How can I fake superscript and subscript with Core Text and an Attributed String?

Question

I'm using an NSMutableAttribtuedString in order to build a string with formatting, which I then pass to Core Text to render into a frame. The problem is, that I need to use superscript and subscript. Unless these characters are available in the font (most fonts don't support it), then setting the property kCTSuperscriptAttributeName does nothing at all.

So I guess I'm left with the only option, which is to fake it by changing the font size and moving the base line. I can do the font size bit, but don't know the code for altering the base line. Can anyone help please?

Thanks!

EDIT: I'm thinking, considering the amount of time I have available to sort this problem, of editing a font so that it's given a subscript "2"... Either that or finding a built-in iPad font which does. Does anyone know of any serif font with a subscript "2" I can use?

Answer 1

There is no baseline setting amongst the CTParagraphStyleSpecifiers or the defined string attribute name constants. I think it's therefore safe to conclude that CoreText does not itself support a baseline adjust property on text. There's a reference made to baseline placement in CTTypesetter, but I can't tie that to any ability to vary the baseline over the course of a line in the iPad's CoreText.

Hence, you probably need to interfere in the rendering process yourself. For example:

• create a CTFramesetter, e.g. via CTFramesetterCreateWithAttributedString
• get a CTFrame from that via CTFramesetterCreateFrame
• use CTFrameGetLineOrigins and CTFrameGetLines to get an array of CTLines and where they should be drawn (ie, the text with suitable paragraph/line breaks and all your other kerning/leading/other positioning text attributes applied)
• from those, for lines with no superscript or subscript, just use CTLineDraw and forget about it
• for those with superscript or subscript, use CTLineGetGlyphRuns to get an array of CTRun objects describing the various glyphs on the line
• on each run, use CTRunGetStringIndices to determine which source characters are in the run; if none that you want to superscript or subscript are included, just use CTRunDraw to draw the thing
• otherwise, use CTRunGetGlyphs to break the run into individual glyphs and CTRunGetPositions to figure out where they would be drawn in the normal run of things
• use CGContextShowGlyphsAtPoint as appropriate, having tweaked the text matrix for those you want in superscript or subscript

I haven't yet found a way to query whether a font has the relevant hints for automatic superscript/subscript generation, which makes things a bit tricky. If you're desperate and don't have a solution to that, it's probably easier just not to use CoreText's stuff at all — in which case you should probably define your own attribute (that's why [NS/CF]AttributedString allow arbitrary attributes to be applied, identified by string name) and use the normal NSString searching methods to identify regions that need to be printed in superscript or subscript from blind.

For performance reasons, binary search is probably the way to go on searching all lines, the runs within a line and the glyphs within a run for those you're interested in. Assuming you have a custom UIView subclass to draw CoreText content, it's probably smarter to do it ahead of time rather than upon every drawRect: (or the equivalent methods, if e.g. you're using a CATiledLayer).

Also, the CTRun methods have variants that request a pointer to a C array containing the things you're asking for copies of, possibly saving you a copy operation but not necessarily succeeding. Check the documentation. I've just made sure that I'm sketching a workable solution rather than necessarily plotting the absolutely optimal route through the CoreText API.

Answer 2

Here is some code based on Tommy's outline that does the job quite well (tested on only single lines though). Set the baseline on your attributed string with @"MDBaselineAdjust", and this code draws the line to offset, a CGPoint. To get superscript, also lower the font size a notch. Preview of what's possible: http://cloud.mochidev.com/IfPF (the line that reads "[Xe] 4f¹⁴...")

Hope this helps :)

NSAttributedString *string = ...;
CGPoint origin = ...;

CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString((CFAttributedStringRef)string);
CGSize suggestedSize = CTFramesetterSuggestFrameSizeWithConstraints(framesetter, CFRangeMake(0, string.length), NULL, CGSizeMake(CGFLOAT_MAX, CGFLOAT_MAX), NULL);
CGPathRef path = CGPathCreateWithRect(CGRectMake(origin.x, origin.y, suggestedSize.width, suggestedSize.height), NULL);
CTFrameRef frame = CTFramesetterCreateFrame(framesetter, CFRangeMake(0, string.length), path, NULL);
NSArray *lines = (NSArray *)CTFrameGetLines(frame);
if (lines.count) {
    CGPoint *lineOrigins = malloc(lines.count * sizeof(CGPoint));
    CTFrameGetLineOrigins(frame, CFRangeMake(0, lines.count), lineOrigins);

    int i = 0;
    for (id aLine in lines) {
        NSArray *glyphRuns = (NSArray *)CTLineGetGlyphRuns((CTLineRef)aLine);

        CGFloat width = origin.x+lineOrigins[i].x-lineOrigins[0].x;

        for (id run in glyphRuns) {
            CFRange range = CTRunGetStringRange((CTRunRef)run);
            NSDictionary *dict = [string attributesAtIndex:range.location effectiveRange:NULL];
            CGFloat baselineAdjust = [[dict objectForKey:@"MDBaselineAdjust"] doubleValue];

            CGContextSetTextPosition(context, width, origin.y+baselineAdjust);

            CTRunDraw((CTRunRef)run, context, CFRangeMake(0, 0));
        }

        i++;
    }

    free(lineOrigins);
}
CFRelease(frame);
CGPathRelease(path);
CFRelease(framesetter);

`

Answer 3

You can mimic subscripts now using TextKit in iOS7. Example:

NSMutableAttributedString *carbonDioxide = [[NSMutableAttributedString alloc] initWithString:@"CO2"];
[carbonDioxide addAttribute:NSFontAttributeName value:[UIFont systemFontOfSize:8] range:NSMakeRange(2, 1)];
[carbonDioxide addAttribute:NSBaselineOffsetAttributeName value:@(-2) range:NSMakeRange(2, 1)];