Question

I am creating a drawing application like this, but I have one issue. The problem is that when I draw a line with one color and then draw a line with another color, it gives me combination of both colors where the lines intersect.

- (void)drawRect:(CGRect)rect
{

    [curImage drawAtPoint:CGPointMake(0, 0)];
    CGPoint mid1 = midPoint(previousPoint1, previousPoint2); 
    CGPoint mid2 = midPoint(currentPoint, previousPoint1);

    CGContextRef context = UIGraphicsGetCurrentContext(); 

    [self.layer renderInContext:context];

    CGContextMoveToPoint(context, mid1.x, mid1.y);
    CGContextAddQuadCurveToPoint(context, previousPoint1.x, previousPoint1.y, mid2.x, mid2.y); 
    CGContextSetLineCap(context, kCGLineCapRound);
    CGContextSetLineWidth(context, self.lineWidth);
    CGContextSetStrokeColorWithColor(context, self.lineColor.CGColor);

    CGContextStrokePath(context);

    [super drawRect:rect];

    [curImage release];

}

The rest of the project is available on GitHub.

Answer 1

Maybe have a look at blend modes.

https://developer.apple.com/library/mac/#documentation/graphicsimaging/Reference/CGContext/Reference/reference.html#//apple_ref/c/tdef/CGBlendMode