DGL
https://delphigl.com/forum/

polygon clipping (csg) 2d
https://delphigl.com/forum/viewtopic.php?f=19&t=7448		 Seite 1 von 1
Autor:  noeska [ So Apr 20, 2008 20:41 ]
Betreff des Beitrags:  polygon clipping (csg) 2d
Delphi 2005 pro
C conversion
Polygon
Clipping
CSG
2D

Hello,

2D gives more chalenges then i expected.
Now i am trying to merge 2 polygon shapes to a new one.
Doing a search on the inet i found the following: http://davis.wpi.edu/~matt/courses/clipping/ That looks doable. But looking at the c source i get somewhat confused again as:

Code:
  1.  
  2. typedef struct _node
  3. {
  4.   int x, y;
  5.   struct _node *next;
  6.   struct _node *prev;
  7.   struct _node *nextPoly;   /* pointer to the next polygon */
  8.   struct _node *neighbor;   /* the coresponding intersection point */
  9.   int intersect;            /* 1 if an intersection point, 0 otherwise */
  10.   int entry;                /* 1 if an entry point, 0 otherwise */
  11.   int visited;              /* 1 if the node has been visited, 0 otherwise */
  12.   float alpha;              /* intersection point placemet */
  13. } node;
  14.  

In delphi that could be written as:
Code:
  1.  
  2. type
  3. tnode = class;
  4. pnode = ^tnode;
  5.  
  6. tnode = class
  7.   x, y: single;
  8.   next: pnode;
  9.   prev: pnode;
  10.   nextPoly: pnode;   // pointer to the next polygon */
  11.   neighbor : pnode;   // the coresponding intersection point */
  12.   intersect: bool;            // 1 if an intersection point, 0 otherwise */
  13.   entry: bool;                // 1 if an entry point, 0 otherwise */
  14.   visited: bool;              // 1 if the node has been visited, 0 otherwise */
  15.   alpha: single;              // intersection point placemet */
  16. end;
  17.  

Now i would rather use a record. But this does not work:
Code:
  1.  
  2. tnode = record;
  3. pnode = ^tnode;
  4.  


Now this is initialized as:
Code:
  1. node *s=0, *c=0, *root=0;

What is happening here assing 0 to record? I gues i have just to create my object and/or record here.
The following also confuses me ( i think i either adds point for polygon a or for polygon b).

Code:
  1.  
  2. void add(Widget w, int which_button, int x, int y, void *data)
  3. {
  4.   node *new;
  5.   if (!DRAW) return;
  6.  
  7.   if (which_button == 1)
  8.   {
  9.         new = malloc(sizeof(node));
  10.         new->x = x;
  11.         new->y = y;
  12.         new->prev = 0;        /* not need to initialize with 0 after malloc ... */
  13.         new->nextPoly = 0;
  14.         new->neighbor = 0;
  15.         new->intersect = 0;
  16.         new->entry = 0;
  17.         new->visited = 0;
  18.         new->alpha = 0.;
  19.         if (DRAW == 1)
  20.         {
  21.                 new->next = s;
  22.                 if (s) s->prev = new;
  23.                 s = new;
  24.         }
  25.         else /* DRAW == 2 */
  26.         {
  27.                 new->next = c;
  28.                 if (c) c->prev = new;
  29.                 c = new;
  30.         }
  31.         redisplay(W[3], X, Y, NULL);
  32.   }
  33.   else if (which_button == 3)
  34.   {
  35.         DRAW = DRAW==1 ? 2:0;
  36.         redisplay(W[3], X, Y, NULL);
  37.   }
  38. }

What is happinging here:
Code:
  1.  
  2.         if (DRAW == 1)
  3.         {
  4.                 new->next = s;
  5.                 if (s) s->prev = new;
  6.                 s = new;
  7.         }

the s that was created empty before is assigned to next.
now the prev of s is compared to new that is the point that is currently added? Or is s compared to prev of s and if that is the case it is made the new value? aarghh....
finaly the new is made the new s. I am getting confused here.
Thank for your help in advance.
Autor:  noeska [ So Apr 20, 2008 21:09 ]
Betreff des Beitrags: 
for the time being i wrote it like this:
Code:
  1.  
  2. type
  3. pnode = ^tnode;
  4. tnode = record
  5.   x, y: single;
  6.   next: pnode;
  7.   prev: pnode;
  8.   nextPoly: pnode;   // pointer to the next polygon */
  9.   neighbor : pnode;   // the coresponding intersection point */
  10.   intersect: bool;            // 1 if an intersection point, 0 otherwise */
  11.   entry: bool;                // 1 if an entry point, 0 otherwise */
  12.   visited: bool;              // 1 if the node has been visited, 0 otherwise */
  13.   alpha: single;              // intersection point placemet */
  14. end;
  15.  
  16. var
  17.   s: pnode;
  18.  
  19. procedure Add(x,y: single);
  20. var
  21.   newnode : pnode;
  22. begin
  23.         new(newnode);
  24.         newnode.x := x;
  25.         newnode.y := y;
  26.         newnode.prev := nil;        // not need to initialize with 0 after malloc ...
  27.         newnode.nextPoly := nil;
  28.         newnode.neighbor := nil;
  29.         newnode.intersect := false;
  30.         newnode.entry := false;
  31.         newnode.visited := false;
  32.         newnode.alpha := 0.0;
  33.  
  34.         newnode.next := s;
  35.         if (s<>nil) then
  36.           s.prev := newnode;
  37.         s := newnode;
  38. end;

Only to me it seems that prev and next should be named the oposite. e.g. prev should be next and next should be prev?
Autor:  Stucuk [ Mo Apr 21, 2008 13:12 ]
Betreff des Beitrags: 
From the looks of things they should be flipped(Next <-> Prev).
Seite 1 von 1 Alle Zeiten sind UTC + 1 Stunde
Powered by phpBB® Forum Software © phpBB Group
https://www.phpbb.com/