Question

I am using following code to create the paint brush in iphone.

@interface Canvas : UIImageView {
    CGPoint location;
}

@property CGPoint location;
.m file
@synthesize location;

- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];   
    self.location = [touch locationInView:self];    
}

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint currentLocation = [touch locationInView:self];  

    UIGraphicsBeginImageContext(self.frame.size); 
    CGContextRef ctx = UIGraphicsGetCurrentContext();  
    //CGContextSetBlendMode(ctx, kCGBlendModeOverlay);


    [self.image drawInRect:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)];
    CGContextSetLineCap(ctx, kCGLineCapRound);
    CGContextSetBlendMode(ctx, kCGBlendModeNormal);
    CGContextSetLineWidth(ctx, 5.0);
    CGContextSetRGBStrokeColor(ctx, 1.0, 0.0, 0.0, 1.0);
    //CGContextSetBlendMode(ctx, kCGBlendModeOverlay);
    CGContextBeginPath(ctx);
    CGContextMoveToPoint(ctx, location.x, location.y);
    CGContextAddLineToPoint(ctx, currentLocation.x, currentLocation.y);
    CGContextStrokePath(ctx);
    self.image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    location = currentLocation;
}

- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint currentLocation = [touch locationInView:self];

    UIGraphicsBeginImageContext(self.frame.size);
    CGContextRef ctx = UIGraphicsGetCurrentContext();  
  //  CGContextSetBlendMode(ctx, kCGBlendModeOverlay);

    [self.image drawInRect:CGRectMake(0, 0, self.frame.size.width, self.frame.size.height)];
    CGContextSetBlendMode(ctx, kCGBlendModeNormal);
    CGContextSetLineCap(ctx, kCGLineCapRound);
    CGContextSetLineWidth(ctx, 5.0);
    CGContextSetRGBStrokeColor(ctx, 1.0, 0.0, 0.0, 1.0);
    CGContextBeginPath(ctx);
    CGContextMoveToPoint(ctx, location.x, location.y);
    CGContextAddLineToPoint(ctx, currentLocation.x, currentLocation.y);
    CGContextStrokePath(ctx);
    self.image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    location = currentLocation;     
}

It’s working but while drawing there are some dots after certain distance in line which is being drawn.i want to remove those dots and want smooth straight line.How can i achieve this?

Answer 1

You are using the same image all over again and again multiplying it on the old image, right?
I would try to use a fresh context on every touch. Then you multiply it on the existing image.
Perhaps thats the reason for the strange look?

Also you could add checks, if the start and endpoint are different.

And you should make an extra method for drawing. You have redundant code in TouchesMoved and TouchesEnded.