How does line spacing work in Core Text? (and why is it different from NSLayoutManager?)

Question

I'm trying to draw text using Core Text functions, with a line spacing that's as close as possible to what it would be if I used NSTextView.

Take this font as an example:

NSFont *font = [NSFont fontWithName:@"Times New Roman" size:96.0];

The line height of this font, if I would use it in an NSTextView is 111.0.

NSLayoutManager *lm = [[NSLayoutManager alloc] init];
NSLog(@"%f", [lm defaultLineHeightForFont:font]); // this is 111.0

Now, if I do the same thing with Core Text, the result is 110.4 (assuming you can calculate the line height by adding the ascent, descent and leading).

CTFontRef cFont = CTFontCreateWithName(CFSTR("Times New Roman"), 96.0, NULL);
NSLog(@"%f", CTFontGetDescent(cFont) + CTFontGetAscent(cFont) + 
             CTFontGetLeading(cFont)); // this is 110.390625

This is very close to 111.0, but for some fonts the difference is much bigger. E.g. for Helvetica, NSLayoutManager gives 115.0 whereas CTFont ascent + descent + leading = 96.0. Clearly, for Helvetica, I wouldn't be able to use ascent + descent + leading to calculate the spacing between lines.

So I thought I'd use CTFrame and CTFramesetter to layout a few lines and get the linespacing from that. But that also gives different values.

CTFontRef cFont = CTFontCreateWithName(CFSTR("Times New Roman"), 96.0, NULL);
NSDictionary *attrs = [NSDictionary dictionaryWithObject:(id)cFont forKey:(id)kCTFontAttributeName];
NSAttributedString *threeLines = [[NSAttributedString alloc] initWithString:@"abcdefg\nabcdefg\nabcdefg" attributes:attrs];

CTFramesetterRef threeLineFramesetter =  CTFramesetterCreateWithAttributedString((CFAttributedStringRef)threeLines);
CGMutablePathRef path = CGPathCreateMutable();
CGPathAddRect(path, NULL, CGRectMake(0.0, 0.0, 600.0, 600.0));
CTFrameRef threeLineFrame = CTFramesetterCreateFrame(threeLineFramesetter, CFRangeMake(0, 0), path, NULL);

CGPoint lineOrigins[3];
CTFrameGetLineOrigins(threeLineFrame, CFRangeMake(0, 0), lineOrigins);
NSLog(@"space between line 1 and 2: %f", lineOrigins[0].y - lineOrigins[1].y); // result: 119.278125
NSLog(@"space between line 2 and 3: %f", lineOrigins[1].y - lineOrigins[2].y); // result: 113.625000

So the line spacing is now even more different from the 111.0 that was used in my NSTextView, and not every line is equal. It seems that the line breaks add some extra space (even though the default value for paragraphSpacingBefore is 0.0).

I'm working around this problem now by getting the line height via NSLayoutManager and then individually drawing each CTLine, but I wonder if there's a better way to do this.

Answer 1

OK, so I took a good look at what goes on in the guts of NSLayoutManager, and it appears, based on my reading of the disassembly, that the code it uses boils down to something like this:

CGFloat ascent = CTFontGetAscent(theFont);
CGFloat descent = CTFontGetDescent(theFont);
CGFloat leading = CTFontGetLeading(theFont);

if (leading < 0)
  leading = 0;

leading = floor (leading + 0.5);

lineHeight = floor (ascent + 0.5) + floor (descent + 0.5) + leading;

if (leading > 0)
  ascenderDelta = 0;
else
  ascenderDelta = floor (0.2 * lineHeight + 0.5);

defaultLineHeight = lineHeight + ascenderDelta;

This will get you the 111.0 and 115.0 values for the two fonts you mention above.

I should add that the correct way, according to the OpenType specification, is just to add the three values (being careful, if you’re using an API that doesn’t make them all positive, to get the sign of the descent value correct).