When converting text to image, the converted images were different between macOS and iOS. The main differences are font weight and line height.

Font Weight

Thought the font weight and the fonts are the same, on macOS the font result is always thicker. I don't know why. But in my experience, if you use "HelveticaNeue-Light" for iOS, use "HelveticaNeue-Thin" for macOS.

Line Height

The line height of font is even tricky.

Equation

In Apple's doc, Apple gives below graph. We could draw a simple equation from the graph.

line height = ascent + decent + line gap (leading)

So I did two tests on both iOS and macOS in Playgound.

// macOS
func getFontInfo(_ name:String) {
    let font = NSFont(name: name, size: 17.0)!
    print(font.ascender) // 13.09033203125
    print(font.descender) // -3.90966796875
    print(font.leading) // 0.0
    print(font.ascender - font.descender + font.leading) // 17.0
    
    let layoutManager = NSLayoutManager()
    print(layoutManager.defaultLineHeight(for: font)) // 20.0
}

getFontInfo("Helvetica")

// iOS
func getFontInfo(_ name:String) {
    let font = UIFont(name: name, size: 17.0)!
    print(font.ascender) // 15.64033203125
    print(font.descender) // -3.90966796875
    print(font.leading) // 0.0
    print(font.ascender - font.descender + font.leading) // 19.55
    print(font.lineHeight) // // 19.55
}

getFontInfo("Helvetica")

From the two tests, we could draw two conclusions:

1. The equation on iOS was balanced, but on macOS was not.
2. For the same font with the same weight, the ascender were different.

I didn't know why those happened. So I sent an "Apple Developer Technical Support". Here was the reply from Apple.

According to Apple, if I wanted to use the equation, I should use Core Text framework. But in fact Apple didn't provide line height in Core Text.

Then I did another two tests.

// macOS
func getLineHeightForFontName(_ name:String) {
    let font = CTFontCreateWithName(name as CFString, 17.0, nil)
    
    print(CTFontGetAscent(font)) // 13.09033203125
    print(CTFontGetDescent(font)) // 3.90966796875
    print(CTFontGetLeading(font)) // 0.0
}

getLineHeightForFontName("Helvetica")

// iOS
func getLineHeightForFontName(_ name:String) {
    let font = CTFontCreateWithName(name as CFString, 17.0, nil)
    
    print(CTFontGetAscent(font)) // 13.09033203125
    print(CTFontGetDescent(font)) // 3.90966796875
    print(CTFontGetLeading(font)) // 0.0
}

getLineHeightForFontName("Helvetica")

From all four tests, we could get the conclusions:

1. Though on iOS, the equation was balanced. The ascent property was modified by Apple.
2. On macOS, the line height was modified by Apple.
3. From the above two conclusions, both NSFont and UIFont were not trusted. The only trusted line height was something we get from Core Text.

Line Height

#if os(macOS)
func getLineHeight(_ font:NSFont) -> CGFloat {
    let ctFont = CTFontCreateWithName(font.fontName as CFString, font.pointSize, nil)
    return CTFontGetAscent(ctFont) + CTFontGetDescent(ctFont) + CTFontGetLeading(ctFont)
}
#else
func getLineHeight(_ font:UIFont) -> CGFloat {
    let ctFont = CTFontCreateWithName(font.fontName as CFString, font.pointSize, nil)
    return CTFontGetAscent(ctFont) + CTFontGetDescent(ctFont) + CTFontGetLeading(ctFont)
}
#endif

Others

NSTextView Best Practice

Text Programming Guide for iOS

Cocoa Text Architecture Guide

Core Text - Calculating line heights