Put border around partially transparent Image being drawn on CGContext

Question

I have an image with a yellow vase in the foreground and transparent background:

I'm drawing it on a CGContext:

CGContextDrawImage(context, CGRectMake(0, 0, 100, 100), myImage.CGImage);

I can draw a shadow around it by using the following statement before CGContextDrawImage:

CGContextSetShadowWithColor(context, CGSizeMake(0,0), 5, [UIColor blueColor].CGColor);

But I want to put a stroke around the image, so that it'll looks like following:

If I did this:

CGContextSetRGBStrokeColor(shadowContext, 0.0f, 0.0f, 1.0f, 1.0f);
CGContextSetLineWidth(shadowContext, 5);
CGContextStrokeRect(shadowContext, CGRectMake(0, 0, 100, 100));

It (obviously) draws a rectangular border around the whole iamge like this:

Which is not what I need.

But what's the best way to draw the border as in the third image?

Please note that it's not possible to use UIImageView in this case, so using the properties of CALayer of UIImageView is not applicable.

Answer 1

One way to do this is to use the mathematical morphology operator of dilation to "grow" the alpha channel of the image outward, then use the resulting grayscale image as a mask to simulate a stroke. By filling the dilated mask, then drawing the main image on top, you get the effect of a stroke. I've created a demo showing this effect, available on Github here: https://github.com/warrenm/Morphology (all source is MIT licensed, should it prove useful to you).

And here's a screenshot of it in action:

Note that this is staggeringly slow (dilation requires iteration of a kernel over every pixel), so you should pick a stroke width and precompute the mask image for each of your source images in advance.