Re: advancementForGlyph problem
Re: advancementForGlyph problem
- Subject: Re: advancementForGlyph problem
- From: "David F." <email@hidden>
- Date: Sun, 2 May 2010 19:01:03 -0600
On Apr 30, 2010, at 7:20 AM, Graham Cox wrote:
>
> On 30/04/2010, at 9:08 PM, steven Hooley wrote:
>
>>> From your later response it seems like your question is really
>> regarding drawing strikethroughs and underlines. If so, the strike
>> through and underline are not part of the glyph or font - they are
>> just a line drawn from the start point of a range of glyphs to the end
>> point of a range of glyphs. You cannot draw them piecemeal, a glyph at
>> a time, and expect them to perfectly match up without overlap or
>> underlap (apologies for making up a word), as the amount that any 2
>> glyphs overlap is particular to those 2 glyhps. ie. The strikethrough
>> of an 'A' followed by a 'V' would need to be a different length than
>> that of an 'A' followed by a 'j'.
>
>
> Not that this will tell you anything about what Cocoa does, but when I had to implement underlines and strikethroughs for text-on-path in DrawKit, I did so by working a line at a time and knocking out any places where descenders would hit the line. For text-on-path, drawing a line per glyph looks absolutely terrible, being straight and not curved, and having gaps where the character curves away from its neighbour.
>
> I would imagine that for performance reasons as well as aesthetic, Cocoa draws whole lines for strikethroughs and underlines as well.
If you zoom your screen by Ctrl-Scrollwheel with TextEdit open, you can see that characters (and their strikethrough/underline) appear to be drawn individually when the character is first inserted (you can see the seams in the strikethrough/underline between characters) but then the line appears to get redrawn all at once about a second later and the seams (mostly) disappear.
> I did also discover that relying on the values exactly returned by a font didn't always give similar results to Cocoa's underline positioning - I never did quite work out its metrics but arrived at certain fudge factors empirically that gave almost identical results with most fonts.
I was able to get correct metrics for glyphs with ATSUI. But since that is now deprecated I'm rewriting my code to use Cocoa text. I wish I could say that the rewrite was easy, but I'm dying a death of 1000 papercuts.
David
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