How to draw a blurred shape? - cocoa

Question

How do I draw a blurred shape in Cocoa? Think of a shadow with a blurRadius accompanying a filled path, but without sharp-edged foreground path shape.
What I tried is using a filled path with a shadow, and setting the fill color to transparent (alpha 0.0). But that makes the shadow invisible as well, as it is apparently taking the shadow casting "object's" alpha into account.

Answer 1

This is actually reasonably tricky. I struggled with this for a while until I came up with this category on NSShadow:
#implementation NSShadow (Extras)
//draw a shadow using a bezier path but do not draw the bezier path
- (void)drawUsingBezierPath:(NSBezierPath*) path alpha:(CGFloat) alpha
{
[NSGraphicsContext saveGraphicsState];
//get the bounds of the path
NSRect bounds = [path bounds];
//create a rectangle that outsets the size of the path bounds by the blur radius amount
CGFloat blurRadius = [self shadowBlurRadius];
NSRect shadowBounds = NSInsetRect(bounds, -blurRadius, -blurRadius);
//create an image to hold the shadow
NSImage* shadowImage = [[NSImage alloc] initWithSize:shadowBounds.size];
//make a copy of the shadow and set its offset so that when the path is drawn, the shadow is drawn in the middle of the image
NSShadow* aShadow = [self copy];
[aShadow setShadowOffset:NSMakeSize(0, -NSHeight(shadowBounds))];
//lock focus on the image
[shadowImage lockFocus];
//we want to draw the path directly above the shadow image and offset the shadow so it is drawn in the image rect
//to do this we must translate the drawing into the correct location
NSAffineTransform* transform=[NSAffineTransform transform];
//first get it to the zero point
[transform translateXBy:-shadowBounds.origin.x yBy:-shadowBounds.origin.y];
//now translate it by the height of the image so that it draws outside the image bounds
[transform translateXBy:0.0 yBy:NSHeight(shadowBounds)];
NSBezierPath* translatedPath = [transform transformBezierPath:path];
//apply the shadow
[aShadow set];
//fill the path with an arbitrary black color
[[NSColor blackColor] set];
[translatedPath fill];
[aShadow release];
[shadowImage unlockFocus];
//draw the image at the correct location relative to the original path
NSPoint imageOrigin = bounds.origin;
imageOrigin.x = (imageOrigin.x - blurRadius) + [self shadowOffset].width;
imageOrigin.y = (imageOrigin.y - blurRadius) - [self shadowOffset].height;
[shadowImage drawAtPoint:imageOrigin fromRect:NSZeroRect operation:NSCompositeSourceOver fraction:alpha];
[shadowImage release];
[NSGraphicsContext restoreGraphicsState];
}
#end

Answer 2

I found one implementation which does exactly what I meant online. Written by Sean Patrick O'Brien. It is a category on NSBezierPath, like so:
#interface NSBezierPath (MCAdditions)
- (void)drawBlurWithColor:(NSColor *)color radius:(CGFloat)radius;
#end
#implementation NSBezierPath (MCAdditions)
- (void)drawBlurWithColor:(NSColor *)color radius:(CGFloat)radius
{
NSRect bounds = NSInsetRect(self.bounds, -radius, -radius);
NSShadow *shadow = [[NSShadow alloc] init];
shadow.shadowOffset = NSMakeSize(0, bounds.size.height);
shadow.shadowBlurRadius = radius;
shadow.shadowColor = color;
NSBezierPath *path = [self copy];
NSAffineTransform *transform = [NSAffineTransform transform];
if ([[NSGraphicsContext currentContext] isFlipped])
[transform translateXBy:0 yBy:bounds.size.height];
else
[transform translateXBy:0 yBy:-bounds.size.height];
[path transformUsingAffineTransform:transform];
[NSGraphicsContext saveGraphicsState];
[shadow set];
[[NSColor blackColor] set];
NSRectClip(bounds);
[path fill];
[NSGraphicsContext restoreGraphicsState];
[path release];
[shadow release];
}
#end
This code is downloadable from this blog post. I didn't find it separately as code anywhere online.