polygon clipping (csg) 2d

[noeska]

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:

typedef struct _node 
{ 
  int x, y; 
  struct _node *next; 
  struct _node *prev; 
  struct _node *nextPoly;   /* pointer to the next polygon */ 
  struct _node *neighbor;   /* the coresponding intersection point */ 
  int intersect;            /* 1 if an intersection point, 0 otherwise */ 
  int entry;                /* 1 if an entry point, 0 otherwise */ 
  int visited;              /* 1 if the node has been visited, 0 otherwise */ 
  float alpha;              /* intersection point placemet */ 
} node;

In delphi that could be written as:

type
tnode = class;
pnode = ^tnode;

tnode = class
  x, y: single;
  next: pnode;
  prev: pnode;
  nextPoly: pnode;   // pointer to the next polygon */
  neighbor : pnode;   // the coresponding intersection point */
  intersect: bool;            // 1 if an intersection point, 0 otherwise */
  entry: bool;                // 1 if an entry point, 0 otherwise */
  visited: bool;              // 1 if the node has been visited, 0 otherwise */
  alpha: single;              // intersection point placemet */
end;

Now i would rather use a record. But this does not work:

tnode = record;
pnode = ^tnode;

Now this is initialized as:

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).

void add(Widget w, int which_button, int x, int y, void *data) 
{ 
  node *new; 
  if (!DRAW) return; 

  if (which_button == 1) 
  { 
        new = malloc(sizeof(node)); 
        new->x = x; 
        new->y = y; 
        new->prev = 0;        /* not need to initialize with 0 after malloc ... */ 
        new->nextPoly = 0; 
        new->neighbor = 0; 
        new->intersect = 0; 
        new->entry = 0; 
        new->visited = 0; 
        new->alpha = 0.; 
        if (DRAW == 1) 
        { 
                new->next = s; 
                if (s) s->prev = new; 
                s = new; 
        } 
        else /* DRAW == 2 */ 
        { 
                new->next = c; 
                if (c) c->prev = new; 
                c = new; 
        } 
        redisplay(W[3], X, Y, NULL); 
  } 
  else if (which_button == 3) 
  { 
        DRAW = DRAW==1 ? 2:0; 
        redisplay(W[3], X, Y, NULL); 
  } 
}

What is happinging here:

        if (DRAW == 1) 
        { 
                new->next = s; 
                if (s) s->prev = new; 
                s = new; 
        }

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.

[noeska]

for the time being i wrote it like this:

type
pnode = ^tnode;
tnode = record
  x, y: single;
  next: pnode;
  prev: pnode;
  nextPoly: pnode;   // pointer to the next polygon */
  neighbor : pnode;   // the coresponding intersection point */
  intersect: bool;            // 1 if an intersection point, 0 otherwise */
  entry: bool;                // 1 if an entry point, 0 otherwise */
  visited: bool;              // 1 if the node has been visited, 0 otherwise */
  alpha: single;              // intersection point placemet */
end;

var
  s: pnode;

procedure Add(x,y: single);
var
  newnode : pnode;
begin
        new(newnode);
        newnode.x := x;
        newnode.y := y;
        newnode.prev := nil;        // not need to initialize with 0 after malloc ...
        newnode.nextPoly := nil;
        newnode.neighbor := nil;
        newnode.intersect := false;
        newnode.entry := false;
        newnode.visited := false;
        newnode.alpha := 0.0;

        newnode.next := s;
        if &#40;s<>nil&#41; then
          s.prev &#58;= newnode;
        s &#58;= newnode;
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?