What's the CoreText equivalent to AppKit's NSObliquenessAttributeName?

Question

I'm drawing some text in Mac/iOS cross-platform code using CoreText. I may be using fonts that do not have a real "Italic" version installed in the OS for all users, but they need to be aware that the text is italic even then.

With AppKit's NSAttributedString -drawAtPoint:, I can use NSObliquenessAttributeName to make the text slanted (and thus look italic -- well, oblique). CoreText doesn't seem to have an equivalent for this attribute. At least I found none in CTStringAttributes.h (not that there's any documentation even years after CoreText was released).

Does anyone know how I can get oblique text with CoreText on iOS?

Answer 1

I'd try using the affine transform argument to CTFontCreateWithName() with a shear matrix. For instance

CGAffineTransform matrix = { 1, 0, 0.5, 1, 0, 0 };
CTFontRef myFont = CTFontCreateWithName(CFSTR("Helvetica"), 48, &matrix);

That will create quite an extreme skew (assuming I got it right), but you get the idea.

Update:

In fact, the documentation appears to imply that this is the right way to do things .

Answer 2

Haven't tried, but according to iOS Programming Pushing The Limits , passing kCTFontItalicTrait to CTFontCreateCopyWithSymbolicTraits will choose true italic if available, and oblique otherwise. There's also kCTFontSlantTrait for manual decimal slant up to 30 degrees.

Answer 3

Displaying a font that has no italic trait as italic is generally a bad idea. However, I can understand that there are some cases where this has to be enforced anyways.

The only solution that comes to my mind right now is to create a custom font with a sheared font matrix:

CGAffineTransform matrix = CGAffineTransformMake(1, tan(degreesToRadians(0)), tan(degreesToRadians(20)), 1, 0, 0);  
CTFontRef myfont = CTFontCreateWithName(CFSTR("Helvetica"), 48, &matrix);

You'll have to play with the matrix and see what brings the best results. (Please not that this is a fake code mix out of my head and the internet.)