How to draw a stroke outline using CGContext API?

I have an icon image I would like to draw a 1px, colored 0.5 opacity line around, inside the image (similar to Stroke layer style with position:inside in Photoshop). So far I've tried this:

UIGraphicsBeginImageContextWithOptions(inCompositeSize, NO, inBaseImage.scale);
            context = UIGraphicsGetCurrentContext();

        CGContextDrawImage(context, frame, image.CGImage);
        CGColorSpaceRef cs = CGColorSpaceCreateDeviceRGB();
        CGContextSetStrokeColorSpace(context, cs);
        CGColorSpaceRelease(cs);
        CGContextSetStrokeColorWithColor(context, [UIColor blueColor].CGColor);
        CGContextStrokePath(context);
//...

This seems to have no effect. 开发者_JS百科I'm guessing this is because I'm working with an image and not a path. How can I produce an outline stroke effect on the image?

Clarification: I'm trying to draw an outline on the opaque icon area only. The icon background is 0 alpha.

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StrokePath strokes a path, and you haven't plotted a path. That's why it strokes nothing.

You need to do either of two things:

1. Vectorize the raster image and stroke that path. Core Image's edge-detection filter will help, but even so, this is hard. There is no built-in API for this; you'll have to find a library that does it or do it yourself.
2. Fake it by drawing the image with a shadow. Note that shadows can be any color; it doesn't have to look like a shadow.