問題描述
CATextLayer 中不需要的填充 (Unwanted padding in CATextLayer)
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)?
參考解法
方法 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.
方法 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.
(by GorillaPatch、omz、Martin Winter)