Question

In my code:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
mouseSwiped = YES;

UITouch *touch = [touches anyObject];   
CGPoint currentPoint = [touch locationInView:drawImage];

UIGraphicsBeginImageContext(drawImage.frame.size);
[drawImage.image drawInRect:CGRectMake(0, 0, drawImage.frame.size.width, drawImage.frame.size.height)];
CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 10.0);
CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), r, g, b, a);
CGContextBeginPath(UIGraphicsGetCurrentContext());
CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
CGContextStrokePath(UIGraphicsGetCurrentContext());
drawImage.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

lastPoint = currentPoint;

mouseMoved++;

if (mouseMoved == 10) {
    mouseMoved = 0;
}
}

I’m able to draw a line in drawImage, but if I want draw a line that remove the other line? as a eraser that cancel the others line drawn. is it possible?

Answer 1

If you want to “erase” pixels by drawing a line that replaces what’s underneath with transparency, then you need to make your stroke colour fully transparent but also change the blend mode to kCGBlendModeCopy (with the default blend mode, drawing with a fully transparent stroke colour has of course no effect).

CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 0, 0, 0, 0);
CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeCopy);

After that, you can draw the “eraser” line just like you are doing in your code snippet (CGContextMoveToPoint, CGContextAddLineToPoint, CGContextStrokePath)