iOS Core Graphics: Draw ONLY shadows of a CGPath - objective-c

Question

I am drawing a simple path in iOS 5 with Core Graphics:
CGMutablePathRef path = CGPathCreateMutable();
CGPathMoveToPoint( path, NULL, center.x , topMargin );
CGPathAddLineToPoint(path, NULL, center.x+20, topMargin+50);
CGPathAddLineToPoint(path, NULL, center.x , topMargin+40);
CGPathAddLineToPoint(path, NULL, center.x-20, topMargin+50);
CGPathAddLineToPoint(path, NULL, center.x , topMargin );
Now i want to fill it in Overlay mode like so:
[[UIColor colorWithRed:0 green:0 blue:0 alpha:0.4] setFill];
CGContextAddPath(context, path);
CGContextSetBlendMode (context, kCGBlendModeOverlay);
CGContextFillPath(context);
Which gives me exactly the expected result. But next, i want to create an embossed effect. I thought of using a white and a black drop shadow in order to achieve this effect like so:
[[UIColor colorWithRed:0 green:0 blue:0 alpha:0] setFill];
CGContextAddPath(context, path);
CGContextSetShadowWithColor(context, CGSizeMake(1, 1), 1.0, highlightColor);
CGContextSetBlendMode (context, kCGBlendModeNormal);
CGContextFillPath(context);
[[UIColor colorWithRed:0 green:0 blue:0 alpha:0] setFill];
CGContextAddPath(context, path);
CGContextSetShadowWithColor(context, CGSizeMake(-1, -1), 1.0, shadowColor);
CGContextSetBlendMode (context, kCGBlendModeNormal);
CGContextFillPath(context);
The problem is, the shadows are not drawn when alpha is set to 0.
Now the question: Is there a way to draw only the shadows without the fill color but in full alpha? Can I somehow prevent the inside of my path from being drawn? Or is there perhaps a simpler way of drawing two shadows for one path?

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.

Answer 2

The problem is, the shadows are not drawn when alpha is set to 0.
Yes, the shadow relies on the shape's alpha value to render.
Now the question: Is there a way to draw only the shadows without the fill color but in full alpha? Can I somehow prevent the inside of my path from being drawn? Or is there perhaps a simpler way of drawing two shadows for one path?
You can draw the shadow only without rendering the main shape. The idea is:
add extra offset width to the shadow, the offset should be big enough, i.e. the context/canvas width. So that the shadow is separated from the main shape.
Translate the context with a negative same horizontal offset, so that the main shape is moved out of the rendering area, and the shadow is moved back.
For the shadow only layer, you probably want to use beginTransparencyLayer(auxiliaryInfo:) and saveGState() to push the drawing into a separated context.
Here is a comparison:
For code snippet, you can check my answer for another question here: https://stackoverflow.com/a/74844890/3164091