Creating a stroke/outline of a .PNG with CALayer?

Question

I would like to apply a "stroke" or outline to a png, identically to how Photoshop does it. I have a feeling this can be done with CALayer, but after some tinkering, it is not immediately obvious. setBorderWidth + setBorderColor is almost what I want, except that it only adds a border to the entire dimension of the image, rather than the outline of the png image itself.

Once the stroke is applied, I'd like to also knockout the fill of the png, leaving only an outlined border of the initial shape.

Answer 1

There is no automatic way to do what you're asking. You have to know the path of the shape within your png that you want to "knockout". Once you've defined that, you can create a CAShapeLayer, which accepts a CGPathRef, containing your points. You can stroke and fill the path layer with whatever color you choose and then add it to the layer hierarchy of the displaying view or use it to define a mask of one of the layers in your view.