Objective-C: Touch Draw Opacity - objective-c

Question

I am trying to make a drawing app that has a control on the opacity of the brush but when i tried to lower the opacity the result is like this. I used core graphics. (check the image).
How will i resolve this issue?
Here are some of my codes.
-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event//upon touches
{
UITouch *touch = [touches anyObject];
previousPoint1 = [touch locationInView:self.view];
previousPoint2 = [touch locationInView:self.view];
currentTouch = [touch locationInView:self.view];
}
-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event//upon moving
{
UITouch *touch = [touches anyObject];
previousPoint2 = previousPoint1;
previousPoint1 = currentTouch;
currentTouch = [touch locationInView:self.view];
CGPoint mid1 = midPoint(previousPoint2, previousPoint1);
CGPoint mid2 = midPoint(currentTouch, previousPoint1);
UIGraphicsBeginImageContext(CGSizeMake(1024, 768));
[imgDraw.image drawInRect:CGRectMake(0, 0, 1024, 768)];
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetLineCap(context,kCGLineCapRound);
CGContextSetLineWidth(context, slider.value);
CGContextSetBlendMode(context, blendMode);
CGContextSetRGBStrokeColor(context,red, green, blue, 0.5);
CGContextBeginPath(context);
CGContextMoveToPoint(context, mid1.x, mid1.y);
CGContextAddQuadCurveToPoint(context, previousPoint1.x, previousPoint1.y, mid2.x, mid2.y);
CGContextStrokePath(context);
imgDraw.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsGetCurrentContext();
endingPoint=currentTouch;
}

Answer 1

Rather than updating the image for every touchesMoved, I'd suggest you don't resave the image every time, but rather add each touchesMoved to an array of data points. Then, in your drawing routine (as NSResponder suggests) pull up the original image and then redraw the whole path mapped out by your array of points. If you want to update your image at some point, do it on touchesEnded, but not for every touchesMoved.

Answer 2

Don't do your drawing in -touchesMoved:withEvent:. In any event method, you should be updating what needs to be drawn, and then send -setNeedsDisplay.
As your code is written above, you're creating a path between each pair of locations you're getting from the event messages.
You should create a path in -touchesBegan:withEvent:, and add to it in -touchesMoved:withEvent:.