Question

I’m completely in the dark with Core Text’s line spacing. I’m using NSAttributedString and I specify the following attributes on it:
– kCTFontAttributeName
– kCTParagraphStyleAttributeName

From this the CTFrameSetter gets created and drawn to context.

In the paragraph style attribute I’d like to specify the height of the lines.

When I use kCTParagraphStyleSpecifierLineHeightMultiple each line receives padding at the top of the text, instead of the text being displayed in the middle of this height.

When I use kCTParagraphStyleSpecifierLineSpacing a padding is added to the bottom of the text.

Please help me achieve a specified line height with the text(glyphs) in the middle of that height, instead of the text sitting either at the bottom or the top of the line.

Is this not possible without going down the route of explicitly creating CTLine ‘s and so forth?

Answer 1

I’m still not 100% confident in my following statements, but it seems to make sense. Please correct me where I am wrong.

The line height (leading) refers to the distance between the baselines of successive lines of type. The baseline here can be interpreted as the imaginary line which the text sits on.

Spacing is the space between lines. The space appears after the line of text.

I ended up using the following solution to my problem:

// NOT SURE WHAT THE THEORY BEHIND THIS FACTOR IS. WAS FOUND VIA TRIAL AND ERROR.
    CGFloat factor = 14.5/30.5;
    CGFloat floatValues[4];
    floatValues[0] = self.lineHeight * factor/(factor + 1);
    floatValues[1] = self.lineHeight/(factor + 1);
    floatValues[2] = self.lineHeight;

This matrix is used with the paragraph style parameter for NSAttributedString:

CTParagraphStyleSetting paragraphStyle[3];

paragraphStyle[0].spec = kCTParagraphStyleSpecifierLineSpacing;
paragraphStyle[0].valueSize = sizeof(CGFloat);
paragraphStyle[0].value = &floatValues[0];

paragraphStyle[1].spec = kCTParagraphStyleSpecifierMinimumLineHeight;
paragraphStyle[1].valueSize = sizeof(CGFloat);
paragraphStyle[1].value = &floatValues[1];

paragraphStyle[2].spec = kCTParagraphStyleSpecifierMaximumLineHeight;
paragraphStyle[2].valueSize = sizeof(CGFloat);
paragraphStyle[2].value = &floatValues[2];

CTParagraphStyleRef style = CTParagraphStyleCreate((const CTParagraphStyleSetting*) &paragraphStyle, 3);
[attributedString addAttribute:(NSString*)kCTParagraphStyleAttributeName value:(id)style range:NSMakeRange(0, [string length])];
CFRelease(style);

Hope this helps someone. I’ll update this answer as I discover more relevant information.