Question

I have this code into the - (void)drawMapRect:(MKMapRect)mapRect zoomScale:(MKZoomScale)zoomScale inContext:(CGContextRef)context method (into a MKOverlayView subclass) to prevent drawing segments that are less than 10 pixels long on a map overlay :

            CGPoint origin = [self pointForMapPoint:poly.points[0]];
            CGPoint lastPoint = origin;

            CGContextMoveToPoint(context, origin.x, origin.y);

            for (int i=1; i<poly.pointCount; i++) {
                CGPoint point = [self pointForMapPoint:poly.points[i]];

                CGFloat xDist = (point.x - lastPoint.x);
                CGFloat yDist = (point.y - lastPoint.y);
                CGFloat distance = sqrt((xDist * xDist) + (yDist * yDist)) * zoomScale;

                if (distance >= 10.0) {
                    lastPoint = point;
                    CGContextAddLineToPoint(context, point.x, point.y);
                }
            }

will the test >= 10.0 will take care about the screen resolution, or may I introduce some [UIScreen mainScreen].scale parameter ?

Answer 1

I believe that test >= 10.0 does not take into account the screen resolution. Apple does most of their drawing arithmetic using “points” instead of pixels- that way code does not have to change for a retina display compared to a normal display.

If you want to draw something just 10.0 pixels wide, you will need to take into account the screen resolution; however, if you do this you’ll have to write the method to support both retina display and normal display.