Unwanted padding in CATextLayer - cocoa

Question

I am having a problem of drawing a character in a CATextLayer such that the layer is exactly the size of the character.
I use the code below to get the size of the glyph corresponding to the character in the string. At the moment I am neglecting diacretics, so that I am assuming that there is a one to one correlation between glyphs and characters.
I also get nice bounding box values for several characters for the Helvetica font at size 128pt:
Character | x | y | width | height |
B | 9.4 | 0.0 | 70.8 | 91.8 |
y | 1.3 | -27.4 | 61.2 | 96.0 |
I am not sure where the origin of the coordinate system is located in which the coordinates are expressed. I am assuming that (0,0) is at the very left and vertically located on the baseline of the font. That is why 'y' has a negative y value.
I am using this code to calculate the size of a capital B and resize its CATextLayer accordingly.
- (CATextLayer *) testTextLayer
{
CATextLayer *l = [CATextLayer layer];
l.string = #"B";
NSUInteger len = [l.string length];
l.fontSize =128.f;
CGColorRef blackColor = CGColorCreateGenericGray(0.f, 1.f);
l.foregroundColor = blackColor;
CGColorRelease(blackColor);
// need to set CGFont explicitly to convert font property to a CGFontRef
CGFontRef layerFont = CGFontCreateWithFontName((CFStringRef)#"Helvetica");
l.font = layerFont;
// get characters from NSString
UniChar *characters = (UniChar *)malloc(sizeof(UniChar)*len);
CFStringGetCharacters((__bridge CFStringRef)l.string, CFRangeMake(0, [l.string length]), characters);
// Get CTFontRef from CGFontRef
CTFontRef coreTextFont = CTFontCreateWithGraphicsFont(layerFont, l.fontSize, NULL, NULL);
// allocate glyphs and bounding box arrays for holding the result
// assuming that each character is only one glyph, which is wrong
CGGlyph *glyphs = (CGGlyph *)malloc(sizeof(CGGlyph)*len);
CTFontGetGlyphsForCharacters(coreTextFont, characters, glyphs, len);
// get bounding boxes for glyphs
CGRect *bb = (CGRect *)malloc(sizeof(CGRect)*len);
CTFontGetBoundingRectsForGlyphs(coreTextFont, kCTFontDefaultOrientation, glyphs, bb, len);
CFRelease(coreTextFont);
l.position = CGPointMake(200.f, 100.f);
l.bounds = bb[0];
l.backgroundColor = CGColorCreateGenericRGB(0.f, .5f, .9f, 1.f);
free(characters);
free(glyphs);
free(bb);
return l;
}
This is the result I am getting from the above code. It seems to me that the size is correct, however there is some kind of padding taking place around the character.
Now my questions
Am I right with the assumption of the origin of the bounding box of the glyph?
How can one draw the letter such that it fits neatly into the layer, without this padding? Or alternatively, how can one control this padding?
Maybe I am missing an obvious point here. Is there now way after setting the size and the font of the layer to shrink wrap the layer around the character in a defined way (meaning with optional padding, a bit like in CSS)?

Answer 1

How about creating a CGPath from a glyph with CTFontCreatePathForGlyph and then getting its bounding box with CGPathGetBoundingBox?
An alternative would be to create a CTRun somehow and use the CTRunGetImageBounds function which also returns a "tight" bounding box, but this probably requires more lines of code than the aforementioned approach and you'd need to have a graphics context.

Answer 2

I assume this has to do with the built-in space around letters. Their bounding box usually includes some amount of space that serves as a general spacing if you assemble glyphs in a line. These are then optimized with kerning tables.
And yes, extracting the actual bezier path is a very good method of getting a tight bounding box. I have been doing that for years, though I have no experience using CATextLayers.