CTFrame dropping glyphs at the edge

Question

I am trying to render Arabic text in my iOS app with custom TTF font (scheherazade) using core-text, which works for the most part - however certain glyphs at the edge of the CTFrame are dropped.

When I adjust the frame-size to make the dropped-glyphs appear in the interior of the frame, they display corretly, which leads me believe something is going wrong in inside CTFrameDraw. Below is the code I'm using to render the Arabic-text:

CGContextRef context = UIGraphicsGetCurrentContext();

// Flip the coordinate system
CGContextSetTextMatrix(context, CGAffineTransformIdentity);
 CGContextTranslateCTM(context, 0, v.textFrame.size.height);
CGContextScaleCTM(context, 1.0, -1.0);

CGMutablePathRef path = CGPathCreateMutable(); //1
 CGPathAddRect(path, NULL, v.textFrame );

CGFloat minLineHeight = 60.0;
CGFloat maxLineHeight = 60.0;

CTTextAlignment paragraphAlignment = kCTRightTextAlignment;
CTLineBreakMode lineBrkMode = kCTLineBreakByWordWrapping;

CTParagraphStyleSetting setting[4] = {
{kCTParagraphStyleSpecifierAlignment, sizeof(CTTextAlignment), &paragraphAlignment},
{kCTParagraphStyleSpecifierMinimumLineHeight, sizeof(CGFloat), &minLineHeight},
{kCTParagraphStyleSpecifierMaximumLineHeight, sizeof(CGFloat), &maxLineHeight},
{kCTParagraphStyleSpecifierLineBreakMode, sizeof(CTLineBreakMode), &lineBrkMode}
};


CTParagraphStyleRef paragraphStyle = CTParagraphStyleCreate(setting, 4);
NSDictionary *attr        = [NSDictionary dictionaryWithObjectsAndKeys:
(id)v.arabicFont, (id)kCTFontAttributeName,
paragraphStyle, (id)kCTParagraphStyleAttributeName,
nil];

CFRelease(paragraphStyle);

NSAttributedString* attString = [[[NSAttributedString alloc]
initWithString:v.verseText attributes:attr] autorelease]; //2

CTFramesetterRef framesetter =
CTFramesetterCreateWithAttributedString((CFAttributedStringRef)attString); //3
CTFrameRef frame =
CTFramesetterCreateFrame(framesetter,
CFRangeMake(0, [attString length]), path, NULL);

CTFrameDraw(frame, context); //4

CFRelease(frame); //5
CFRelease(path);
CFRelease(framesetter);

Also attached are the screenshots showing the problem I face. Any help would be much appreciated. Thanks.

invalid: http://stellarbeacon.com.au/invalid.png valid : http://stellarbeacon.com.au/valid.png

Answer 1

There are some bugs in CoreText related to determining the correct frame size. Some of these where fixed in iOS 6, e.g. http://www.cocoanetics.com/2012/02/radar-coretext-line-spacing-bug/

If your problem still exists there then you should file a Radar with the specifics. Also you should open a call with Apple DTS which can probably provide you with a workaround. Often - if your problem is indeed a bug - then you get your DTS call credited back.

PS: try to display your text with my DTCoreText views which do manual layouting and display and see if the problem can be reproduced there. If not then you have your workaround.