Building 'DrawSomething' style app for neuroscience project research purposes - ios

Question

I am trying to implement a view that draws the users handwriting (curser position) for the iPad (4). I saw Apple's sample code, that uses OpenGL, however, there were parts I couldn't understand, so, I tried implementing this using core graphics.
#import "PaintView.h"
#include <stdlib.h>
#implementation PaintView
- (id)initWithCoder:(NSCoder *)aDecoder {
self = [super initWithCoder:aDecoder];
if(self) {
//
pointsToDraw = [[NSMutableArray alloc] init];
}
return self;
}
- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event
{
UITouch *touch = [touches anyObject];
NSLog(#"%#", touch);
CGPoint location = [touch locationInView:self];
CGPoint previousLocation = [touch previousLocationInView:self];
Ink *ink = [[Ink alloc] initWithPoint:previousLocation toPoint:location time:touch.timestamp];
// UITouch *newTouch = [touch copy];
[pointsToDraw addObject:ink];
[self setNeedsDisplay];
}
- (void)drawLine:(CGPoint)startingPoint toPoint:(CGPoint)endingPoint context:(CGContextRef)context
{
// Drawing code
CGContextSetStrokeColorWithColor(context, [UIColor redColor].CGColor);
CGRect temp = CGRectMake(10, 10, 100, 100);
// Draw them with a 2.0 stroke width so they are a bit more visible.
CGContextSetLineWidth(context, 2.0);
CGContextMoveToPoint(context, startingPoint.x,startingPoint.y); //start at this point
CGContextAddLineToPoint(context, endingPoint.x, endingPoint.y); //draw to this point
// and now draw the Path!
}
- (void)drawRect:(CGRect)rect
{
[super drawRect:rect];
// [self drawLine:CGPointMake(10, 10) toPoint:CGPointMake(30, 30)];
CGContextRef context = UIGraphicsGetCurrentContext();
for (Ink *ink in pointsToDraw){
[self drawLine:ink.point toPoint:ink.previousPoint context:context];
}
CGContextStrokePath(context);
}
#end
The problem is, every touch I draw everything (ink is a class that contains two CGPOINT's and a time stamp), and after a while this dramatically slows things down creating substantial lag.
My goal is to be able to both capture handwriting in a precise way, and play it back precisely.
Another things to consider, is that I am using a stylus which gives pressure information so I need to be able to draw my line in changing widths.
Any advice will be greatly appreciated.

Answer 1

Instead of storing the points in an array, store them in an array and a UIBezierPath. Then you only need to draw the bezier path within drawRect: instead of setting up the whole scheme.
The stylus doesn't give pressure information - at least not on iOS. iPhones have capacitive touch screens not resistive. The standard algorithm to change width is to use the speed as the factor and draw using little triangles that you fill in to create your line.
Funny! Here is a relevant article: http://www.nearinfinity.com/blogs/jason_harwig/2012/11/06/capture-a-signature-on-ios.html