iOS: outlining the opaque parts of a partly-transparent image

Question

I have an application which requires that a solid black outline be drawn around a partly-transparent UIImage. Not around the frame of the image, but rather around all the opaque parts of the image itself. I.e., think of a transparent PNG with an opaque white "X" on it -- I need to outline the "X" in black.

To make matters trickier, AFTER the outline is drawn, the opacity of the original image will be adjusted, but the outline must remain opaque -- so the outline I generate has to include only the outline, and not the original image.

My current technique is this:

• Create a new UIView which has the dimensions of the original image.
• Duplicate the UIImage 4 times and add the duplicates as subviews of the UIView, with each UIImage offset diagonally from the original location by a couple pixels.
• Turn that UIView into an image (via the typical UIGraphicsGetImageFromCurrentImageContext method).
• Using CGImageMaskCreate and CGImageCreateWithMask, subtract the original image from this new image, so only the outline remains.

It works. Even with only the 4 offset images, the result looks quite good. However, it's horribly inefficient, and causes a good solid 4-second delay on an iPhone 4.

So what I need is a nice, speedy, efficient way to achieve the same thing, which is fully supported by iOS 4.0.

Any great ideas? :)

Answer 1

I would like to point out that whilst a few people have suggested edge detection, this is not an appropriate solution. Edge detection is for finding edges within image data where there is no obvious exact edge representation in the data.

For you, edges are more well defined, you are looking for the well defined outline. An edge in your case is any pixel which is on a fully transparent pixel and next to a pixel which is not fully transparent, simple as that! iterate through every pixel in the image and set them to black if they fulfil these conditions.

Alternatively, for an anti-aliased result, get a boolean representation of the image, and pass over it a small anti-aliased circle kernel. I know you said custom filters are not supported, but if you have direct access to image data this wouldn't be too difficult to implement by hand...

Cheers, hope this helps.

Answer 2

For the sake of contributing new ideas:

A variant on your current implementation would use CALayer's support for shadows, which it calculates from the actual pixel contents of the layer rather than merely its bounding rectangle, and for which it uses the GPU. You can try amping up the shadowOpacity to some massive value to try to eliminate the feathering; failing that you could to render to a suitable CGContext, take out the alpha layer only and manually process it to apply a threshold test on alpha values, pushing them either to fully opaque or fully transparent.

You can achieve that final processing step on the GPU even under ES 1 through a variety of ways. You'd use the alpha test to apply the actual threshold, you could then, say, prime the depth buffer to 1.0, disable colour output and the depth test, draw the version with the shadow at a depth of 0.5, draw the version without the shadow at a depth of 1.0 then enable colour output and depth tests and draw a solid black full-screen quad at a depth of 0.75. So it's like using the depth buffer to emulate stencil (since the GPU Apple used before the ES 2 capable device didn't support a stencil buffer).

That, of course, assumes that CALayer shadows appear outside of the compositor, which I haven't checked.

Alternatively, if you're willing to limit your support to ES 2 devices (everything 3GS+) then you could upload your image as a texture and do the entire process over on the GPU. But that would technically leave some iOS 4 capable devices unsupported so I assume isn't an option.