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Asked: 2026-05-25T20:54:30+00:00

I have an unusual problem that caused my mind to stuck. I have created

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I have an unusual problem that caused my mind to stuck. I have created a drawing application that uses CGPathes to be created by a pen tool on the fly via the following view controller code.

@implementation WWLGrabManager
- (void) touchesBegan:(NSSet*)touchesIgnore withEvent:(UIEvent*)event {
     currentPath = CGPathCreateMutable();
     CGPathMoveToPoint(currentPath, NULL, pt.x, pt.y); 
}

- (void) touchesMoved:(NSSet*)touchesIgnore withEvent:(UIEvent*)event {
     CGPathAddLineToPoint(currentPath, NULL, pt.x, pt.y);
}

- (void) touchesEnded:(NSSet*)touches withEvent:(UIEvent*)event {
    currentPath = [self newSmoothedPathWithPath:currentPath];
}
@end

After the drawing I iterate once again over the CGPath in order to smooth the drawn edges of the path. This shall make the path to not look so ugly by building the middle of each three points and applying a curve to each point. The code for smoothing edges mainly consists of the following pseudo code:

-(CGMutablePathRef)newSmoothedPathWithPath:(CGPathRef)path {
      // This is pseudo code
      CGMutablePathRef newPath = CGPathCreateMutable();
      foreach (CGPoint point in path)
            CGPathAddCurveToPoint(newPath, NULL, 
                    ctrlPt1.x,ctrlPt1.y,ctrlPt2.x,ctrlPt2.y,
                    ((*preLastPoint).x + (*lastPoint).x + (point).x)/3, 
                    ((*preLastPoint).y + (*lastPoint).y + (point).y)/3); 
      }
      return newPath;
 }

Now, after applying the smoothing function, the new CGPath causes fragments like in the picture here. I have double checked the control points, but I cannot figure out why this is happening.

For debugging reasons I have printed out a log of the CGPathes points and control points below.

 MainPoint //      ControlPoint1 //  ControlPoint2

 66.00, 91.00 //   67.00,87.00 //    64.25,95.75 
 59.00,110.00 //   60.75,105.25 //   58.00,113.75 
 55.00,125.00 //   56.00,121.25 //   54.00,128.75 
 51.00,140.00 //   52.00,136.25 //   50.25,144.25 
 48.00,157.00 //   48.75,152.75 //   47.25,161.00 
 45.00,173.00 //   45.75,169.00 //   44.75,176.75 
 44.00,188.00 //   44.25,184.25 //   43.75,191.75 
 43.00,203.00 //   43.25,199.25 //   43.00,207.00 
 43.00,219.00 //   43.00,215.00 //   43.00,223.00 
 43.00,235.00 //   43.00,231.00 //   43.00,239.25 
 43.00,252.00 //   43.00,247.75 //   43.00,256.00 
 43.00,268.00 //   43.00,264.00 //   44.25,272.00 
 48.00,284.00 //   46.75,280.00 //   49.50,287.50 
 54.00,298.00 //   52.50,294.50 //   56.75,300.75 
 65.00,309.00 //   62.25,306.25 //   68.75,310.75 
 80.00,316.00 //   76.25,314.25 //   84.00,316.50 
 96.00,318.00 //   92.00,317.50 //   101.50,318.25 
 118.00,319.00 //  112.50,318.75 //  124.75,319.00 
 145.00,319.00 //  138.25,319.00 //  151.25,319.00 
 170.00,319.00 //  163.75,319.00 //  175.50,318.25 
 192.00,316.00 //  186.50,316.75 //  199.50,314.00 
 222.00,308.00 //  214.50,310.00 //  226.75,306.75 
 241.00,303.00 //  236.25,304.25 //  245.00,301.75 
 257.00,298.00 //  253.00,299.25 //  260.50,295.75 
 271.00,289.00 //  267.50,291.25 //  273.25,285.25 
 280.00,274.00 //  277.75,277.75 //  280.50,270.25 
 282.00,259.00 //  281.50,262.75 //  282.00,254.50 
 282.00,241.00 //  282.00,245.50 //  280.50,237.25 
 276.00,226.00 //  277.50,229.75 //  273.50,222.75 
 266.00,213.00 //  268.50,216.25 //  263.00,208.75 
 254.00,196.00 //  257.00,200.25 //  249.75,192.50 
 237.00,182.00 //  241.25,185.50 //  234.00,179.75 
 225.00,173.00 //  228.00,175.25 //  221.75,170.50 
 212.00,163.00 //  215.25,165.50 //  208.50,160.25 
 198.00,152.00 //  201.50,154.75 //  194.00,148.75 
 182.00,139.00 //  186.00,142.25 //  178.00,136.75 
 166.00,130.00 //  170.00,132.25 //  162.25,128.50

Update: I used the algorithm described in this link, which was translated to Objective-C as follows:

 CGPoint ctrl2 = controlPointForPoints(*lastPoint,*preLastPoint,currentPoint);
 CGPoint ctrl1 = controlPointForPoints(*lastPoint,currentPoint,*preLastPoint);

 static CGPoint controlPointForPoints(CGPoint pt, CGPoint pre, CGPoint post) {
     CGPoint ctrlPt = CGPointMake(
         middleOfPoints(middleOfPoints(pt.x, pre.x), middleOfPoints(symmetryOfPoints(pt.x, post.x), pt.x)),
         middleOfPoints(middleOfPoints(pt.y, pre.y), middleOfPoints(symmetryOfPoints(pt.y, post.y), pt.y))
     );

     return ctrlPt;
 }

 static float symmetryOfPoints(float a,float b) {
     return a - ((b-a)*smoothingFactor) / 100.0;
 }

 static float middleOfPoints(float a, float b) {
     return (a+b) / 2.0;
 }

However, exchanging the two control points does not lead to satisfactory results but increases the number of fragments enourmously. I’d appreciate any further help.

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1 Answer

  1. Editorial Team

    Unfortunately, your code doesn’t show how the control points of the smoothed path are calculated. But they are the problem. Most of them are on the wrong side of the start or end point of a curve segment. As a consequence, your path makes a very tight loop or S curve at many main points.

    Another indication of this is that while the path generally turns left, many path segments are to the right.

    I don’t quite understand why you have these white half circles. It seems the even-odd rule is used to draw the path instead of the non-zero winding rule, which probably cannot be changed. Anyway, the problem will disappear if you fix the above problem.

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