Ios – iPhone smooth sketch drawing algorithm

drawingiosiphonequartz-graphics

I am working on a sketching app on the iPhone.
I got it working but not pretty as seen here
enter image description here

And I am looking for any suggestion to smooth the drawing
Basically, what I did is when user places a finger on the screen I called

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event

then I collect a single touch in an array with 

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event

and when the user lefts a finger from the screen, I called

- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event

then I draw all the points in the array using

NSMutableArray *points = [collectedArray points];   

CGPoint firstPoint;
[[points objectAtIndex:0] getValue:&firstPoint];

CGContextMoveToPoint(context, firstPoint.x, firstPoint.y);
CGContextSetLineCap(context, kCGLineCapRound);
CGContextSetLineJoin(context, kCGLineJoinRound);

for (int i=1; i < [points count]; i++) {
    NSValue *value = [points objectAtIndex:i];
    CGPoint point;
    [value getValue:&point];    
    CGContextAddLineToPoint(context, point.x, point.y);

} 

CGContextStrokePath(context);
UIGraphicsPushContext(context);

And now I want to improve the drawing tobe more like "Sketch Book" App
enter image description here

I think there is something to do with signal processing algorithm to rearrange all the points in the array but I am not sure. Any Help would be much appreciated.

Thankz in advance 🙂

Best Answer

CGPoint midPoint(CGPoint p1, CGPoint p2)
{

    return CGPointMake((p1.x + p2.x) * 0.5, (p1.y + p2.y) * 0.5);

}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{

    UITouch *touch = [touches anyObject];

    previousPoint1 = [touch previousLocationInView:self];
    previousPoint2 = [touch previousLocationInView:self];
    currentPoint = [touch locationInView:self];

}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{

    UITouch *touch = [touches anyObject];

    previousPoint2 = previousPoint1;
    previousPoint1 = [touch previousLocationInView:self];
    currentPoint = [touch locationInView:self];


    // calculate mid point
    CGPoint mid1 = midPoint(previousPoint1, previousPoint2); 
    CGPoint mid2 = midPoint(currentPoint, previousPoint1);

    UIGraphicsBeginImageContext(self.imageView.frame.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    [self.imageView.image drawInRect:CGRectMake(0, 0, self.imageView.frame.size.width, self.imageView.frame.size.height)];

    CGContextMoveToPoint(context, mid1.x, mid1.y);
    // Use QuadCurve is the key
    CGContextAddQuadCurveToPoint(context, previousPoint1.x, previousPoint1.y, mid2.x, mid2.y); 

    CGContextSetLineCap(context, kCGLineCapRound);
    CGContextSetLineWidth(context, 2.0);
    CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 1.0);
    CGContextStrokePath(context);

    self.imageView.image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

}
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