Is there a way to create a CGPath matching outline of a SKSpriteNode?

Question

My goal is to create a CGPath that matches the outline of a SKSpriteNode.

This would be useful in creating glows/outlines of SKSpriteNodes as well as a path for physics.

One thought I have had, but I have not really worked much at all with CIImage, so I don't know if there is a way to access/modify images on a pixel level.

Then maybe I would be able to port something like this to Objective-C :

http://www.sakri.net/blog/2009/05/28/detecting-edge-pixels-with-marching-squares-algorithm/

Also very open to other approaches that make this process automated as opposed to me creating shape paths for every sprite I make for physics or outline/glow effects.

Answer 1

What you're looking for is called a contour tracing algorithm. Moore neighbor tracing is popular and works well for images and tilemaps. But do check out the alternatives because they may better fit your purposes.

AFAIK marching squares and contour tracing are closely related, if not the same (class of) algorithms.

An implementation for tilemaps (to create physics shapes from tiles) is included in Kobold Kit. The body of the algorithm is in the traceContours method of KKTilemapLayerContourTracer.m.

It looks more complex than it really is, on the other hand it takes a while to wrap your head around it because it is a "walking" algorithm, meaning the results of prior steps is used in the current step to make decisions.

The KK implementation also includes a few minor fixes specifically for tilemaps (ie two or more horizontally or vertically connected tiles become a single line instead of dividing the line into tile-sized segments). It was also created with a custom point array structure, and when I ported it to SK I decided it would be easier to continue with that and only at the end convert the point arrays to CGPath objects.

You can make certain optimizations if you can safely assume that the shape you're trying to trace is not going to touch the borders, and there can not be any tiles that are only connected diagonally. All of this becomes clearer when you're actually implementing the algorithm for your own purposes.

But as far as a ready-made, fits-all-purposes solution goes: there ain't none.