compare coords and get octant...

octant.js

var x = 13
var y = 12;
var centerX = 10;
var centerY = 10;


function getOctant( x, y ) {
     x-=centerX;
     y -= centerY;
    var dir = 0;
    if( x < 0 ) {
      if( y < 0 ) {
	dir = 4;
	      if( -x < -y ) dir += 1;
     }else {
	dir = 2;
	      if( -x > y ) dir += 1;
    }
} else 
      if( y < 0 ) {
	dir = 6;
	      if( x > -y ) dir += 1;
	}
      else{
	    dir = 0;
	      if( y > x ) dir += 1;
	}
    
	return dir;    
}

console.log( getOctant( 17, 15 ) );
console.log( getOctant( 12, 25 ) );
console.log( getOctant( 8, 25 ) );
console.log( getOctant( 3, 15 ) );

console.log( getOctant( 5, 8 ) );
console.log( getOctant( 5, 2 ) );
console.log( getOctant( 15, 2 ) );
console.log( getOctant( 15, 8 ) );