Question

I’m writing an application that will calculate a CGPoint and show a mark in an envelope (a diagram if you like). My envelope is just part of the background image in an UIImageView. What I want to do is to construct a sort of “line”, corresponding to the envelopes limits (they’re not straight lines, but curves) so that if the calculated CGPoint is to the left of this line, or to the right of another line, then the calculated point is not approved. Where it to be in the middle of these two, it’s approved.

I was first thinking of drawing lines using CoreGraphics, but I’m not sure if one could check whether the calculated CGPoint is to the right or left of those lines.

The envelope is only 149px high, so I was also thinking of putting together a dictionary, where the keys where the y position and the values where the x position of the pixels that represented that defining boundary line.

The application is rather simple and is not animating anything. Does anybody have an idea of how to best come up with a solution for this sort of behavior?

Answer 1

You can do this by creating a CGPath that represents your boundary lines (the outline of your envelope) and testing that a point is contained in it with CGPathContainsPoint.

You’ll have to do some trial and error to construct a CGPath that matches your envelope shape, try filling it in the drawRect method to see what your path actually is.

Here’s an example with a circle path:

CGPoint viewCenter = CGPointMake(100,100);
CGPoint checkPoint = CGPointMake(110,110);
UIBezierPath *bpath = [UIBezierPath bezierPathWithArcCenter:viewCenter radius:50 startAngle:0 endAngle:DEGREES_TO_RADIANS(360) clockwise:YES];
CGPathRef path = [bpath CGPath];
BOOL inPath = CGPathContainsPoint(path, NULL, checkPoint, NO);

Here I have DEGREES_TO_RADIANS defined like this:

#define DEGREES_TO_RADIANS(angle) ((angle) / 180.0 * M_PI)