iOS Core Graphics: Draw ONLY shadows of a CGPath

Question

I am drawing a simple path in iOS 5 with Core Graphics:

CGMutablePathRef path = CGPathCreateMutable();
CGPathMoveToPoint(   path, NULL, center.x   , topMargi         


        

Answer 1

I suggest you set the context's clipping path to the inverse of the shape's path, configure the shadow, and fill the shape normally, with full opacity. The clipping path will mask out the fill color, and only the shadow would remain.

CGContextSaveGState(context);
CGRect boundingRect = CGContextGetClipBoundingBox(context);
CGContextAddRect(context, boundingRect);
CGContextAddPath(context, path);
CGContextEOClip(context);

[[UIColor blackColor] setFill];
CGContextAddPath(context, path);
CGContextSetShadowWithColor(context, CGSizeMake(1, 1), 1.0, highlightColor);
CGContextSetBlendMode (context, kCGBlendModeNormal);
CGContextFillPath(context);

CGContextAddPath(context, path);
CGContextSetShadowWithColor(context, CGSizeMake(-1, -1), 1.0, shadowColor);
CGContextSetBlendMode (context, kCGBlendModeNormal);
CGContextFillPath(context);

CGContextRestoreGState(context);

The trick is using CGContextEOClip and an additional rectangle subpath to set the clipping area to whatever is not covered by the original path. This will work for any path that is not self-intersecting.