Losing NSTextEffectLetterpressStyle when drawing into CGBitmapContext

Question

I am drawing text on top of a solid background color, and while it is appearing, the NSTextEffectLetterpressStyle effect applied to the attributed string isn't showing up - you can only see the text color not the additional white outline that provides the desired look. It looks essentially the exact same as if I didn't apply the letterpress effect. Why is this, how can I draw the letterpress text effect correctly?

let bitmapContext = CGBitmapContextCreate(nil, UInt(imageRect.width), UInt(imageRect.height), UInt(8), UInt(imageRect.width * 16), CGColorSpaceCreateDeviceRGB(), CGBitmapInfo(CGImageAlphaInfo.PremultipliedLast.rawValue))

CGContextSetFillColorWithColor(bitmapContext, UIColor.blackColor().CGColor)
let fullBox = CGRectMake(0, 0, imageRect.width, imageRect.height)
CGContextAddRect(bitmapContext, fullBox)
CGContextFillPath(bitmapContext)

CGContextSetTextDrawingMode(bitmapContext, kCGTextFill)
CGContextSetTextPosition(bitmapContext, drawPoint.x, drawPoint.y)
let coloredAttributedString = NSAttributedString(string: "20", attributes:[NSFontAttributeName: myFont, NSForegroundColorAttributeName: textColor, NSTextEffectAttributeName: NSTextEffectLetterpressStyle])
let displayLineTextColored = CTLineCreateWithAttributedString(coloredAttributedString)
CTLineDraw(displayLineTextColored, bitmapContext)

let cgImage = CGBitmapContextCreateImage(bitmapContext)
var myImage = CIImage(CGImage: dateStampCGImage)

This is the result:

This is what I expected it to be (notice the white-ish outline):

Answer 1

Both CFAttributedString and NSAttributedString can apply arbitrary attributes to ranges. The attributed string types don't inherently interpret the attributes.

Rather, the string drawing technologies interpret the attributes. Core Text understands a different set of attributes than UIKit or AppKit. The set of attributes supported by Core Text is documented here. It does not list (an equivalent of) NSTextEffectAttributeName.

If you set up a current graphics context using UIGraphicsBeginImageContextWithOptions() and draw into that using the NSAttributedString drawing methods, that should work, since it's using UIKit to do the drawing.