Graphical intersection of lines in C

Question

I have a homework.

The homework is: There are 3 lines, theirs end with squares. Firstly The program has to view a circle is the lines are cut each other. (In other way: Take the intersection of the 3 lines). Secondly, the program has to change the background, along the lines. Each line's both side define the background with a colour. And how rotating the lines, together with them, the background colour changing. There are 3 lines, and 6 background colour. The border of background color is along the lines.

The programming enviroment is the DevC++ (we must use the c++ console applicaton, but in the lesson we not coding in c++, just c...)

Youtube video about the exercise/homework

I've tried implement of the lines' intersection, but It doesn't work very well. And I don't have any idea, how can I implement of the colourful background change.

What kind of knowledge is needed for it?

I would like to if somebody can suggest to me some: algorithm, webpage, tutorial, sourecode, anything what can help me. Or what is the name of my homework in english ( to google search) Cause I don't think, my solution is the best way to prepare my homework (maybe it won't succes)

Here the code, that I have did up to now (but it not perfect. The intersection of lines is not perfectly. It's not a beautiful solution, sorry I am not an expert C programmer):

sourcode in english

• PONT = point, dot
• PONTH = aggregation of points
• atir = rewrite
• metszilleszt = fitting of intersection
• szakasz = section, phase... (there is too many in english-hungarian) or platoon :-D
• eger = mouse
• egérkezelés = mouse control

• balgomb = left button of mouse

  # include "graphics.h"
  # include <conio.h>
  #include <stdio.h>
  typedef struct {
          float x1,x2,x3;
  } PONTH;
  
  typedef struct {
          double x,y;
  }PONT;
  
  PONTH atir(PONT A){
        PONTH C;
        C.x1=A.x;
        C.x2=A.y;
        C.x3=1;
  return C;
  }
  
  PONTH metszilleszt(PONTH A,PONTH B){
        PONTH C;
        C.x1=(A.x2*B.x3)-(A.x3*B.x2);
        C.x2=-(A.x1*B.x3)+(A.x3*B.x1);
        C.x3=(A.x1*B.x2)-(A.x2*B.x1);
     return C;
  }
  int main()
  {
  //PONT szakasz[4]={100,50,300,200,30,130,140,170};
  PONT szakasz[6]={100,50,300,200,30,130,140,170,30,70,210,40};
  
  int ap;
  int gd,gm;
  int page =0;
  gd=VGA;gm=VGAMED;
  initgraph(&gd,&gm,"");
  PONTH A,B,C,D,E,F;
  PONTH tmp1,tmp2,tmp3,tmp4,tmp5,tmp6;
  PONT pont;
  
  for(;;){
   setactivepage(page);
   cleardevice();
  
   A=atir(szakasz[0]);
   B=atir(szakasz[1]);
   C=atir(szakasz[2]);
   D=atir(szakasz[3]);
   E=atir(szakasz[4]);
   F=atir(szakasz[5]);
  
   tmp1=metszilleszt(A,B);
   tmp2=metszilleszt(C,D);
   tmp3=metszilleszt(E,F);
  
   tmp4=metszilleszt(tmp2,tmp1);
  
   tmp5=metszilleszt(tmp3,tmp1);
  
   tmp6=metszilleszt(tmp3,tmp2);
  
   pont.x=int (tmp3.x1/tmp3.x3);
   pont.y=int (tmp3.x2/tmp3.x3);
   //printf("%f %f\n",pont.x,pont.y);
  
  
  // good
   if((((tmp4.x2/tmp4.x3)>=szakasz[0].y) && ((tmp4.x2/tmp4.x3)<=szakasz[1].y)) &&
     (((tmp4.x1/tmp4.x3)>=szakasz[0].x) && ((tmp4.x1/tmp4.x3)<=szakasz[1].x)) ||
     (((tmp4.x2/tmp4.x3)>=szakasz[0].y) && ((tmp4.x2/tmp4.x3)<=szakasz[1].y)) &&
     (((tmp4.x1/tmp4.x3)<=szakasz[0].x) && ((tmp4.x1/tmp4.x3)>=szakasz[1].x)))
     {
      setcolor(RED);
      fillellipse(int (tmp4.x1/tmp4.x3),int (tmp4.x2/tmp4.x3),5,5); 
     }
  
