how to get buffer zone around a uibezierpath

i have some uibezierpaths. as paths, they don't really have thickness. but, I am hoping to find a way to define an area around a path like the grayish areas around the lines in this picture

basically, i want to test whether drawn lines fall within the buffer zone around the lines. i thought this would be simple, but it's turning out to be much more complex than i thought. I can use the CGPathApply function to examine the points along my path, and then getting a range +or- each point, but it's more complicated than that with angles and curves. any ideas?

Expanding the width of a path is actually quite difficult. However, you could just stroke it with a thicker width and get pretty much the same effect. Something like...

CGContextSetRGBStrokeColor(context, 0.4, 0.4, 0.4, 1.0);
[path setLineWidth:15];
[path stroke];
CGContextSetRGBStrokeColor(context, 0.0, 0.0, 0.0, 1.0);
[path setLineWidth:3];
[path stroke];

...would produce a picture like the one in your question. But I doubt that's news to you.

The real trick is the test of "whether drawn lines fall within the buffer zone." That problem is very similar to one which I just answered for myself in another question. Take a look at the LineSample.zip code I shared there. This implements a bitmap/bitwise data comparison to detect hits on lines much like you need. You could just draw the thicker "buffer" paths into the bitmap for testing and show the thinner lines in your view.