Create a circle or a disk with antialiasing for retina display

Question

I have creating a circle using CGContextFillEllipseInRect from CoreGraphic.

I'm using this circle (which is actually a disk) to replace the thumbImage of an UISlider. The antialiasing is applied by default.

But the result on my iPhone 6 is clearly not good. I can clearly see the pixels ,not as much as with the antialiasing off, but way more than the pixels of a normal UISlider.

Maybe I'm doing something wrong. So my question is, is there a way to get programmatically the same nice disk than the one used by default for an UISlider?

EDIT:

Here is how I create the disk:

class func drawDisk(rgbValue:UInt32, rectForDisk: CGRect = CGRectMake(0.0, 0.0, 1.0, 1.0)) -> UIImage {
    let color = uicolorFromHex(rgbValue)
    UIGraphicsBeginImageContext(rectForDisk.size)
    let context = UIGraphicsGetCurrentContext()

    CGContextSetFillColorWithColor(context, color.CGColor)
    CGContextFillEllipseInRect(context, rectForDisk)

    let rectForCircle = CGRectMake(0.5, 0.5, rectForDisk.size.width - 1, rectForDisk.size.height - 1)
    CGContextSetLineWidth(context, 1.0)
    CGContextSetStrokeColorWithColor(context,
            UIColor.blackColor().CGColor)
    CGContextAddEllipseInRect(context, rectForCircle)
    CGContextStrokePath(context)

    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    return image
}

Answer 1

The problem is that you are creating a non-retina graphics context when using UIGraphicsBeginImageContext, as mentioned in the documentation:

This function is equivalent to calling the UIGraphicsBeginImageContextWithOptions function with the opaque parameter set to NO and a scale factor of 1.0.

Instead you should be using UIGraphicsBeginImageContextWithOptions to create your image context. You can keep passing NO for the opaque parameter if you want an image that supports transparency (same as what you are implicitly doing now).

In most cases you can pass 0.0 for the scale factor. This sets the scale factor to that of the device's main screen. Again, as mentioned in the documentation:

If you specify a value of 0.0, the scale factor is set to the scale factor of the device’s main screen.

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So, in short, you should create your image context like this:

UIGraphicsBeginImageContextWithOptions(rectForDisk.size, false, 0.0) // false for Swift