      if((((tmp5.x2/tmp5.x3)>=szakasz[0].y) && ((tmp5.x2/tmp5.x3)<=szakasz[1].y)) &&
     (((tmp5.x1/tmp5.x3)>=szakasz[0].x) && ((tmp5.x1/tmp5.x3)<=szakasz[1].x)) ||
     (((tmp5.x2/tmp5.x3)>=szakasz[0].y) && ((tmp5.x2/tmp5.x3)<=szakasz[1].y)) &&
     (((tmp5.x1/tmp5.x3)<=szakasz[0].x) && ((tmp5.x1/tmp5.x3)>=szakasz[1].x)))
     {
      setcolor(RED);
      //fillellipse(int (tmp5.x1/tmp5.x3),int (tmp5.x2/tmp5.x3),5,5); 
      fillellipse(int (tmp5.x1/tmp5.x3),int (tmp5.x2/tmp5.x3),5,5); 
     }
  
      if((((tmp6.x2/tmp6.x3)>=szakasz[0].y) && ((tmp6.x2/tmp6.x3)<=szakasz[1].y)) &&
     (((tmp6.x1/tmp6.x3)>=szakasz[0].x) && ((tmp6.x1/tmp6.x3)<=szakasz[1].x)) ||
     (((tmp6.x2/tmp6.x3)>=szakasz[0].y) && ((tmp6.x2/tmp6.x3)<=szakasz[1].y)) &&
     (((tmp6.x1/tmp6.x3)<=szakasz[0].x) && ((tmp6.x1/tmp6.x3)>=szakasz[1].x)))
     {
      setcolor(RED);
      fillellipse(int (tmp6.x1/tmp6.x3),int (tmp6.x2/tmp6.x3),5,5); 
     }
  
     //else{ setcolor(RED);
    // fillellipse(int (tmp3.x1/tmp3.x3),int (tmp3.x2/tmp3.x3),5,5); }
  
   /* Egerkezeles */
    if (!balgomb) ap = getactivepoint((pont2d*)szakasz,6,6);
     if (ap >= 0 && balgomb)
     {
      szakasz[ap].x = egerx;
      szakasz[ap].y = egery;
     }
   /* Egerkezeles vege */
   setcolor(WHITE);
    line((int)szakasz[0].x,(int)szakasz[0].y,(int)szakasz[1].x,(int)szakasz[1].y);
    rectangle((int)szakasz[0].x,(int)szakasz[0].y, (int)szakasz[0].x+4, (int)szakasz[0].y+4);
    rectangle((int)szakasz[1].x,(int)szakasz[1].y, (int)szakasz[1].x+4, (int)szakasz[1].y+4);
    line((int)szakasz[2].x,(int)szakasz[2].y,(int)szakasz[3].x,(int)szakasz[3].y);
    rectangle((int)szakasz[2].x,(int)szakasz[2].y, (int)szakasz[2].x+4, (int)szakasz[2].y+4);
    rectangle((int)szakasz[3].x,(int)szakasz[3].y, (int)szakasz[3].x+4, (int)szakasz[3].y+4);
    line((int)szakasz[4].x,(int)szakasz[4].y,(int)szakasz[5].x,(int)szakasz[5].y);
    rectangle((int)szakasz[4].x,(int)szakasz[4].y, (int)szakasz[4].x+4, (int)szakasz[4].y+4);
    rectangle((int)szakasz[5].x,(int)szakasz[5].y, (int)szakasz[5].x+4, (int)szakasz[5].y+4);
  
   setvisualpage(page);
   page = 1-page;
   if (kbhit()) break;
  }
   getch();
   closegraph();
   return(0);
  }
  

Answer 1

Regarding the intersections of lines, you should read the article of Loren Shure and Lucio Cetto from Matworks on Loren's blog:

• Part 1
• Part 2

It's in Matlab but the principles are the same.