Drawing Smooth Curves - Methods Needed

Question

How do you smooth a set of points in an iOS drawing app WHILE MOVING? I have tried UIBezierpaths but all I get are jagged ends where they intersect, when I just shift the po

Answer 1

@Rakesh is absolutely right - you dont need to use Catmull-Rom algorithm if you just want a curved line. And the link he suggested does exacly that. So here's an addition to his answer.

The code bellow does NOT use Catmull-Rom algorithm & granularity, but draws a quad-curved line (control points are calculated for you). This is essentially what's done in the ios freehand drawing tutorial suggested by Rakesh, but in a standalone method that you can drop anywhere (or in a UIBezierPath category) and get a quad-curved spline out of the box.

You do need to have an array of CGPoint's wrapped in NSValue's

+ (UIBezierPath *)quadCurvedPathWithPoints:(NSArray *)points
{
    UIBezierPath *path = [UIBezierPath bezierPath];

    NSValue *value = points[0];
    CGPoint p1 = [value CGPointValue];
    [path moveToPoint:p1];

    if (points.count == 2) {
        value = points[1];
        CGPoint p2 = [value CGPointValue];
        [path addLineToPoint:p2];
        return path;
    }

    for (NSUInteger i = 1; i < points.count; i++) {
        value = points[i];
        CGPoint p2 = [value CGPointValue];

        CGPoint midPoint = midPointForPoints(p1, p2);
        [path addQuadCurveToPoint:midPoint controlPoint:controlPointForPoints(midPoint, p1)];
        [path addQuadCurveToPoint:p2 controlPoint:controlPointForPoints(midPoint, p2)];

        p1 = p2;
    }
    return path;
}

static CGPoint midPointForPoints(CGPoint p1, CGPoint p2) {
    return CGPointMake((p1.x + p2.x) / 2, (p1.y + p2.y) / 2);
}

static CGPoint controlPointForPoints(CGPoint p1, CGPoint p2) {
    CGPoint controlPoint = midPointForPoints(p1, p2);
    CGFloat diffY = abs(p2.y - controlPoint.y);

    if (p1.y < p2.y)
        controlPoint.y += diffY;
    else if (p1.y > p2.y)
        controlPoint.y -= diffY;

    return controlPoint;
}

Here's the result: