Weird anti-aliasing in custom view
-
12-09-2019 - |
Question
I have a custom UIView
which is drawn using its -[drawRect:]
method.
The problem is that the anti-aliasing acts very weird as black lines horizontal or vertical lines are drawn very blurry.
If I disable anti-aliasing with CGContextSetAllowsAntialiasing
, everything is drawn as expected.
Anti-Aliasing:
alt text http://dustlab.com/stuff/antialias.png
No Anti-Aliasing (which looks like the expected result with AA):
alt text http://dustlab.com/stuff/no_antialias.png
The line width is exactly 1, and all coordinates are integral values.
The same happens if I draw a rectangle using CGContextStrokeRect
, but not if I draw exactly the same CGRect
with UIRectStroke
.
Solution
Since a stroke expands equal amounts to both sides, a line of one pixel width must not be placed on an integer coordinate, but at 0.5 pixels offset.
Calculate correct coordinates for stroked lines like this:
CGPoint pos = CGPointMake(floorf(pos.x) + 0.5f, floorf(pos.y) + 0.5f);
BTW: Don't cast your values to int and back to float to get rid of the decimal part. There's a function for this in C called floor
.
OTHER TIPS
in your view frames, you probably have float values that are not integers. While the frames are precise enough to do fractions of a pixel (float), you will get blurriness unless you cast to an int
CGRect frame = CGRectMake((int)self.frame.bounds..., (int)...., (int)...., (int)....);