How to make a C4 object glow, using shadows

C4WorkSpace.m

//
//  C4WorkSpace.m
//  glow
//
//  Created by Travis Kirton on 12-04-04.
//  Copyright (c) 2012 POSTFL. All rights reserved.
//

#import "C4WorkSpace.h"

@implementation C4WorkSpace

-(void)setup {
    C4Shape *bg = [C4Shape rect:self.canvas.frame];
    bg.lineWidth = 0.0f;
    bg.fillColor = [UIColor blackColor];
    [self.canvas addShape:bg];
    
    C4Shape *ellipse = [C4Shape ellipse:CGRectMake(284, 412, 200, 200)];
    ellipse.lineWidth = 0.0f;
    ellipse.shadowOffset = CGSizeZero;
    ellipse.shadowColor = [UIColor yellowColor];
    ellipse.shadowOpacity = 0.8;
    ellipse.shadowRadius = 55.0f;
    [self.canvas addShape:ellipse];
}

@end