What is wrong with my coffeescript translation of this JS? - coffeescript

Question

Trying to improve my Coffeescript skills, so thought I'd covert this jcrop demo. However it's not working as expect. Specifically, the redraw function does not appear to be being called.
This works fine.
// Create a new Selection object extended from Selection
var CircleSel = function(){ };
// Set the custom selection's prototype object to be an instance
// of the built-in Selection object
CircleSel.prototype = new $.Jcrop.component.Selection();
// Then we can continue extending it
$.extend(CircleSel.prototype,{
zoomscale: 1,
attach: function(){
this.frame.css({
background: 'url(' + $('#target')[0].src.replace('750','750') + ')'
});
},
positionBg: function(b){
var midx = ( b.x + b.x2 ) / 2;
var midy = ( b.y + b.y2 ) / 2;
var ox = (-midx*this.zoomscale)+(b.w/2);
var oy = (-midy*this.zoomscale)+(b.h/2);
//this.frame.css({ backgroundPosition: ox+'px '+oy+'px' });
this.frame.css({ backgroundPosition: -(b.x+1)+'px '+(-b.y-1)+'px' });
},
redraw: function(b){
// Call original update() method first, with arguments
$.Jcrop.component.Selection.prototype.redraw.call(this,b);
this.positionBg(this.last);
return this;
},
prototype: $.Jcrop.component.Selection.prototype
});
But when I try to write this in Coffescript it fails
CircleSel.prototype = new ($.Jcrop.component.Selection)
$.extend CircleSel.prototype,
zoomscale: 1
attach: ->
#frame.css background: 'url(' + $('#target')[0].src.replace('750', '750') + ')'
return
positionBg: (b) ->
midx = (b.x + b.x2) / 2
midy = (b.y + b.y2) / 2
ox = -midx * #zoomscale + b.w / 2
oy = -midy * #zoomscale + b.h / 2
#frame.css backgroundPosition: -(b.x + 1) + 'px ' + -b.y - 1 + 'px'
return
# this redraw function is not being called, everything else appears to work fine
redraw: (b) ->
$.Jcrop.component.Selection::redraw.call this, b
#positionBg #last
this
prototype: $.Jcrop.component.Selection.prototype
What have I done wrong?