CALayer: Single pixel line looks like 2 pixels

Question

This is my code:

int columns 3;
int columnWidth = self.layer.bounds.size.width / 3;

for (int c = 1; c < columns; c++) {
    CALayer *layer  = [CALayer layer];
    layer.frame = (CGRectMake(columnWidth * c + 0.5, 0.5, 1, self.layer.bounds.size.height));
    layer.backgroundColor = myColor;
    [grid addSublayer:layer];
}

I understand that I have to shift the x and y 0.5 pixels, which is what I have done, yet it still shows as a 2 pixel line instead of 1.

Answer 1

Set the layer Frame as

layer.frame = (CGRectMake(columnWidth * c + 0.5, 0.5, 0.5, self.layer.bounds.size.height));

Yeah it will work for retina also

Check this post Section 1 Point Lines and Pixel Boundaries.It is well explained there

Answer 2

I went ahead and made an explanatory picture, in case it's helpful to anyone

The lower line on exact pixel coordinates and will look blurry... each half of the width of the line lies in a different row of pixels...

The upper line is offset by line_width/2.0 and will look sharp... it lies completely within a row of pixels.