         {
         Line[i][1]= Line[i][1] + ( (Xmin - Line[i][0] ) * Slope) ;
         Line[i][0]=Xmin;
         }
         else if(digit_b2==1)
         {
         Line[i][3]= Line[i][3] + ( (Xmin - Line[i][2] ) * Slope) ;
         Line[i][2]=Xmin;
         }
         break;
 
         } //End of Case
 
 
         b1/=10;
         b2/=10;
 
      } //End of inner For Loop
 
 
      b1=CheckBit(Line[i][0],Line[i][1]);
      b2=CheckBit(Line[i][2],Line[i][3]);
      b1ANDb2=AND(b1,b2);
 
 
      if( b1!=50000 || b2!=50000 )
      { i--; continue; }
 
 
} //End of Inner If-Else-if
} //End of Outer If-Else-if
} //End of Outer For Loop
}
 
 
void main()
{
clrscr();
 
UnClipped_Input();
Draw();
 
Clip();
Draw();
}


OUTPUT:







ċ
Testing_CLIPPER.txt
(7k)
Neil Mathew,
Jan 25, 2012, 3:19 AM
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