Detecting Touch on PNGs with Alpha

Question

I am trying to detect a touch event on a PNG image loaded into a UIImageView. I have everything working fine except that the touch is being tested for the bounding rectangle around the image (as expected). What I would like to do is test if the user has selected part of the visible PNG as opposed to the UIImageView itself.

For example if I have a horseshoe image, I want it to only respond to touches when you select the sides and not the center part where nothing is being drawn. I am kind of at a loss on this one, google reveals a number of people with the same issue but not even a hint towards where to begin looking.

Answer 1

Two ways:

a) you examine a pixel data of your image to determine if the touched pixel is a transparent pixel. You have to draw your image to an offline buffer to make this possible. Use CGContextDrawImage and CGBitmapContextGetData to get access to pixel data from UIImage.CGImage This Apple's Q&A explains the basic method to access pixel data.

b) you have a polygon representation of the horseshoe and use polygon hit testing to determine if the horseshoe was touched. Google for "point in polygon" for algorithms.

a) is probably less work if you need this just for a few images, but if you have a lot of hit testing (game with a lot of movement) b) might be better.