iOS Core Graphics How to erase a drawing

Question

I am following this tutorial for my doodle application.

http://www.raywenderlich.com/18840/how-to-make-a-simple-drawing-app-with-uikit

his erase function is using white color drawing. But I am using image background and when I erase, I really need to erase.

I have tried

CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);

But it's not working. Here is my code:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    _mouseSwiped = NO;
    UITouch *touch = [touches anyObject];
    _lastPoint = [touch locationInView:self.tempImageView];
}

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

    _mouseSwiped = YES;
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self.tempImageView];

    UIGraphicsBeginImageContext(self.tempImageView.frame.size);
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height)];
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width );


    CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, 1.0);

    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }
    else {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
    }

    CGContextStrokePath(UIGraphicsGetCurrentContext());
    self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    [self.tempImageView setAlpha:_alpha];
    UIGraphicsEndImageContext();

    _lastPoint = currentPoint;
}

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

    CGSize size = self.tempImageView.frame.size;

    if(!_mouseSwiped) {
        UIGraphicsBeginImageContext(self.tempImageView.frame.size);
        [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width);


        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, _alpha);
        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());
        CGContextFlush(UIGraphicsGetCurrentContext());
        self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }




    UIGraphicsBeginImageContext(self.drawImageView.frame.size);
    [self.drawImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:1.0];

    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }

    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height) blendMode:kCGBlendModeNormal alpha:_alpha];


    self.drawImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    self.tempImageView.image = nil;
    UIGraphicsEndImageContext();
}

Answer 1

solved this problem by swapping tempImageView and drawImageView when touch began if it's erasing

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    _mouseSwiped = NO;
    UITouch *touch = [touches anyObject];
    _lastPoint = [touch locationInView:self.tempImageView];

    if (_isErasing) {
        self.tempImageView.image = self.drawImageView.image;
        self.drawImageView.image = nil;
    }

}



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

    _mouseSwiped = YES;
    UITouch *touch = [touches anyObject];
    CGPoint currentPoint = [touch locationInView:self.tempImageView];

    UIGraphicsBeginImageContext(self.tempImageView.frame.size);
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, self.tempImageView.frame.size.width, self.tempImageView.frame.size.height)];
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
    CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width );


    if (_isErasing) {
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
    }
    else {
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, 1.0);
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
    }

    CGContextStrokePath(UIGraphicsGetCurrentContext());
    self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    [self.tempImageView setAlpha:_alpha];
    UIGraphicsEndImageContext();

    _lastPoint = currentPoint;
}





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

    CGSize size = self.tempImageView.frame.size;

    if(!_mouseSwiped) {
        UIGraphicsBeginImageContext(self.tempImageView.frame.size);
        [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height)];
        CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), _width);


        if (_isErasing) {
            CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);
        }
        else {
            CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), _red, _green, _blue, _alpha);
            CGContextSetBlendMode(UIGraphicsGetCurrentContext(),kCGBlendModeNormal);
        }

        CGContextMoveToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), _lastPoint.x, _lastPoint.y);
        CGContextStrokePath(UIGraphicsGetCurrentContext());
        CGContextFlush(UIGraphicsGetCurrentContext());
        self.tempImageView.image = UIGraphicsGetImageFromCurrentImageContext();
        UIGraphicsEndImageContext();
    }




    UIGraphicsBeginImageContext(self.drawImageView.frame.size);
    [self.drawImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:1.0];
    [self.tempImageView.image drawInRect:CGRectMake(0, 0, size.width, size.height) blendMode:kCGBlendModeNormal alpha:_alpha];
    self.drawImageView.image = UIGraphicsGetImageFromCurrentImageContext();
    self.tempImageView.image = nil;
    UIGraphicsEndImageContext();
}

Answer 2

If anybody is still intereted this is how it worked for me in Swift 3. Thanks for the help OMGPOP

override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
    swiped = false
    hideAllButtons()
    if let touch = touches.first {
        lastPoint = touch.location(in: self.view)
    }

    if isErasing {
        self.tempImageView.image = self.mainImageView.image
        self.mainImageView.image = nil
    }
}

override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
    swiped = true
    if let touch = touches.first {
        let currentPoint = touch.location(in: view)
        drawLineFrom(fromPoint: lastPoint, toPoint: currentPoint)

        lastPoint = currentPoint
    }

}

override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
    showAllButtons()
    if !swiped {
        drawLineFrom(fromPoint: lastPoint, toPoint: lastPoint)
    }

    UIGraphicsBeginImageContext(mainImageView.frame.size)
    mainImageView.image?.draw(in: CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: .normal, alpha: 1.0)
    tempImageView.image?.draw(in:  CGRect(x: 0, y: 0, width: view.frame.size.width, height: view.frame.size.height), blendMode: .normal, alpha: opacity)
    mainImageView.image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    tempImageView.image = nil
}

// Draws a line between two points on the view
func drawLineFrom(fromPoint: CGPoint, toPoint: CGPoint) {
    UIGraphicsBeginImageContext(view.frame.size)
    let context = UIGraphicsGetCurrentContext()
    tempImageView.image?.draw(in: CGRect(x:0, y:0, width: view.frame.size.width, height: view.frame.size.height))

    context?.move(to: fromPoint)
    context?.addLine(to: toPoint)

    context?.setLineCap(.round)
    context?.setLineWidth(brushWidth)

    if isErasing {
        context?.setBlendMode(.clear)
    } else {
        context?.setStrokeColor(red: red, green: green, blue: blue, alpha: 1.0)
        context?.setBlendMode(.normal)
    }
    context?.strokePath()

    tempImageView.image = UIGraphicsGetImageFromCurrentImageContext()
    tempImageView.alpha = opacity
    UIGraphicsEndImageContext()

}

Answer 3

Here's how I got the eraser working. In the touchesEnded function I'm drawing on the context with the following code:

//draw the main image on the context
self.mainImage.image?.draw(in: CGRect(x:0, y:0, width:self.view.frame.size.width, height:self.view.frame.size.height), blendMode: .normal, alpha: 1.0)
    if(erasing == false) {
        //draw the current stroke normally since we're not erasing
        self.tempDrawImage.image?.draw(in: CGRect(x:0, y:0, width:self.view.frame.size.width, height:self.view.frame.size.height), blendMode: .normal, alpha: opacity!)
    } else {
        //let's create a mask of what we would like to erase by using the destinationIn function which according to the docs is the following
        //R = D*Sa
        //Destination * Source_Alpha
        self.tempDrawImage.image?.draw(in: CGRect(x:0, y:0, width:self.view.frame.size.width, height:self.view.frame.size.height), blendMode: .destinationIn, alpha: 1.0)
        //Let's apply this mask to the current drawing using XOR blending which removes any drawing underneath our mask but leaves everything else alone
        self.mainImage.image?.draw(in: CGRect(x:0, y:0, width:self.view.frame.size.width, height:self.view.frame.size.height), blendMode: .xor, alpha: 1.0)
    }
    //let's apply the changes we've made to our mainImage
    self.mainImage.image = UIGraphicsGetImageFromCurrentImageContext();
    self.tempDrawImage.image = nil;

To get a better idea of how it works you can comment out the draw calls individually to see how they look.

You might find some aliasing issues where you're getting little bits left behind on the edge of strokes. To remove that when drawing in erase mode turn of alisasing for the context:

let ctx = UIGraphicsGetCurrentContext()
    ctx?.setShouldAntialias(false)

Answer 4

Why don't you paint/draw on a separate view ?

TopView = Paint + gesture ...., BottomView = background image.

erase color = ClearColor (transparent)

At the end, if you want to save your composition, up to you to combine both pictures.