Edge Tracing

InitGraph();
rows=31;cols=47;
ScaleWindow(0, 0,cols+1, rows+1);
Colour(Black);
BoxFill(0,0,cols+1,rows+1);

pic=new Array(cols+1);marked=new Array(cols+1);
for(var i = 0;i<=cols;i++){
   pic[i]=new Array(rows+1);marked[i]=new Array(rows+1);
}

var white = true, black = false;
var yellow = "yellow";
var d, i, j, il, jl, istep, jstep, xold, yold;
var lightest=100, darkest=100, greyline100, fade=1;
var trail=0, straight=0, diff=0, kk=false;
var here=false, beenthere=false;
var thresh = 5; //1024 / cols * 4;

vi=new Array(1, 1, 0,-1,-1,-1, 0, 1);
vj=new Array(0, 1, 1, 1, 0,-1,-1,-1); //directions N, NE etc.

function Spot () {
c = pic[i][j];
ColourRGB(c,c,c);
BoxFill (i - .5, j - .5, 1, 1);
}

function Dot(p, q) {
LineStart(p+.5, q+.5);
}

function Look (x) { //move in direction x and look at new point
var r;
hop (x);
l = pic[i][j];      //value of new point

if (l > greyline) { //compare with mid level
   r=white;
   Colour(Red);
   Dot(i,j);            //red dot if lighter
 }else{
   r=black;
   Colour(Green);
   Dot(i,j);            //green dot if darker
 }

//update lightest and darkest to track extremes,
//fading together slowly

if(l>lightest){lightest=l;}else{lightest=lightest-fade;};
 if(l<darkest) {darkest=l;} else{darkest =darkest +fade;};

greyline = (lightest + darkest) / 2; //mid level

diff = lightest - darkest; //represents local gradient
 
 if(diff>thresh){return r}else{return here;};
 //if too flat return same as last time
}

trail = -4;            //allow turns
}

function turn (x) {
 trail = trail + 1;        //how far since last edge?
 straight = straight + 1;  //how far since last turn?
 if(8 * straight > trail){ //if far enought then
   d = (d + x) & 7;       //turn in direction x
   straight = 0;          //and restart counting from turn
 }
}

//Set up picture with a diamond on shaded ground
m=(rows+cols)/2;
for (i = 0;i<=cols;i++){ 
   for (j = 0; j<=rows;j++) {
      p = Math.abs(i - cols/2);
      q = Math.abs(j - rows/2);
      if ((p+q)<rows/3) {k =100;}else{k=60;};
      if((p<cols*.4)&(q<rows*.4)){k+=40;};
      k = k + 2 * i;
      pic [i][j] = k;
      marked [i][j] = false; 
      c =  k;
      ColourRGB(c, c, c);
      BoxFill(i, j, 1, 1);
   }
}

// Start searching from an arrray of points

for(ii = 10;ii<cols;ii+=20){
  for(jj = 10;jj<rows;jj+=20){
    i = ii;
    j = jj;
    kk = false;
    beenthere = false;
    d = 0;
    here = white;
    lightest = pic[i][j];
    darkest = lightest;
    greyline = (lightest + darkest) / 2;
    diff=0;

while(!beenthere){        //This is the search algorithm
     if(here == white){       //if first point is white
       here2 = Look(d);       //second point in direction d
       if(here2 == here){     //if it's the same, white, then
          turn(1);            //turn clockwise for next move
       }else{                 //otherwise you've crossed a threshold
          notepoint();        //so mark it
       };
     }else{                   //if first point was black,
       here2 = Look(d - 3);   //second point in direction d-3
       if(here2 == here){     //if it's the same, black, then
          turn (-1);          //turn anticlockwise
       }else{                 //otherwise you've crossed a threshold
          notepoint();        //so mark it
       };
     };
    here=here2;
    }                        //keep going until you hit an old
  }                          //marked point
}