Core Graphics is not drawing a line at the correct width

Question

I\'m trying to combine a border on an UIView with a line drawn in drawRect. I use for both the same width. But the problem is that sometimes the resulting width of the lines dra

Answer 1

I'm guessing the line you are drawing is not pixel aligned. If I'm wrong then ignore my tangent here. But it's useful information nonetheless.

I'm going to borrow some screenshots from this WWDC 2011 video:

"1-29 Session 129 - Practical Drawing for iOS Developers"

I suggest you watch the whole thing. The relevent section starts around the 20:50 mark.

Basically, imagine you want to draw a 1pt line from (1,2) to (4,2)

Core graphics will try and center it along the y value which is 2.0. This means it will try and draw the line with its top edge at y=1.5 and the bottom edge at y=2.5:

On a retina display this will work OK, because y=1.5 and y=2.5 are pixel aligned:

However on a non-retina display the graphics system will be forced to fill 2 vertical pixels at half-intensity to get the closest match it can:

Note that this effect will still be seen on retina displays when you are dealing with smaller fractional point values. So this problems can still be seen on retina displays depending on your view's frame.

To fix this problem, any time you have an odd line width, you need to offset the point value where you draw it by 0.5. You need to offset it up/down/left/right depending on the situation.

I hope this helps. Without a screenshot it's hard to say what your problem is.