Kerning issue when rendering string using the CoreText

Question

I am trying to render string using the CoreText api. Each character is rendered in the CAShapeLayer and i am getting the layer offset (x coordinate) for each character like this:

let offset = CTLineGetOffsetForStringIndex(line, glyphIndex, nil)

But the resulting offset doesn't seem to respect kerning between letters. Here is a an image of what i mean - the top label is a UILabel, the bottom one is my custom rendering:

As you can see the "A" letter in my custom label is placed further from the "W" letter.

I would really appreciate any help. Thanks in advance.

Answer 1

In case someone has the same problem, i used an approach from this place: https://github.com/MichMich/XMCircleType/blob/master/XMCircleType/Views/XMCircleTypeView.m, the kerningForCharacter function. Here is the function itself:

- (float)kerningForCharacter:(NSString *)currentCharacter afterCharacter:(NSString *)previousCharacter
{
    //Create a unique cache key
    NSString *kerningCacheKey = [NSString stringWithFormat:@"%@%@", previousCharacter, currentCharacter];

    //Look for kerning in the cache dictionary
    NSNumber *cachedKerning = [self.kerningCacheDictionary objectForKey:kerningCacheKey];

    //If kerning is found: return.
    if (cachedKerning) {
        return [cachedKerning floatValue];
    }

    //Otherwise, calculate.
    float totalSize = [[NSString stringWithFormat:@"%@%@", previousCharacter, currentCharacter] sizeWithAttributes:self.textAttributes].width;
    float currentCharacterSize = [currentCharacter sizeWithAttributes:self.textAttributes].width;
    float previousCharacterSize = [previousCharacter sizeWithAttributes:self.textAttributes].width;

    float kerning = (currentCharacterSize + previousCharacterSize) - totalSize;

    //Store kerning in cache.
    [self.kerningCacheDictionary setValue:@(kerning) forKey:kerningCacheKey];

    //Return kerning.
    return kerning;
}