CGContext line width

Question

I'm trying to draw a simple line, the problem is that it is coming out not as 1 pixel in width but 2. The docs state that user space units will translate to a pixel, if I read it correctly.

The code is as simple as it gets, yet my line is always 2 pixels wide.

//Get the CGContext from this view
CGContextRef context = UIGraphicsGetCurrentContext();
//Set the stroke (pen) color
CGContextSetStrokeColorWithColor(context, [UIColor blueColor].CGColor);

//Set the width of the pen mark
CGContextSetLineWidth(context, 1);

for (int i = 1; i <= sections - 1; i++)
{
    UIBezierPath *path = [UIBezierPath bezierPath];
    [path moveToPoint:CGPointMake(0.0, i * kSectionHeight)];

    CGContextMoveToPoint(context, 0.0, i * kSectionHeight); //Start point
    CGContextAddLineToPoint(context, self.frame.size.width, i * kSectionHeight);
}

CGContextStrokePath(context);

Condorcet

possible duplicate of UIBezierPath stroke 1px line and fill 1px width rectangle - different results. – Ejectment

Answer 1

No points don't translate to pixels. If your line is too thick change the value.

Smoothshaven

To expand on it. Points are device resolution independent in the API. Apple will determine what one point equals for a given apple device. Displays do not only have pixels but pixel size and spacing. Apple will determine what best represents a point on their hardware. – Smoothshaven

Ok, herein lies the rub. How do I change the value? If I give a value of .5 instead of 1, the width of the line is the same, but simply a lighter shade? Must admit I'm lost here... – Condorcet

So there you are running into antialiasing. The line is thiner, but not pixel aligned. see #11177060 – Smoothshaven

Also look at books.google.co.jp/… – Smoothshaven

Yup, that's it. Used to dealing with that with strings, just didn't occur to me here. Thanks! – Condorcet

Answer 2

I tried the following in a test app:

CGFloat desiredWidthInPixels = 1.0f;
CGFloat lineWidth = desiredWidthInPixels / [[UIScreen mainScreen] scale];
NSLog(@"lineWidth = %f", lineWidth);

I got a lineWidth of 0.5f, which should translate accurately to a single pixel width on the screen (since scale is 2.0, indicating a "Retina" device). This should adapt correctly across devices of different screen sizes.

***Caveat: Of course, this only works as long as [[UIScreen mainScreen] scale] operates as it currently does.

Sardis

As I mentioned above, when I give a value of .5, the line is the same width. If, however, I use the code above and the retina version of the simulator, then the line is 1 pixel wide. So it seems to do the correct thing in a retina screen but not in a non-retina. – Condorcet

On a non-retina screen, the code above should give a line width of 1.0, since scale will be 1.0. Is that not what you're seeing? – Sardis

Yes, 1 on non-retina, .5 on retina. But still wasn't drawing correctly due to the anti-aliasing discussed in the post above. Thanks! – Condorcet